Stormwater Runoff Control Guidelines
Longview-Kelso Urban Area
July, 2004
STORMWATER RUNOFF AND EROSION CONTROL GUIDELINES
TABLE OF CONTENTS
Title
Page No.
SECTION 17.80 INTRODUCTION
.010 All Construction, Excavation and Creation ........................................2
.020 Stormwater Runoff and Erosion Control Plans ..................................2
.030 Submission of Plans............................................................................2
.040 Reserved..............................................................................................3
.050 Categorical Exemptions ......................................................................3
.060 Maintenance of Stormwater Runoff and Erosion Control ..................3
.070 City May Contract...............................................................................4
.080 Definitions...........................................................................................4
SECTION 17.80.090 STORMWATER RUNOFF GUIDELINES
.010 Authority and Purpose ........................................................................7
.020 Engineering Policy..............................................................................7
.030 Applicability .......................................................................................8
.040 Standard Specifications/Standard Plans ..............................................8
.050 Approval of Alternate Materials or Methods......................................8
.060 Special Design Problems ....................................................................8
.070 Revisions to Stormwater Runoff Control Guidelines .........................8
.080 Definitions...........................................................................................9
SECTION 17.80.090 CONSTRUCTION PLANS
.100 General Information ..........................................................................14
.110 Plan Preparation ................................................................................14
.120 Sheet Size .........................................................................................14
.130 Scale of Plans....................................................................................14
.140 Required Sheets ................................................................................14
.150 Title Sheet .........................................................................................15
.160 Plan Sheet..........................................................................................15
.170 Profile Sheet ......................................................................................16
.180 Detail Sheets .....................................................................................17
.190 Supporting Information.....................................................................17
.200 Facility Plan ......................................................................................17
.210 Erosion Control Plan.........................................................................17
.220 Plan Submittal ...................................................................................18
.230 As-Built Plan Requirements .............................................................18
.240 Storm Drains .....................................................................................19
SECTION 17.80.092 STORM DRAINAGE
.010 General Design Requirements ..........................................................21
.020
.030
.040
.050
.060
Site Drainage Plans ...........................................................................22
Pipe Materials and Size ....................................................................23
Minimum Design Criteria .................................................................24
Culverts .............................................................................................29
Bridges ..............................................................................................29
SECTION 17.80.092 ALIGNMENT AND COVER
.070 Right-of-Way Location.....................................................................31
.080 Curvature...........................................................................................31
.090 Minimum Cover................................................................................31
.100 Easements .........................................................................................31
.110 Relation to Watercourses ..................................................................32
SECTION 17.80.092 STRUCTURE LOCATION
.120 Manholes...........................................................................................34
.130 Catch Basins......................................................................................34
SECTION 17.80.140 STORMWATER DETENTION/RETENTION
.150 Development Not Requiring Detention ............................................36
.155 Off-site Regional Retention/Detention Facilities............................ 36
.160 Floodplain Information .....................................................................37
.170 Detention Volume.............................................................................37
.180 Emergency Overflow ........................................................................38
.190 Detention Facilities ...........................................................................38
.200 Dry Pond ...........................................................................................38
.210 Dry Vault/Tank .................................................................................41
.220 Erosion Control.................................................................................43
.230 Erosion Control - Application...........................................................43
SECTION 17.80.092 PRIVATE DRAINAGE SYSTEMS
.240 Subdivisions......................................................................................45
.250 Subsurface Drainage .........................................................................45
SECTION 17.80.092.260 ON-SITE WATER QUALITY CONTROL MEASURES
.260 On-Site Water Quality Control Measures.........................................47
.270 Water Quality Design Storm.............................................................47
.280 Oil/Water Separators.........................................................................47
Figure .290 Oil/Water Separation .......................................................................50
.300 Biofiltration Swale ............................................................................51
.310 Vegetative Filter Strip.......................................................................54
Figure .320 Biofiltration/Vegetated Filter Strip ..................................................56
.330 Wet Pond...........................................................................................57
Figure .340 Detention/Wet Pond Schematic .......................................................60
Figure .350 Section AA Detention/Wet Pond .....................................................61
Figure .360 Detention/Wet Pond Detail ..............................................................62
.370 Nutrient Control Wet Pond ...............................................................63
.380 Wet Vault/Tank.................................................................................66
Figure .390 Typical Detention Tank ...................................................................69
SECTION 17.80.094 EROSION CONTROL GUIDELINES
.010 Introduction ......................................................................................71
.020 Erosion Control Plan Submittal Requirements . ...............................71
.030 Considerations in Planning for Erosion Control...............................72
.040 Permit Application Requirements:
Construction on Existing Lots of Record..........................................73
.050 Other Private Developments Construction .......................................73
.060 Private Construction in Public Rights-of-Way .................................74
.070 Public Works Construction ..............................................................74
SECTION 17.80.094 RECOMMENDED EROSION CONTROL MEASURES
.080 Introduction and General Plan Requirements ...................................77
.090 Erosion Control Matrices .................................................................77
Table .100 Erosion Control Matrix – Single Family/Duplex Residential ..........79
Table .110 Erosion Control Matrix –
Commercial, Subdivision & Large Site Construction ....................80
Table .120 Erosion Control Matrix –
Utilities Construction and Stock Piles/Ditches/Swales Protection ..81
.130 Recommended Design Criteria .........................................................82
.140 Gravel Construction Entrances .........................................................82
Figure .150 Gravel Construction Entrance ..........................................................84
Figure .160 Wheel Wash .....................................................................................85
.170 Temporary Sediment Fences.............................................................86
Figure .180 Sediment Fence ................................................................................87
.190 Straw Bale Sediment Barrier ............................................................88
Figure .200 Straw Bale Sediment Barrier ...........................................................90
Figure .210 Straw Bale Sediment Barrier ...........................................................91
.220 Sidewalk Subgrade Gravel Barrier ...................................................92
.230 Undisturbed Buffers..........................................................................92
.240 Establishing Temporary Grasses & Permanent Vegetative Cover ...93
.250 Straw Mulch .....................................................................................95
.260 Erosion Blankets ...............................................................................95
.270 Plastic Sheet Covering .....................................................................96
.280 Sediment Traps & Ponds ..................................................................97
Figure .290 Sediment Trap ..................................................................................98
Figure .300 Sediment Pond ...............................................................................100
.300 Temporary Interceptor Dikes and Swales ......................................101
Figure .310 Interceptor Swales and Dikes .........................................................103
.320 Storm Drain Inlet Protection ..........................................................104
Figure .330 Filter Fabric Inlet Barrier ...............................................................105
Figure .340 Block and Gravel Inlet Barrier .......................................................106
Figure .350 Gravel and Wire Mesh Inlet Barrier ..............................................107
.360 Check Dams ...................................................................................108
Figure .370 Check Dams ...................................................................................109
SECTION 17.80.096 FORMS AND RESOURCES
.010 Standard Forms ..............................................................................111
.020 SCS Universal Soil Loss Equation ................................................112
.030 Recommended Standard Notes For Erosion Control Plans ...........113
.040 Standard Notes For Sediment Fences ............................................114
.050 Informational Resources ................................................................115
.098 Violations; penalties .......................................................................115
STORMWATER RUNOFF CONTROL GUIDELINES
LIST OF FIGURES
Title
Page No.
STORM DRAINAGE 17.80.092
.290 Oil/Water Separation ........................................................................50
.320 Biofiltration / Vegetated Filter Strip .................................................56
.340 Detention / Wet Pond Schematic ......................................................60
.350 Section AA Detention / Wet Pond ....................................................61
.360 Detention / Wet Pond Detail .............................................................62
.390 Typical Detention Tank ....................................................................69
EROSION CONTROL 17.80.094
.150 Gravel Construction Entrances .........................................................84
.160 Wheel Wash ......................................................................................85
.180 Sediment Fence.................................................................................87
.200 Straw Bale Sediment Barrier ............................................................90
.210 Straw Bale Sediment Barrier ............................................................91
.290 Sediment Trap...................................................................................98
.300 Sediment Pond ................................................................................100
.310 Interceptor Swales and Dikes..........................................................103
.330 Filter Fabric Inlet Barrier ................................................................105
.340 Block and Gravel Inlet Barrier........................................................106
.350 Gravel and Wire Mesh Inlet Barrier ...............................................107
.370 Check Dams ....................................................................................109
LIST OF TABLES
Title
Page No.
EROSION CONTROL 17.80.094
.100 Erosion Control Matrix - Single Family/Duplex Residential ..........79
.110 Erosion Control Matrix –
Commercial, Subdivision, & Large Site Construction .....................80
.120 Erosion Control Matrix –
Utilities Construction and Stock Piles/Ditches/Swales Protection ...81
SECTION 17.80
INTRODUCTION
SECTION 17.80.010 INTRODUCTION
Section 17.80.010 All construction, excavation and creation of impervious surfaces
to conform to the “Guidelines”.
All subdivisions of land, construction of buildings and structures, excavations of land,
and the creation of impervious land surfaces hereafter performed within the City of
Longview shall conform to the requirements of the “Stormwater Runoff Control
Guidelines for the Longview-Kelso Urban Area” and the “Erosion Control Guidelines for
the Longview-Kelso Urban Area” as set forth in this chapter.
Section 17.80.020 Stormwater runoff and erosion control plans to be approved prior
to the issuance of any building permit, excavation permit, or approval of any
preliminary plat or short subdivision of land
No building permit required by Title 17 of this code shall be issued until plans for
stormwater runoff and erosion control have been approved in writing by the Stormwater
Plans Examiner. No permit for excavation, or for construction, repair and maintenance
work within any public right of way shall be granted until plans for stormwater runoff
and erosion control have been approved in writing by the Stormwater Plans Examiner.
No permit for driveway construction shall be granted until plans for stormwater runoff
and erosion control have been approved in writing by the Stormwater Plans Examiner.
No short subdivision under chapter 19.67 of this code shall be approved until plans for
stormwater runoff and erosion control have been approved in writing by the Stormwater
Plans Examiner. No public improvement plans for any preliminary plat under chapter
19.80 of this code shall be approved until plans for stormwater runoff and erosion control
have been approved in writing by the Stormwater Plans Examiner.
Section 17.80.030 Submission of plans for stormwater runoff and erosion control
Any applicant for:
•
•
•
•
•
•
A building permit under Title 16 of this code,
An excavation permit,
A permit to perform construction, repair and maintenance work within any public
right of way,
A permit for the construction of a driveway under Title 17 of this code,
Approval of a short subdivision under Chapter 19.67 of this code, or
Approval of a public improvement drawing under Title 19 of this code
shall prepare and submit plans for stormwater runoff and erosion control as required by
the Guidelines. If the project is “categorically exempt” from the requirements of the
Guidelines as provided in Section 17.80.050, such plans shall be marked or stamped as
categorically exempt from the provisions hereof by the Stormwater Plans Examiner
within five days thereafter. If such plans are not categorically exempt, the Stormwater
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Plans Examiner shall, within 30 days after receipt of such plans, either approve the same,
reject such plans, or require modifications therein to conform to the Guidelines.
Section 17.80.040 Reserved
Section 17.80.050 Categorical exemptions, filing fee
The following work, construction, and development is categorically exempt from the
provisions of this Ordinance:
1. Commercial, agricultural, and forest practices regulated under Title 222 WAC,
except for Class IV General Forest Practices that are conversions of/or from
timberland to other uses, are exempt from the provisions of this chapter.
2. Development undertaken by the Washington State Department of Transportation
in State highway rights-of-way is regulated by Chapters 173-270 WAC, the Puget
Sound Highway Runoff Program.
3. Any work, construction, or other activities that creates less than 5,000 square feet
of new impervious area and cause less than 5000 square feet of ground surface
disturbance is exempt from the provisions of this chapter.
All other new development and redevelopment is subject to the requirements of this
chapter.
In order to claim a categorical exemption, plans shall be submitted in accordance with
this Ordinance. If an examination of such plans reveals that the proposed project is
categorically exempt as provided in this section, such plans shall be marked or “stamped”
as categorically exempt and returned to the applicant.
Section 17.80.060 Maintenance of stormwater runoff and erosion control facilities
and structures
1. All stormwater runoff and erosion control facilities and structures situated within
the City of Longview and on property owned by the City, or within rights-of-way
of the City shall be maintained as required by the Guidelines, by the city.
2. All stormwater runoff and erosion control facilities and structures situated on real
property within the City of Longview, except as provided in Subsection 1 of this
section, shall be maintained, as required by the Guidelines, by the owner (or
occupant) of the real property upon which such facilities or structures are situated.
To assure notice thereof to future owners of such real property, after final
inspection and prior to the issuance of a certificate of occupancy or certificate of
completion, the owner of the land shall make, execute and record a document
with the auditor of Cowlitz County, Washington, setting forth the legal
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description of the land, a description of the stormwater runoff and erosion control
facility or structure, the location thereof, and a notification that the owner of the
land, and any successor owner thereof, is responsible for maintaining said facility
or structure in accordance with the requirements of the Guidelines.
3. Inspection of all work and construction of stormwater runoff and erosion control
facilities shall be performed as provided in the Guidelines, and a final inspection
shall occur upon completion thereof. Certificates of occupancy or completion
shall not be issued until final inspection of such stormwater runoff and erosion
control facilities has occurred, and the inspector has approved such construction
in writing.
4. The city shall have the right to enter upon the land and buildings as necessary for
the purposes of maintenance, operation and inspection.
Section 17.80.070 City may contract for review of plans, issuance of permits,
performance of inspections and approval of stormwater runoff control plans,
facilities and structures
The City Manager is hereby authorized to enter into a contract with Consolidated Diking
Improvement District No. 1 of Cowlitz County, Washington, (CDID #1), or with a
private consultant, for the review and approval of stormwater runoff and erosion control
plans, the performance of inspections and reinspections, and for the issuance of
certificates of completion. Said contract may provide that all fees paid under this chapter
may be paid to and retained by CDID #1 in partial consideration for its services. For the
purposes of appeal only, the performance of such services shall be deemed the
performance by the City Department of Planning and Building, and appeals from
determinations by the Stormwater Plans Examiner or the Department of Public Works
may be permitted under Section 19.12.060 as if such determinations were made by the
Building Official or the Director of the Department of Community Development.
Section 17.80.080 Definitions
•
“Stormwater Plans Examiner” means the Director of Community Development, the
Building Official, or a designated employee of CDID #1 pursuant to a contract
entered into pursuant to Section 17.80.070.
•
“Applicant” means any person, firm or corporation desiring to perform any of the
work described in Section 17.80.020 or obtains approval of any short subdivision or
any preliminary plat, as provided in Chapter 19.67.
•
“Stormwater runoff and erosion control facilities and structures” and “Stormwater
runoff control system” means and includes: any and all culverts, bridges, pipes,
watercourses, manholes, catch basins, dry ponds, detention facilities, oil/water
separators, biofiltrations, swales, vegetative filter strips, wet ponds, nutrient control
wet ponds, wet vaults, wet tanks, sediment fences, straw bale sediment barriers,
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sidewalk subgrade gravel barriers, undisturbed buffers, temporary grasses, permanent
vegetative cover, straw mulch, erosion blankets, plastic sheet coverings, sediment
traps, sediment ponds, temporary interceptor dikes and swales, storm drain inlet
projections, check dams, and any all other devices, structures, land configurations and
excavations, and land fills required or permitted by the Guidelines.
•
“Off-site regional public retention/detention facilities” means man-made
retention/detention ponds and related facilities constructed and maintained by a
governmental agency for the detention of stormwater, into which stormwater runoff
may be directed through waterways, canals, culverts, ditches or other facilities.
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SECTION 17.80.090
STORMWATER RUNOFF GUIDELINES
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SECTION 17.80.90 STORMWATER RUNOFF GUIDELINES
Section 17.80.090.010 Authority and Purpose
The purpose of these Stormwater Runoff Control Guidelines is to provide a consistent
policy under which physical aspects of stormwater facility design will be implemented.
Most of the elements contained in this document are Public Works oriented and it is
intended that they apply to public improvements under jurisdiction contract, public
improvements under private contract designated herein, and private improvements where
deemed applicable by the Stormwater Plans Examiner.
These Stormwater Runoff Control Guidelines cannot provide for all situations. They are
intended to assist but not to substitute for competent work by design professionals. It is
expected that engineers will bring to each project the best of skills from their respective
disciplines.
The Stormwater Runoff Control Guidelines are also not intended to limit design
innovation resulting in compliance with the design criteria framework contained in these
guidelines. However, any proposed departure from the Guidelines will be judged on the
likelihood that such a departure will produce a compensating or comparable result
adequate to meet the stormwater protections contained in these guidelines. Alternate
materials and methods will be considered for approval by the Stormwater Plans Examiner
as the need arises and conditions warrant modification. For situations falling outside of
these Guidelines, Washington Department of Transportation Highway Runoff Manual
and Hydraulics Manual, or those documents described in Section 17.80.096.050 may be
used upon approval from the Stormwater Plans Examiner.
Section 17.80.90.020 Engineering Policy
It shall be the policy of the jurisdiction to require compliance with Washington
Administrative Code (Title 196 WAC) for professional engineers. All engineering plans,
reports, or documents shall be prepared by a registered professional engineer, or by a
subordinate employee under the engineer’s direction, and shall be signed by the engineer
and stamped with the engineer’s seal to indicate the engineer’s responsibility for them. It
shall be the engineer’s responsibility to review any proposed public facility extension,
modification, or other change with the Stormwater Plans Examiner prior to engineering
or proposed design work to determine any special requirements or whether the proposal
is permissible. A “Preliminary Review” and/or a “Plans Approved for Construction”
stamp of the jurisdiction on the plans, for any project, does not in any way relieve the
engineer of responsibility to meet all requirements of the jurisdiction or obligation to
protect life, health, and property of the public. The plan for any project shall be revised or
supplemented at any time it is determined that the full requirements of the jurisdiction
have not been met.
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Section 17.80.90.030 Applicability
These Stormwater Runoff Control Guidelines shall govern all construction and upgrading
of all public and privately financed stormwater facilities and applicable work within the
adopting jurisdiction service areas. In addition, these Guidelines are applicable to all
divisions of land and divisions of land prior to any short plat.
Section 17.80.90.040 Standard Specifications / Standard Plans
Except as otherwise provided by these Guidelines, all construction design detail,
workmanship, and materials shall be in accordance with the most recent edition of
Standard Specifications and Standard Plans for Road, Bridge, and Municipal
Construction by Washington State Department of Transportation in cooperation with the
American Public Works Association.
Section 17.80.90.050 Approval of Alternate Materials or Methods
Any substitution material or alternate method not explicitly approved herein will only be
considered for approval as set forth in Section 17.80.090.010. Persons seeking such
approvals shall make application in writing. Approval of any deviation from these
Guidelines must be provided by the Stormwater Plans Examiner.
Any substitution must meet or exceed the minimum requirements set in these Guidelines.
The written application is to include, but is not limited to, the manufacturer’s
specifications and testing results, design drawings, calculations, and other pertinent
information. Any deviations or special problems shall be reviewed on a case-by-case
basis and approved by the Stormwater Plans Examiner. When requested by the
Stormwater Plans Examiner, full design calculations shall be submitted for review with
the request for approval.
Section 17.80.90.060 Special Design Problems
Special applications not covered in these Guidelines require review and approval by the
Stormwater Plans Examiner. Submittal of full design calculations, supplemental
drawings, and information will be required prior to any approval. Such applications
which may occur requiring special review and approval are among, but not limited to, the
following: U.S. Army Corps of Engineers.
Section 17.80.90.070 Revisions to Stormwater Runoff Control Guidelines
It is anticipated that revisions to these Guidelines will be made from time to time. The
date appearing on the title page is the date of the latest revision. Users should apply the
latest published issue to the work contemplated. Parenthetical notations at the end of
8
sections indicate the most recent change to those sections. All sections without notations
are from the original Guidelines as adopted. Some sections may be changed more than
once and it shall be the user’s responsibility to maintain his/her copy of these Guidelines
with the latest changes.
Section 17.80.90.080 Definitions
As-Built Plans - Plans signed and dated by the project engineer indicating that the plans
have been reviewed and revised by post-construction survey, if necessary, to accurately
show all as-built construction details, as required by the Stormwater Plans Examiner.
City - The City of Longview, Washington.
Cul-de-Sac - A dead-end street having a turnaround area at the end.
Cut Sheets - Sheets of tabulated data, indicating stationing, structures, fittings, angle
points, beginning of curve, points on curve, end of curves, storm drain slope, staking
offset, various elevations, offset cuts, and storm drain depths for streets, and storm drains.
Datum - The vertical elevation control for the City of Longview and for the City of
Kelso is “The North American Vertical Datum of 1988” (NAVD '88).
Definition of Words - That, whenever, in these Standards, the words “directed”,
“required”, “permitted”, “ordered”, “designated”, or words of like importance are used,
they shall be understood to mean the direction, requirement, permission, or order of
designation of the Stormwater Plans Examiner. Similarly, the words “approved”,
“acceptable”, or “satisfactory”, shall mean approved by, acceptable to, or satisfactory to
the Stormwater Plans Examiner.
Detention - The holding of run-off for a short period of time and then releasing it to
downstream drainage facilities.
District - Consolidated Diking Improvement District No.1
Drainage Facilities - Pipes, ditches, detention basins, natural creeks, streams,
depressions, natural and man-made channels, culvert bridges, etc., used singularly or in
combination with each other for the purpose of conveying or storing storm water run-off.
Easement - Areas located outside of dedicated rights-of-way which are granted to the
jurisdiction for special uses.
Engineer - The engineer, including the City’s engineer, licensed by the State of
Washington as a Professional Engineer under whose direction plans, profiles, and details
for the work are prepared and submitted to the jurisdiction for review and approval, or
who is in charge of and responsible for construction of the improvement.
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French Drain or Leach Line - A covered underground excavated trench filled with
washed gravel that surrounds a perforated delivery pipe used to receive storm water,
wherein the sides and bottom of the trench are porous, permitting the storm water to seep
into the ground.
Grade - The degree of inclination of a road, ground slope, or slope of pipe invert or
channel flowline.
Guidelines - This set of stormwater runoff control and erosion control guidelines, its
adopting ordinance, and all documents included by reference, plus future amendments.
Impervious Areas - Hard surface areas located upon real property which either prevent
or retard saturation of water into the land surface, as existed under natural conditions preexistent to development, and cause water to run off the land surface in greater quantities
or at an increased rate of flow from that present under natural conditions pre-existent to
development.
Jurisdiction - For purposes of these Guidelines, a governmental body that has adopted
these Guidelines.
Local or Residential Street - A facility designated to primarily serve direct access to
abutting land and offers the lowest level of traffic mobility. Through-traffic movement is
deliberately discouraged.
Master Plan - The “Lower Cowlitz River Flood Control Master Plan”.
Natural Grade - The grade of the land in an undisturbed state.
Off-Site Regional Public Retention/Detention Facilities - Man-made
retention/detention ponds and related facilities constructed and maintained by a
governmental agency for the detention of stormwater, into which stormwater runoff may
be directed through waterways, canals, culverts, ditches or other facilities.
On-Site Detention - The storage of excess run-off on the development site prior to its
entry into a downstream stormwater system and gradual release of the stored run-off in
compliance with the design criteria established in these guidelines.
Owner - The owner of record of real property as shown on the latest tax rolls or deed
records of the county, and includes a person who furnishes evidence that he is purchasing
a parcel of property under a written recorded land sale contract.
Person - Individual, firm, corporation, association, agency or other entity.
Peak Run-off - The maximum water run-off rate, as measured in cubic feet per second
(cfs), determined for the design storm.
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Plans - Construction plans, including system plans, storm sewer plans, and profiles,
cross-sections, detailed drawings, etc., or reproductions thereof, approved or to be
approved by the Stormwater Plans Examiner, which show the location, character,
dimensions, and details for the work to be done, and which constitute a supplement to
these standards.
Private Storm Drain - A storm drain located on private property not owned or
maintained by the jurisdiction.
Public Storm Drain - Any storm drain in public right-of-way or easement operated and
maintained by the jurisdiction.
Release Rate - The controlled rate of release of drainage, storm, and run-off water from
property, storage pond, run-off detention pond, or other facility during and following a
storm event.
Retention - The process of collecting and holding surface and stormwater runoff with no
surface outflow.
Right-of-Way - All land or interest therein which (by deed, conveyance, agreement,
easement, dedication, usage, or process of law) is reserved for or dedicated to the use of
the public for sidewalk, utility, and or roadway purposes for which the jurisdiction has
sole responsibility to maintain.
Sedimentation - Disposition of erosional debris-soil sediment transported by water from
a higher elevation to an area of lower gradient where sediments are deposited as a result
of slack water.
Short-Plat - To divide an area or tract of land into less than five parcels within a calendar
year when such area or tract of land exists as a unit or contiguous units of land under
single ownership at the beginning of such year.
Silt - Fine textured soil particles, including clay and sand, as differentiated from coarse
particles of sand and gravel.
Siltation - Deposition of (silt) waterborne sediments - fine textured sedimentation.
Standard Drawings - The drawings of structures or devices commonly used on public
improvements and referred to on construction plans.
Stormwater Plans Examiner - The individual (or agent) designated by the City as
having authority for interpreting and administering these guidelines. (e.g. City Engineer,
City Public Works Director, or other, etc.)
Structures - Those structures designated on the standard plans such as catch basins,
manholes, etc.
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Subdivision - To divide an area or tract of land into five or more lots within a calendar
year when such area or tract of land existed as a unit or contiguous units of land under a
single ownership at the beginning of such year.
Uniform Plumbing Code - The Uniform Plumbing Code adopted by the International
Association of Plumbing and Mechanical Officials (currently adopted by jurisdictions),
as revised by the State of Washington, called the “Washington State Plumbing Specialty
Code”.
Wetlands - Those lands adjacent to watercourses or isolated therefrom which may
normally or periodically be inundated by the waters from the watercourse or the drainage
waters from the drainage basin in which it is located. These include swamps, bogs, sinks,
marshes, and lakes, all of which are considered to be part of the watercourse and drainage
system of the jurisdiction and shall include the headwater areas where the watercourse
first surfaces. They may be, but are not necessarily, characterized by special soils such as
peat, muck, and mud.
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SECTION 17.80.090
CONSTRUCTION PLANS
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SECTION 17.80.90 CONSTRUCTION PLANS
Section 17.80.90.100 General Information
Prior to any construction work and plan approval, complete construction plans, erosion
control plans, specifications and all other necessary submittals shall be submitted to the
Stormwater Plans Examiner for approval.
Section 17.80.90.110 Plan Preparation
Construction plans and specifications shall be prepared and stamped in accordance with
Sections 17.80.90.120-.180 by a professional engineer licensed in the State of
Washington.
Section 17.80.90.120 Sheet Size
All construction plans shall be clearly and legibly drawn in ink on engineering mylar
sheets measuring 22 x 34 inches. Sheets shall have a 1-1/2 inch clear margin on the left
edge and a 1/2 inch margin on all other edges. Construction plans may also be submitted
in a CADD/GIS format approved by the Stormwater Plans Examiner.
Section 17.80.90.130 Scale of Plans
The following scale of drawings is suggested:
•
One (1) inch = 40 feet horizontal, one (1) inch = four (4) feet vertical.
•
*Subdivision street plans, when combined with other proposed facilities listed above,
may be drawn at one (1)” = 40’ scale.
•
** When a scale is used which is smaller than one (1)”=20’ (i.e., 1” = 40’)
intersection details showing fittings and valving shall be provided at a larger scale.
•
Architectural scales (e.g., 1/4” = 1’0) are not permitted, unless approved.
Variance from these standards shall require prior approval from the Stormwater Plans
Examiner. A variance shall be considered when, in the opinion of the Stormwater Plans
Examiner, another scale is needed to show appropriate detail.
Section 17.80.90.140 Required Sheets
Construction plan submittals shall contain the following minimum sheets: Title sheet
(unless not required by the Stormwater Plans Examiner), plan and profile sheet (s), and
detail sheet(s).
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Section 17.80.90.150 Title Sheet
All subdivision projects and multiple sheet improvement projects shall have a title sheet
as the first page of the construction plans. This sheet shall contain the following
minimum information:
1. Site plan of entire project with street right-of-way and/or subdivision layout at a
one (1)” = 100’ scale. A one (1)” = 200’ scale may be used if project size is too
large. The site plan shall also be a composite utility plan showing all properties
served by storm facilities, in addition to the proposed facility.
2. Vicinity map at a one (1)” = 1000’ scale or greater.
3. Index of sheets.
4. Complete legend of symbols used.
5. General and construction notes pertinent to project.
6. Temporary and/or permanent benchmarks used along with their descriptions,
elevations of benchmark and datum.
7. Engineer’s name, address, phone number, and seal.
8. Developer/owner name, address and phone number for public improvements with
private financing.
9. Contact phone number for affected utility companies.
10. Show tax lot numbers or lot and block designations.
Section 17.80.90.160 Plan Sheet
The plan view of each sheet shall be drawn at the appropriate scale showing the
following minimum information:
1. Adjacent street curbs, property lines, right-of-way lines, utility easements
referenced to property lines, street centerlines, and intersections. Show property
corner, curb elevations and points of disposal for building storm drains.
2. Location of all underground utilities within 100 feet of project (if they are
affected by the project) existing power/telephone poles and guy anchors, valves,
manholes, catch basins, fire hydrants, meter boxes and vaults, signs, etc.
3. Location of all water courses, railroad crossings, culverts, bridges, large water
transmission pipes and gravity sewers, and/or storm drains within 200 feet of
15
proposed gravity sewer and storm drain extensions if they affect the design of the
project. All water courses shall show the 100-year flood plain as indicated on the
U.S. Army Corps of Engineers and Federal Emergency Management Agency
(FEMA) maps.
4. On storm drain plans, each manhole and catch basin, shall be numbered and
stationed. Stationing shall tie to existing street monuments, property corners, or
manholes. Each line shall be stationed continuously upgrade and go from left to
right on the plan sheet. Each separate line shall be separately designated (e.g.
storm line ‘A’, etc.)
5. Each project will tie to the Washington State Coordinate Geodetic Control
System.
Section 17.80.90.170 Profile Sheet
Profiles for construction plans shall be the same horizontal scale as the plan sheet. Where
profiles are drawn on the same sheet as the plan view, the profile shall be immediately
below the plan view. Stationing shall be continuously upgraded from left to right with
lower stations to the left. The following minimum information shall be shown:
1. For sewers and storm drains, show locations of manholes and catch basins with
each numbered and stationed as indicated in Section 17.80.090.160(d).
2. Existing profile at centerline of proposed utility or street.
3. Proposed profile grade, as appropriate, for all storm drains, giving pipe size,
length between structures or fittings, slope, backfill and pipe material, storm
sewer inverts, rim elevations, etc.
4. Existing underground utility that crosses the alignment of the proposed facility.
5. Beginning of all vertical curves, points of vertical intersection, end of vertical
curve, low point of sag curve, and length of vertical curve. Profiles of existing
centerline grade shall extend a minimum of 250 feet beyond the end of the
improvement.
6. Clearly show all potential conflicts with existing public and private utilities (i.e.,
pipes, conduits, vaults, cathodic protection systems, etc.) that impact proposed
design.
Special Note: Jurisdiction as-builts, when available, are only to be used as an aid to the
engineer. When a potential conflict may occur, the engineer shall field locate,
or cause to be located, and verify the alignment, depth, and inverts of all existing
facilities shown on the plans that will be crossed by the proposed facility.
16
Section 17.80.90.180 Detail Sheets
Detailed drawings shall be included with all construction plans where jurisdiction
Standard Specifications Drawings do not exist. If a Standard Drawing, such as storm
sewer manholes, must be modified to fit existing or unique conditions, the modified
drawing shall be shown on the plans. When appropriate, due to required detail
complexity, a separate detail sheet shall be drawn. When jurisdiction Standard Drawing
appurtenances or construction installations are to be used, a reference to the specific
Standard Drawing number shall be made on the title sheet.
Section 17.80.90.190 Supporting Information
The engineer shall submit sufficient supporting information to justify the proposed
design. Such information shall include, but not be limited to, the following:
1. Design calculations.
2. Hydrology and hydraulic calculations with basin maps.
3. Alternate materials specifications including manufacturer’s design application
recommendation.
4. Grading plan support information to include as appropriate:
a. Soils engineering report
b. Hydrology report
c. Engineering geology report
Section 17.80.90.200 Facility Plan
When designing storm sewer facilities, a facility plan shall be submitted with the
construction plans unless exempted by the Stormwater Plans Examiner. This plan shall be
used to identify and analyze the proposed extension of facilities. The topographic plan
shall show all upstream and tributary areas within no less than 200 feet of the proposed
development. The plan shall include existing contours at two (2) foot intervals, or as
approved by the Stormwater Plans Examiner, including location of existing structures and
public and private utilities.
Section 17.80.90.210 Erosion Control Plan
The erosion control plan shall address the measures as required by the Erosion Control
Guidelines, contained in Section 17.80.094 of this ordinance. Construction projects
17
beginning prior to May 1 or those projects anticipating construction activity between
November 1 and April 30 will be required to submit a plan addressing “wet weather”
measures as outlined in the Erosion Control Guidelines. Construction activity is assumed
as “active” until all permanent vegetation and/or erosion protection is established.
The plan shall include existing contours at two (2) foot intervals, or as approved by the
Stormwater Plans Examiner, including location of erosion control facilities (i.e., silt
fence, straw mulch, sediment ponds, etc.), outlet structures (i.e., catch basins, culverts,
creeks, etc), and existing public and private utilities.
Section 17.80.90.220 Plan Submittal
Six (6) sets of construction plans for all public works facility improvements shall be
submitted to the Stormwater Plans Examiner. The Stormwater Plans Examiner will
coordinate the plan review and approval of all construction plans which will include
review by Consolidated Diking Improvement District No.1 and others as determined by
the Stormwater Plans Examiner. The Stormwater Plans Examiner may require a
minimum of 30 days in which to complete its review of a complete submittal.
All plan submittals shall include information required in Section 17.80.90.190, and
Section 17.80.90.200 of these Stormwater Runoff Control Guidelines along with all other
information requested by the Stormwater Plans Examiner. This information is to include,
but not be limited to, construction cost estimates, easement documents, right-of-way
dedications, executed agreements, and a plan check and inspection fee. All submittals
will be reviewed for completeness and the engineer notified if required information is
missing. Submittals should be made in a timely manner as lack of information to the
Stormwater Plans Examiner may impede the review process.
Section 17.80.90.230 As-Built Plan Requirements
For all public works facility improvements, the engineer shall submit certified as-built
drawings for all plans, which were approved for construction. As-built drawings shall
meet the requirements of Sections 17.80.90.110, .140 and .230-240 of these Guidelines
and shall be of archival quality. Original inked mylars or CADD/GIS drawings in a
format approved by the Stormwater Plans Examiner may also be submitted in lieu of
photographic mylars. Final approval of improvements shall be withheld until the “asbuilt” drawings have been submitted and approved by the Stormwater Plans Examiner.
The engineer shall submit, along with the as-built drawings, a statement certifying that all
work for which plans were approved has been completed in accordance with jurisdiction
Design Standards and Specifications.
The words “As-Built Drawing” shall appear as the last entry in the revision block along
with the month, day, and year the as-built drawing was prepared.
18
Note: Actual location and depth from finish grade of any other utilities encountered
during construction shall be shown and noted on both plan and profile of the as-built
plans.
Section 17.80.90.240 Storm Drains
The following minimum information shall be noted on storm drain as-built drawings:
1. Station of wye or tee into main line. Tie end of branch line to nearest property
corner at right-of-way line and distance back from the face of curb.
2. Show alignment changes, grade changes, and changes in construction materials. If
changed alignment results in station changes, a station equation shall be shown as
appropriate at a manhole.
3. Other changes altering the approved plans.
19
SECTION 17.80.092
STORM DRAINAGE
20
SECTION 17.80.092 STORM DRAINAGE
Section 17.80.092.010 General Design Requirements
Performance Standards - Storm Drainage design within a development area must
include provisions to adequately control run-off from all public and private streets and
the roof, footing, and area drains of residential, multi-family, commercial, or industrial
buildings. The design must ensure future extension of the drainage system to the entire
drainage basin in conformance with the adopted Lower Cowlitz River Flood Control
Master Plan and these Guidelines. These provisions include:
1. Surface or subsurface drainage, caused or affected by the changing of the natural
grade of the existing ground or removal of natural ground cover or placement of
impervious surfaces, shall not be allowed to flow over adjacent public or private
property in a volume, flow rate or location different from that which existed
before development occurred, but shall be collected and conveyed in an approved
manner to an approved point of disposal.
2. Surface water entering the subject property shall be received at the naturally
occurring locations and surface water exiting the subject property shall be
discharged at the natural locations with adequate energy dissipaters within the
subject property to minimize downstream damage and with no diversion at any of
these points.
3. The approved point of disposal for all storm water may be a storm drain, existing
open channel, creek, detention or retention pond approved by the Stormwater
Plans Examiner. Discharge to a CDID No.1 facility requires CDID No.1 approval.
Acceptance of suggested systems will depend upon the prevailing site conditions,
capacity of existing downstream facilities, and feasibility of the alternate design.
4. When private property must be crossed in order to reach an approved point of
disposal, it shall be the developer’s responsibility to acquire a recorded drainage
easement (of dimensions in accordance with those included in Section
17.80.92.100). The drainage facility installed must be a closed conduit system.
Temporary drainage ditch facilities, when approved, must be engineered to
contain the storm water without causing erosion or other adverse effects to the
private property.
5. The design peak discharge from the subject property may not be increased from
conditions existing prior to the proposed development, except where it can be
demonstrated to the Stormwater Plans Examiner that there is no adverse impact.
6. Retention/detention facilities will be required where necessary to maintain surface
water discharge rates at or below the existing, pre-development design storm peak
discharge except where it can be demonstrated by the applicant that no adverse
impact will result from not providing said facilities.
21
7. Minimum width of an access easement from an existing public road to a drainage
facility shall be 15 feet. More width may be required to accommodate equipment
access.
8. Drainage from residential roofs, footings, and downspouts may drain directly to a
street through the curb under the following circumstances:
a. The building pad ground elevation is at least two (2) feet above the existing
street curb, and,
b. The existing street is adequately crowned to avoid sheet flow across the street.
This requirement will be waived if Type A curb and gutter (monolithic) is
existing or installed.
9. Vegetation shall be established on areas disturbed by or on areas of construction,
as necessary, to minimize erosion in accordance with Section 17.80.092.220 of
these standards, and as set forth in the Section 17.80.094 Erosion Control
Guidelines.
All storm drain system designs shall make adequate provisions for collecting all storm
water run-off. The system shall accommodate all run-off from upstream tributary areas
whether or not such areas are within the proposed development. The amount of run-off to
be accommodated shall be based upon ultimate development of all upstream tributary
areas, as quantified through hydrologic analysis, as given in Section 17.80.92.040.
Proposed storm drain systems shall not discharge flows into inadequate downstream
systems. The Stormwater Plans Examiner may require analysis of existing downstream
systems to assure capacity.
Public storm lines shall be located within the public right-of-way as directed by the
Stormwater Plans Examiner, per Section 17.80.92.070. These lines are placed in the
public right-of-way for ease of maintenance and access, control of the facility, operation
of the facility, and to provide required replacement and/or repair.
Section 17.80.92.020 Site Drainage Plans
Existing Drainage Plan - Provide a topographical contour map defining existing
conditions to include the following minimum information:
1. Two (2) foot contour intervals; slopes over ten percent may use five foot (5’)
intervals; extend contours a minimum of 100 feet beyond property.
2. All structures, buildings, parking lots, and utilities on the property.
22
3. Location of all existing drainage facilities and water courses, including wetlands
and floodplain areas, and existing drainage sub-basin boundaries. Locations of all
subsurface water outlets (e.g. springs).
4. Show arrows to indicate direction of flow for all drainage information.
Proposed Drainage Plan - Show proposed site grading and drainage facilities on a
topographical contour map. Unless the detail for proposed improvements will obscure the
conditions shown on the existing drainage plan, proposed site grading and drainage may
be shown on the existing drainage plan. The following minimum information shall also
be shown:
1. Finished contours of the property, after development, at two foot (2’) or five foot
(5’) intervals as required.
2. Percent grade for graded slopes. Elevations, dimensions and locations for all
graded slopes. Sub-basin area and boundaries, time of concentration, overland
flow paths, lengths and slopes.
3. Cut/fill areas, structural fill placement areas, erosion/sedimentation control
methods, reseeding areas.
4. All proposed drainage facilities - public and private systems; paved areas, curbs,
sidewalks; drainage ditches and culverts.
Drainage Calculations - Furnish such supporting information as required per Section
17.80.090.190 of these Guidelines.
Detention Requirements - All proposed development will be required to use adequate
drainage management practices. Developments located within a master planned drainage
basin will follow the recommendations adopted in that plan. Developments not located
within master planned drainage basins will minimize (as given below) the rate and
amount of run-off to receiving systems and streams. On-site storm detention will be
required to ensure that new development does not increase flooding or erosion
downstream. Stormwater detention volume and outlet will be sized such that the peak
release rate for the 2-year, 24-hour design storm after development shall not exceed onehalf of the pre-developed 2-year, 24-hour design storm peak runoff rate. Also the peak
release rate for the 25 and 100-year, 24-hour design storms after development shall not
exceed the respective pre-development design storm peak runoff rate.
Section 17.80.92.030 Pipe Materials and Size
All public storm drains shall be constructed with either concrete or HDPE smooth
interior, corrugated exterior pipe. Approved PVC pipe may be used up through 15 inches
in diameter. Where required, for added strength, Class 50 ductile iron pipe will be used.
Concrete pipe strength shall meet the applicable sections of ASTM C-14 and ASTM C76.
23
Corrugated aluminum alloy pipe may be used for culvert applications if material is
specified as having a 50-year design life. Private storm drainpipe shall meet the
appropriate sections of the Uniform Plumbing Code.
All public storm drain main lines and lateral lines to catch basins and other inlet
structures shall be a minimum of twelve inches (12”) in diameter. Storm drain lines
which convey water from building rain drains and/or footing drains only may be a
minimum of four inches (4”) in diameter. Such lines, if hydraulically adequate, however,
must discharge into a drainage structure of the public main drain systems.
Section 17.80.92.040 Minimum Design Criteria
Storm Frequency - All public storm drain systems shall be designed for the 25-year
design storm recurrence interval. Surcharged conditions for pipe systems and culverts
and bank full conditions for open ditches and channels are acceptable only for
demonstrating the adequacy of the conveyance system to convey the peak run-off for the
50-year design storms (as required), provided that:
1. Run-off is contained within defined conveyance system elements, AND
2. The hydraulic grade line does not exceed the elevation of the roadway subgrade,
AND
3. No portions of a building will be flooded.
Conveyance system adequacy shall be demonstrated by performing a backwater analysis.
Time of Concentration - overland flow of run-off to the initial catchment point into the
storm drain system shall be a minimum of five (5) minutes.
Velocity and Slope - All storm drains shall be on a grade which produces a mean
velocity when flowing full of at least three feet (3’) per second.
Velocity in Natural and Man-Made Channels - Control of discharge from developed
areas to natural and man-made channels shall be such that the average velocity resulting
from all design storms less than or equal to the 10-year event remains below the erosive
velocity of the channel.
Manning Equations - When calculating minimum pipe slopes and velocities, the design
engineer shall use the Manning Pipe Friction Formula.
Pipe Coefficient - The storm drainpipe roughness coefficient to be used in the Manning
Formula shall not be less than 0.013. Suggested values are as follows:
•
Concrete Pipe
0.013
24
•
•
•
•
Corrugated Metal Pipe
Ductile Iron Pipe
HDPE Pipe
PVC Pipe
0.024
0.013
0.013
0.013
Stormwater Flows - Several alternative methods are available to design engineers for
estimating peak run-off. Three of these are the “Rational” method, the SCS “Curve
Number” method, and the use of regression equations. These methods will be acceptable
for estimating peak run-off quantities to size storm drainage improvements in those
portions of the drainage basins/storm systems for which the Lower Cowlitz River Flood
Master Plan contains no recommendations. The following pages contain design rainfall
data.
25
Longview-Kelso Urban Area
Rainfall Depth Relationships
Rainfall Depth, Inches
Storm
Duration
2-year
5-year
10-year
25-year
50-year
100-year
Storm
Duration
5-minute
0.14
0.20
0.23
0.28
0.32
0.36
5-minute
10-minute
0.22
0.31
0.36
0.43
0.49
0.55
10-minute
15-minute
0.28
0.39
0.45
0.54
0.62
0.70
15-minute
30-minute
0.39
0.54
0.63
0.75
0.86
0.97
30-minute
1-hour
0.50
0.68
0.80
0.95
1.09
1.23
1-hour
2-hour
0.69
0.90
1.04
1.28
1.47
1.69
2-hour
3-hour
0.87
1.11
1.27
1.59
1.82
2.12
3-hour
6-hour
1.30
1.60
1.80
2.31
2.66
3.14
6-hour
12-hour
1.92
2.35
2.70
3.34
3.91
4.65
12-hour
24-hour
2.54
3.10
3.60
4.37
5.17
6.17
24-hour
26
Longview-Kelso Urban Area
Rainfall Depth, Inches
Time
Minutes
2-yr
5-yr
10-yr
25-yr
50-yr
100-yr
5
10
15
30
60
120
180
240
300
360
420
480
540
600
660
720
780
840
900
960
1020
1080
1140
1200
1260
1320
1380
1440
0.14
0.22
0.28
0.39
0.50
0.69
0.87
1.02
1.16
1.30
1.40
1.51
1.61
1.71
1.82
1.92
1.97
2.02
2.08
2.13
2.18
2.23
2.28
2.33
2.39
2.44
2.49
2.54
0.20
0.31
0.39
0.54
0.68
0.90
1.11
1.27
1.44
1.60
1.73
1.85
1.98
2.10
2.23
2.35
2.41
2.48
2.54
2.60
2.66
2.73
2.79
2.85
2.91
2.98
3.04
3.10
0.23
0.36
0.45
0.63
0.80
1.04
1.27
1.44
1.62
1.80
1.95
2.10
2.25
2.40
2.55
2.70
2.78
2.85
2.93
3.00
3.08
3.15
3.23
3.30
3.38
3.45
3.53
3.60
0.28
0.43
0.54
0.75
0.95
1.28
1.59
1.83
2.07
2.31
2.48
2.66
2.83
3.00
3.17
3.34
3.43
3.51
3.60
3.68
3.77
3.86
3.94
4.03
4.11
4.20
4.28
4.37
0.32
0.49
0.62
0.86
1.09
1.47
1.82
2.10
2.38
2.66
2.86
3.07
3.28
3.49
3.70
3.91
4.02
4.12
4.23
4.33
4.44
4.54
4.65
4.75
4.86
4.96
5.07
5.17
0.36
0.55
0.70
0.97
1.23
1.69
2.12
2.46
2.80
3.14
3.39
3.64
3.89
4.15
4.40
4.65
4.78
4.90
5.03
5.16
5.28
5.41
5.54
5.66
5.79
5.92
6.04
6.17
27
Longview-Kelso Urban Area
Rainfall Intensity, Inches per Hour
Time
Minutes
2-yr
5-yr
10-yr
25-yr
50-yr
100-yr
5
10
15
30
60
120
180
240
300
360
420
480
540
600
660
720
780
840
900
960
1020
1080
1140
1200
1260
1320
1380
1440
1.73
1.35
1.14
0.79
0.50
0.35
0.29
0.25
0.23
0.22
0.20
0.19
0.18
0.17
0.17
0.16
0.15
0.14
0.14
0.13
0.13
0.12
0.12
0.12
0.11
0.11
0.11
0.11
2.37
1.84
1.55
1.07
0.68
0.45
0.37
0.32
0.29
0.27
0.25
0.23
0.22
0.21
0.20
0.20
0.19
0.18
0.17
0.16
0.16
0.15
0.15
0.14
0.14
0.14
0.13
0.13
2.78
2.15
1.82
1.26
0.80
0.52
0.42
0.36
0.32
0.30
0.28
0.26
0.25
0.24
0.23
0.23
0.21
0.20
0.20
0.19
0.18
0.18
0.17
0.17
0.16
0.16
0.15
0.15
3.31
2.56
2.17
1.50
0.95
0.64
0.53
0.46
0.41
0.39
0.35
0.33
0.31
0.30
0.29
0.28
0.26
0.25
0.24
0.23
0.22
0.21
0.21
0.20
0.20
0.19
0.19
0.18
3.80
2.94
2.49
1.72
1.09
0.73
0.61
0.53
0.48
0.44
0.41
0.38
0.36
0.35
0.34
0.33
0.31
0.29
0.28
0.27
0.26
0.25
0.24
0.24
0.23
0.23
0.22
0.22
4.27
3.31
2.80
1.94
1.23
0.84
0.71
0.61
0.56
0.52
0.48
0.46
0.43
0.41
0.40
0.39
0.37
0.35
0.34
0.32
0.31
0.30
0.29
0.28
0.28
0.27
0.26
0.26
28
Section 17.80.92.050 - Culverts
Culverts at road crossings in natural perennial channels shall be designed to pass the peak
discharge for the 25-year design storm such that the headwater:
1. Does not exceed 1.5 times the culvert diameter, OR
2. Remains at least one foot (1’) below the roadway subgrade, whichever is less.
Section 17.80.92.060 - Bridges
New and replacement bridges over natural, perennial channels shall be designed to pass
the 100 year peak discharge from the tributary area assuming full development. Vertical
clearance between the design water surface and the bottom of any part of the bridge shall
be a minimum of two feet.
29
SECTION 17.80.092
ALIGNMENT AND COVER
30
SECTION 17.80.92.070 ALIGNMENT AND COVER
Section 17.80.92.070 Right-of-Way Location
Storm drain lines shall generally be located five feet (5’) (south and east) from right-ofway centerline. All changes in direction of pipe shall be made at an approved structure,
except as provided in Section 17.80.92.080.
Section 17.80.92.080 Curvature
Storm drain lines shall not be curved between structures. If unusual circumstances are
present, as determined by the Stormwater Plans Examiner, small diameter storm drains
may be curved. Such curves shall conform the street curvature with a minimum radius of
200 feet for 12” diameter pipe with a three and one-half foot (3-1/2’) laying length.
Section 17.80.92.090 - Minimum Cover
All storm drains shall be laid at a depth sufficient to protect against damage by traffic and
to drain building footings where practical. Sufficient depth shall mean the minimum
cover from the top of the pipe to finish grade at the storm drain alignment.
Minimum cover shall be thirty inches (30”) above the top of the pipe in paved areas and
thirty-six inches (36”) at all other locations, unless otherwise approved by the Stormwater
Plans Examiner.
In areas of relatively flat terrain, the design engineer must show that sufficient depth is
provided at the boundary of the development to properly drain the remainder of the
upstream basin area tributary to the site.
Section 17.80.92.100- Easements
1. When it is necessary to locate storm drains in easements, the storm drain shall be
centered in the easement. All storm drain easements shall be exclusive and shall
not be used for any purpose which would interfere with the unrestricted use of the
storm drain line. Exceptions to this requirement will be reviewed on a case by
case basis (e.g. a utility corridor in a new subdivision).
2. Easements for storm drain lines up to thirty-six inches (36”) in diameter shall
have a minimum width of fifteen feet (15’). All pipe lines thirty-six inches (36”)
and greater in diameter shall have a minimum width of twenty feet (20’). Larger
widths may be required for special circumstances, such as excessively deep pipe
or location of building to the easement.
3. Open channels shall have easements sufficient in width to cover the 100-year
Flood Plain Line when a 100-year design storm is required, or fifteen feet (15’)
from the waterway centerline, or ten feet (10’) from the top of the recognized
31
bank, whichever is greater. A fifteen foot (15’) wide access easement shall be
provided on both sides of the channel for channel widths greater than fourteen
feet (14’) at the top of the recognized bank. When considering cleaning needs, an
access easement wider than fifteen feet (15’) is required on one side of the
channel.
4. Easement locations for public storm drains serving a PUD, apartment complex, or
commercial/industrial development shall be in parking lots, private drives, or
similar open areas which will permit unobstructed vehicle access for maintenance.
5. All easements must be furnished to the Stormwater Plans Examiner for review
and approval prior to recording.
Section 17.80.92.110 - Relation to Watercourses
Storm drain lines shall enter a creek or drainage channel at 90 degrees or less to the
direction of flow. The outlet shall have a head wall and scour pad or riprap to prevent
erosion of the existing bank or channel bottom. The size of pipe or channel being entered
will govern which protective measures are required. All protective measures must
conform to the requirements of Section 17.80.92.220 of these Guidelines.
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SECTION 17.80.092
STRUCTURE LOCATION
33
SECTION 17.80.092.120 - STRUCTURE LOCATION
Section 17.80.092.120 - Manholes
1. Manholes shall be located at all changes in slope, alignment, pipe size, and at all
pipe junctions with present or future storm drains.
2. Manhole spacing shall not be greater than 400 feet.
3. Standard manholes are required when rim to crown of pipe elevations exceed four
feet (4’) at pipe junctions. Flat-top manholes shall be used when rim to crown of
pipe elevations are less than four feet (4’).
4. When the downstream pipe size increases, the crown of all upstream pipes shall
not be lower than the crown of the larger downstream pipe.
5. All construction to be in accordance with jurisdiction standards.
Section 17.80.092.130 - Catch Basins
1. Catch basins shall be located in streets at the curb line to receive storm water runoff and convey it to the main storm drain.
2. Catch basins shall be located at the following locations, but in no case be spaced
further than 400 feet (400’):
a. At-curb returns on the upstream side of an intersection.
b. At the ends of all dead-end streets with a descending grade.
c. At intermediate locations so that storm flows at the curb line do not exceed
three feet (3’) in width (measured from the curb face) or three inches (3”) in
depth (measured at the curb face), whichever is less. Catch basins shall be
capable of intercepting completely the design storm flow at the curb.
d. All construction to be in accordance with jurisdiction standards.
34
SECTION 17.80.092
STORMWATER DETENTION/RETENTION
35
SECTION 17.80.092.140 - STORMWATER DETENTION/RETENTION
All proposed development, except as given below, will be required to provide on-site
stormwater detention. Stormwater detention volume will be sized such that the peak
release rate for the 2 year, 24 hour design storm after development shall not exceed onehalf of the pre-developed 2-year, 24-hour design storm peak runoff rate. Also the peak
release rate for the 25- and 100-year, 24-hour design storms after development shall not
exceed the respective pre-development design storm peak runoff rate.
Section 17.80.092.150 - Development Not Requiring Detention
A single development which creates less than 5000 square feet of new impervious cover
will not be required to provide detention.
Section 17.80.092.155 - Off-site Regional Retention/Detention Facilities as
Alternative to On-Site Retention/Detention Requirements
The requirements of on-site retention/detention facilities may be waived in development
areas for which the developer has entered into a contract for diversion of stormwater into
an Off-Site Regional Public Retention/Detention Facility, and an executed copy of such
contract is filed in the office of the Stormwater Plans Examiner provided, however, that
all of the other provisions of this ordinance shall be applicable.
In order for the requirement of an “on-site” detention facility waiver, the following
provisions must be met:
1. Conveyance System Capacity: The conveyance system transporting stormwater
from the development to a primary or secondary ditch must be sized adequately to
handle the additional runoff. The stormwater plans examiner may require the
developer to demonstrate the adequacy of the conveyance system. If required,
supporting documentation must be submitted by a registered Engineer in the State
of Washington.
2. Primary/Secondary Ditches: The following ditches are primary ditches able to
receive and transport stormwater discharge to Off-Site Regional Public
Retention/Detention Facilities in lieu of on-site stormwater detention. The
developer must discharge to one of the following primary or secondary ditches:
•
•
•
•
•
•
•
North Cutoff Slough from Ditch No. 10 to Cutoff Slough
South Cutoff Slough from Ditch No. 10 to Cutoff Slough
Cutoff Slough from North Cutoff Slough/South Cutoff Slough intersection
to Ditch No. 2
Ditch No. 2 from Cutoff Slough to Ditch No. 1
Ditch No. 1 from Ditch No. 2 to Ditch No. 3
Ditch No. 3 from Ditch No. 1 to Ditch No. 5
Ditch No. 5 from Ditch No. 3 to Ditch No. 10
36
•
•
•
Ditch No. 10 from Ditch No. 5 to North Cutoff Slough
Ditch No. 12 from Ditch No. 5 to Ditch No. 13
Ditch No. 13 from Ditch No. 12 to Cutoff Slough
The following ditches are secondary ditches able to receive and transport stormwater
discharge to the primary ditches and to Off-Site Regional Public Retention/Detention
Facilities in lieu of on-site stormwater detention:
•
•
•
•
•
•
•
•
Bypass Ditch from Ditch No. 8 South Cutoff Slough, and
Ditch No. 3 from Oregon Way to Ditch No. 1, and
Ditch No. 8 from Drain No. 16 to Bypass Ditch, and
Ditch No. 8 from Mt. Solo Road to Ditch No. 15, and
Ditch No. 14 from Ditch No. 10 to Reynolds Pump Station, and
Ditch No. 15 from Ditch No. 10 to Branch Creek Drive, and
Drain No. 12 from Oak Street to Cutoff Slough, and
Drain No. 15 from Olympia Way to North Cutoff Slough
3. Storage Factor: The following storage factors shall be used to determine the
required volume of storage in the Off-Site Regional Public Retention/Detention
Facilities that shall be obligated to a specific development area by contract. The
Stormwater Plans Examiner may accept a smaller storage factor upon approval of
a site specific, detailed drainage report prepared by a registered Engineer in the
State of Washington.
4. Single-Family Residential Development: A storage factor of 1600 cubic feet of
storage per acre of development is established for all single-family residential
development with a minimum lot size of 6,000 square feet.
5. Multi-Family/Commercial/Industrial Development: A storage factor of 0.15
cubic feet of storage per square foot of new impervious surface is established for
all multi-family, commercial and industrial development.
Section. 17.80.092.160 - Floodplain Information
Floodplain information, delineating the 100-year floodplain limits, shall be shown where
it occurs within the development. Flood plain limits shall be based on maps prepared by
the U.S. Army Corps of Engineers and the Federal Emergency Management Agency
(FEMA). Where better information is available, it shall be used by the Design Engineer.
Section 17.80.092.170 - Detention Volume
When detention is required, the volume to be detained must limit the developed peak
release rate for the two (2) year, 24-hour design storm to one-half of the pre-developed
two (2) year storm. The peak release rate for the 25- and 100-year, 24-hour design
storms after development shall not exceed the respective pre-development design storm
peak run-off rate.
37
Section 17.80.092.180 - Emergency Overflow
The Design Engineer shall assess the impacts of system failure for on-site detention.
Overflow may occur due to rainfall intensity exceeding the design storm, debris blockage
of storm drain system, or some other reason.
If a system overflows, it shall not cause inundation of neighboring properties. Potential
overflow routes shall be protected from erosion by adequate means.
If surface detention (e.g., detention/retention pond) is used, an overflow system shall be
included to provide controlled discharge of the 100-year, 24-hour design storm event for
developed conditions, without overtopping any part of the pond embankment or
exceeding the capacity of the emergency spillway. The overflow design shall assume
failure of the normal outlet control structure. An emergency spillway shall be able to
safely pass all flows over the pond embankment without overtopping the embankment.
Sufficient armoring will be required to the toe on each face of the embankment to prevent
failure of the embankment from erosion.
Section 17.80.092.190 - Detention Facilities
Detention volume storage methods, include surface storage such as dry and wet detention
ponds and underground storage such as vaults, tanks, and oversized piping, are presented
in the following sections.
Section 17.80.092.200 - Dry Pond
A dry pond is a facility that provides stormwater quantity control by containing excess
run-off in a detention basin, then releasing the run-off at allowable levels.
General Criteria (Dry Pond)
1. Dry pond site selection should consider both the natural topography of the area
and right-of-way boundaries. The planting and preservation of vegetation should
be an integral part of the storage facility design.
2. Dry pond design must take into consideration overflow. Overflow may result
from higher intensity or longer duration storms than the design storm or result
from plugged orifices or inadequate storage due to sediment buildup in the
facility.
3. Interior side slopes up to the maximum water surface shall be no steeper than
3H:1V. Exterior side slopes shall be no steeper than 2H:1V. Slopes should be no
steeper than 4H:1V if they are to be mowed.
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4. Pond walls may be retaining walls, provided that a fence is provided along the top
of the wall and that at least 25 percent of the pond perimeter will be a vegetated
soil slope of not greater than 3H:1V.
5. For berm embankments of six feet (6’) or less (including one foot <1’>
freeboard), the minimum top width shall be six feet (6’).
6. The toe of the exterior slope of pond embankment must be no closer than five feet
(5’) from the right-of-way line.
7. LEFT BLANK INTENTIONALLY
8. Pond berm embankment must be constructed on native consolidated soil, (or
adequately compacted and stable fill soils analyzed by a geotechnical engineer)
free of loose surface soil materials, roots and other organic debris.
9. Pond berm embankments must be constructed by excavating a “key” equal to 50
percent of the berm embankment cross-sectional height and width (except on
highly compacted till soils where the “key” minimum depth can be reduced to one
foot <1’> of excavation into the till).
10. The berm embankment shall be constructed in accordance with Section 203.3(14)C Method C of WSDOT Standard Specifications.
11. Anti-seepage collars must be placed on outflow pipes in berm embankments
impounding water greater than eight (8) feet in depth at the design water surface.
12. Exposed earth on the pond bottom and side slopes shall be sodded or seeded with
the appropriate seed mixture as soon as is practicable. Establishment of
protective vegetative cover shall be ensured with jute mesh or other protection
and reseeded as necessary.
13. The dry pond must be cleaned before becoming a permanent best management
practice if it was used as a retention pond during the construction stage.
14. Design with maintenance in mind. Good maintenance will be crucial to successful
use of the pond.
Design Procedure (Dry Pond)
1. Calculate the pre-project and post project hydrographs for the two-(2) year, 25year and 100-year storms.
2. Estimate the initial pond size based upon the difference between the 25-year preproject and 25-year post-project hydrographs.
39
3. Route the 2-year, 25-year and 100-year post-project storms through the pond, one
storm event at a time. An outlet device such as a weir or an orifice must be
designed to match each release rate. Orifice and weir equations can be used to
design an orifice or analyze a given outlet structure.
4. Continue to alter the volume until the allowable release rates are not exceeded.
5. Identify an overflow route to be used in the event that the pond capacity is
exceeded.
Construction and Maintenance Criteria (Dry Pond)
1. Roads in and around the dry pond shall be constructed according to standard
WSDOT practices and meet the following criteria:
a. Maximum Grade: 15 percent to control structure, 20 percent into pond.
b. Provide 40-foot minimum outside radius on the access road to the control
structure and the turn around to the pond bottom.
c. Fence gates shall be provided for access roads at “straight” sections of road.
Access roads shall be 15 feet in width.
d. Manhole and catch basin lids must be at either edge of an access road or pad
and be at least three feet from a property line.
2. Access shall be limited by a double-posted gate, or by bollards, if a fence is
required. Bollards shall consist of two fixed bollards on each side of the access
road and two removable bollards equally located between the fixed bollards.
3. Maintenance of sediment forebays and attention to sediment accumulation within
the pond is extremely important. Sediment deposition should be continually
monitored in the basin. Significant concentrations of heavy metal (e.g., lead, zinc,
and cadmium) as well as some organics, such as pesticides, may be expected to
accumulate at the bottom of these treatment facilities. Testing of sediment
especially near points of inflow, should be conducted regularly to determine the
leaching potential and level of accumulation of hazardous material before
disposal.
4. Remove litter to keep the dry pond free of external pollution.
5. Check the pond for damage caused by erosion and re-stabilize areas where there
is the potential for continued erosion.
6. Maintain pond shape and depth by removing accumulated sediments that exceed
10 percent of the designed pond depth.
40
Section 17.80.092.210 - Dry Vault/Tank
A dry vault/tank is a facility that treats stormwater for water quantity control by detaining
runoff and then releasing reduced flows at established standards. Dry vaults/tanks are not
to be used for stormwater quality treatment purposes because of their limited pollution
removal capabilities. Dry vaults/tanks must always be preceded by a best management
practice which has been sized to treat the water quality design storm. Because dry
vaults/tanks are underground, they are more difficult to inspect and maintain.
Vaults/tanks shall be permitted for use only on small sites, and then only after it has been
demonstrated that more desirable BMP’s are not practicable.
General Criteria (Dry Vault/Tank)
1. This best management practice must be used in conjunction with a best
management practice that will fully treat runoff from the water quality design
storm.
2. Dry vaults/tanks shall be a minimum of 20 feet from any structure, property line,
and any septic tank/drain field. All facilities shall be a minimum of 50 feet from
any steep slope. A geotechnical report must address the potential impact on a
steep slope.
3. Design with maintenance in mind. Good maintenance will be crucial to successful
use of a vault/tank. Hence, provisions to facilitate maintenance operations must be
built into the project when it is installed. Maintenance must be a basic
consideration in design and in determination of cost.
Design Procedure (Dry Vault/Tank)
1. Calculate the pre-project and post-project hydrographs for the two (2)-year, 25year and 100-year storms.
2. Estimate the initial pond size based upon the difference between the 25-year preproject and 25-year post-project hydrographs.
3. Route the 2-year, 25-year and 100-year post-project storms through the pond, one
storm event at a time. An outlet device such as a weir or an orifice must be
designed to match each release rate. Orifice and weir equations can be used to
design an orifice or analyze a given outlet structure.
4. Continue to alter the volume until the allowable release rates are not exceeded.
5. An overflow outlet must be provided in the event that the vault/tank capacity is
exceeded. This overflow should discharge to the same location as the main outlet.
41
Construction and Maintenance Criteria (Dry Vaults/Tanks)
Materials
Vaults: Minimum 3,000 psi structural reinforced concrete. All construction joints must be
provided with water stops.
Tanks: Pipe material, joints, and protective treatment for tanks shall be in accordance
with WSDOT/APWA Standard Specifications Section 9.05, and AASHTO designations
as noted below:
•
Corrugated iron or steel pipe and pipe arch, Treatment 1 through 6.
•
Aluminized Type 2 corrugated steel pipe and pipe arch (meets AASHTO designations
M274 and M36).
•
Steep spiral rib pipe, Treatment 1 through 6.
•
Aluminum spiral rib pipe.
•
Corrugated aluminum pipe and pipe arch.
•
Reinforced concrete pipe.
Structural Stability
1. Vaults: All vaults shall meet structural requirements for overburden support and
HS-20 traffic loading. Cast-in-place wall sections shall be designed as retaining
walls. Vaults shall be placed on native material with suitable bedding. Vaults shall
not be allowed in fill slopes unless analyzed in a geotechnical report for stability
and construction practices.
2. Tanks: All tanks shall meet structural requirements for overburden support and
traffic loading, if appropriate. HS-20 live loads must be accommodated for tanks
lying under roadways or parking areas. Metal tank end plates must be designed
for structural stability at maximum hydrostatic loading conditions. Flat end plates
generally require thicker gauge material than the pipe and/or require reinforcing
ribs. Tanks shall be placed on native material with a suitable bedding. Tanks shall
not be allowed in fill slopes.
3. Buoyancy (Tanks): In moderately pervious soils where seasonal groundwater may
induce flotation, buoyancy tendencies must be balanced by ballasting with
backfill, providing concrete anchors, increasing the total weight, or by providing
subsurface drains to permanently lower the groundwater table. Calculations must
be submitted which demonstrate stability.
42
Section 17.80.092.220 - Erosion Control
Developments shall provide erosion control methods in accordance with the “Erosion
Control Guidelines” (See Section 17.80.094) to limit the removal of soil materials by
storm run-off during the construction phases of a project.
Section 17.80.092.230- Erosion Control Application
For subdivision plats, temporary erosion control measures shall be utilized by the
applicant during installation of plat improvements and by subsequent builders during
construction of dwellings and other lot improvements.
Prior to the initial clearing and grading of any land development, provisions shall be
made for the interception of all potential silt-laden run-off from discharging from the
proposed land development to downstream properties unless a waiver of any/all of this
requirement is previously approved by the Stormwater Plans Examiner. Said interception
shall cause all silt-laden run-off to be conveyed by open ditch, or other means, to
whatever temporary facility is necessary to remove silt prior to discharge to downstream
properties.
Prior to initial clearing and grading of construction site, an evaluation of the following
factors must be carried out.
43
SECTION 17.80.092
PRIVATE DRAINAGE SYSTEMS
44
17.80.092 PRIVATE DRAINAGE SYSTEMS
Section 17.80.092.240 - Subdivisions
Subdivision lots which drain to the rear should be avoided, but if absolutely necessary, it
shall be necessary for the developer to provide a public drainage system in a public
easement. This system shall be for collection of roof drains, footing drains, and surface
run-off. This system shall be designed to meet the Uniform Plumbing Code requirements.
Section 17.80.092.250 - Subsurface Drainage
Subsurface drains (underdrains) shall be provided at the following locations:
1. For all existing springs and field tile intercepted during construction activity for
other facilities: i.e., sewer, water mains, street excavations, foundations, etc.
Subsurface drains are not needed if the tile is removed.
2. Where high ground water exists or when it is necessary to reduce the piezometric
surface to an acceptable level to prevent land slippage or under-floor flooding of
buildings.
3. The drainage line installed shall begin at a cleanout and terminate at an approved
point of disposal. Open jointed storm drain lines will not be acceptable.
45
17.80.092
ON-SITE WATER QUALITY CONTROL MEASURES
46
Section 17.80.092.260 - On-Site Water Quality Control Measures
On-site water quality control measures shall be provided for new development, with the
exception of residential development with less than 5000 square feet of new impervious
cover. Typical on-site water quality control measures include oil/water separators,
biofiltration swales, vegetated filters, wet ponds and wet tanks/vaults. Typical drawings
are included in this section.
Section 17.80.092.270 - Water Quality Design Storm
The water quality design storm for the Longview-Kelso Urban is represented by one-third
(1/3) of the two (2)-year, 24-hour storm, with an SCS 1a type rainfall distribution. For the
Longview/Kelso Urban Area, the two (2)-year, 24-hour storm event is 2.54 inches,
therefore the water quality design storm is 0.84 inches in 24-hours (2.54 x 0.33).
Section 17.80.092.280 - Oil/Water Separators
There are three general types of separators. One type is the spill control separator (SC). It
is a simple underground vault or manhole with a “T” outlet. The SC separator is effective
at retaining only small spills. The SC separator will not remove diluted oil droplets
spread through the stormwater from oil-contaminated pavement. Two other types of
separators can remove dispersed oil: the American Petroleum Institute (API) separator
and coalescing plate separator (CPS).
The API separator is a long vault or basin with baffles to improve the hydraulic
conditions for treatment. Large API separators may have sophisticated mechanical
equipment for removing oil from the surface and settled solids from the bottom. The CPS
separator contains a bundle of closely spaced plates typically made of fiberglass or
polypropylene. Depending on the manufacturer and/or application, the plates may be
positioned in the bundle at an angle of 45 to 60 degrees from horizontal. The closely
spaced plates improve hydraulic conditions in the CPS separator and thereby promote oil
removal. The primary advantage of the CPS separator is the ability to achieve
theoretically equal removal efficiencies with 20 to 50-percent of the space needed by the
API separator, when designed to remove the same size droplets.
Land uses that must use an API or CPS separator include: Industrial machinery and
equipment, trucks and trailers, aircraft, parts and aerospace, railroad equipment, log
storage and sorting yards, fleet vehicle yards, railroads, gas stations, retail/wholesale
vehicle and equipment dealers, vehicle maintenance and repair, construction business
(paving, heavy equipment storage and maintenance, storage of petroleum products).
General Criteria
1. Separators should precede all other treatment best management practices (BMPs).
47
2. Appropriate removal covers must be provided that allow access for observation
and maintenance.
3. Stormwater from building rooftops and other impervious surface not likely to be
contaminated by oil shall not discharge to the separator.
4. Any pump mechanism shall be installed downstream of the separator to prevent
oil emulsification.
Additional Criteria for API and CPS Separators:
1. Separators are to be sized for the water quality design storm. Larger storms shall
not be allowed to enter the separator. The use of an isolation/diversion structure is
recommended.
2. Separators shall have a forebay to collect floatables and the larger settleable
solids. The surface area shall not be less than 20 square feet per 10,000 square
feet of area draining to the separator.
Additional Criteria for CPS Separators:
1. Plates shall not be less that ¾ inch apart.
2. The angle of the plates shall be from 45 to 60-degrees from horizontal.
Design Procedure - API Separator
API separators are sized using these general guidelines.
•
Horizontal velocity: Three (3) feet per minute or 15 times the rise rate, whichever is
smaller (Rise rate of 0.033 feet per minute is recommended.)
•
Depth of three (3) to eight (8) feet
•
Depth- to-width ratio of 0.30 to 0.50
•
Width of six (6) to 16 feet
•
Baffle height to depth ratios of 0.85 for top baffles and 0.15 for bottom baffles
The separator is initially sized for depth using the equation:
Depth = (Q/2Vh) 1/2
Where Q = Design flow (cubic feet per minute)
Vh= Design horizontal velocity (feet per minute) = 0.50 (15 times 0.033)
48
Calculate the width using the above ratios (i.e., 0.3 to 0.5 depth-to-width ratio). Then
calculate length using the equation:
Length = Depth *
Rise Rate
Vh
= (Q/2Vh) 1/2 *
0.033
0.50 =
(Q/2Vh)1/2
0.066
Design Procedure - CPS Separator
Calculate the projected (horizontal) surface area of plates required using the following
equation:
Ap = Q / Rise rate
Where Ap = Projected surface area of the plate (square feet), note that the actual
surface area Aa = Ap * cosine H
H = Angle of the plates with the horizontal (degrees, usually 45 to 60)
Q = design flow (cubic feet per minute)
Rise rate - Recommended using 0.033 feet per minute.
Construction and Maintenance Criteria (Oil/Water Separators)
There are no special construction considerations. Oil/water separators must be cleaned
frequently to keep accumulated oil from escaping during storms. They must always be
cleaned by October 15 to remove material that has accumulated during the dry season,
and again after a significant storm. In addition: The facility shall be inspected weekly by
the owner:
•
•
•
•
•
Oil absorbent pads are to be replaced as needed but shall always be replaced in the
fall prior to the wet season and in the spring.
The effluent shutoff valve is to be closed during cleaning operations.
Waste oil and residuals shall be disposed in accordance with current local jurisdiction
Health Department requirements.
Any standing water removed during the maintenance operation must be disposed to a
sanitary sewer at a discharge location approved by the jurisdiction.
Any standing water removed shall be replaced with clean water to prevent oil carryover through the outlet weir or orifice.
49
Figure Section 17.80.092.290 - Oil/Water Separation
50
Section 17.80.092.300 Biofiltration Swale
Biofiltration is the simultaneous process of filtration, particle settling, adsorption, and
biological uptake of pollutants in stormwater that occurs when runoff flows over and
through vegetated areas. A biofiltration swale is a sloped, vegetated channel or ditch that
provides both conveyance and water quality treatment to stormwater runoff. It does not
provide stormwater quantity control but can convey runoff to BMP’s designed for that
purpose.
General Criteria (Biofiltration Swale)
1. The length of the swale will be designed such that the hydraulic residence time is
no less than nine (9) minutes and with a minimum length of 100 feet. The
minimum bottom width is 2 feet. The depth of flow must not exceed four (4)
inches during the water quality design storm.
2. The channel slope should be at least one (1) percent and no greater than five (5)
percent.
3. The swale can be sized as both a treatment facility for the water quality design
storm and as a conveyance system to pass the peak hydraulic flows of the 100year storm if it is located “on-line”.
4. The ideal cross-section of the swale should be a trapezoid. The side slopes should
be no steeper than 3:1. Roadside ditches should be regarded as significant
potential biofiltration sites and should be utilized for this purpose whenever
possible.
5. If flow is to be introduced through curb cuts, place pavement slightly above the
biofilter elevation. Curb cuts should be at least 12 inches wide to prevent
clogging.
6. Install low-flow biofiltration swales within ponds where sufficient land does not
exist for both.
7. Biofilters must be vegetated in order to provide adequate treatment of runoff.
8. It is important to maximize water contact with vegetation and the soil surface. For
general purposes, select fine, close-growing, water-resistant grasses.
9. Biofilters should generally not receive construction-stage runoff. If they do,
presettling of sediments should be provided. Such biofilters should be evaluated
for the need to remove sediments and restore vegetation following construction.
51
10.
If possible, divert runoff (other than necessary irrigation) during the period of
vegetation establishment. Where runoff diversion is not possible, cover graded and
seeded areas with suitable erosion control materials.
Design Procedure (Biofiltration Swale)
1. Determine the peak flow rate to the biofilter from the water quality design storm.
2. Determine the slope of the biofilter. This will be somewhat dependent on where
the biofilter is placed. The slope should be at least 1 percent and shall be no
steeper than 5 percent. When slopes less than 2 percent are used, the need for
underdrainage must be evaluated.
3. Select a swale shape. Trapezoidal is the most desirable shape. The remainder of
the design process assumes that a trapezoidal shape has been selected.
4. Use Manning’s Equation to estimate the bottom width of the biofilter. Manning’s
Equation for English units is as follows:
Q = (1.486A R0.667 S0.5)/n
Where Q = flow (cfs)
2
A = Cross sectional area of flow (ft )
R = Hydraulic radius of flow cross section (ft)
S = Longitudinal slope of biofilter (ft/ft)
n = Manning’s Roughness Coefficient = 0.20 for typical biofilter
For a trapezoid, this equation cannot be directly solved for bottom width. However, for
trapezoidal channels that are flowing very shallow, the hydraulic radius can be set equal
to the depth of flow. Using this assumption, the equation can be altered to:
1.667 0.5
S ))-zy
b = ((0.135Q)/y
Where y = depth of flow
z = the side slope of the biofilter in the form of z:1
Typically the depth of flow is selected to be four (4 inches. It can be set lower, but doing
so will increase the bottom width. Sometimes when the flowrate is very low, the equation
listed above will generate a negative value for b. Since it is not possible to have a
negative bottom width, the bottom width should be set to 1-foot when this occurs.
Biofilters are limited to a maximum bottom width of eight (8) feet. If the required bottom
width is greater than eight (8) feet, parallel biofilters should be used in conjunction with a
device that splits the flow and directs the proper amount to each biofilter.
1. Calculate the cross sectional area of flow for the given channel using the
calculated bottom width and the selected side slopes and depth.
52
2. Calculate the velocity of flow in the channel using:
V = Q/A
If V is less than or equal to 1 ft/sec, the biofilter will function correctly with the
selected bottom width. Proceed to design step 7.
If V is greater than one (1) ft/second, the biofilter will not function correctly.
Increase the bottom width, recalculate the depth using Manning’s Equation and
return to design step 5.
3. Select a location where a biofilter with the calculated width and a length of no
less than 100 feet will fit. If a length of 100 feet is not possible, the width of the
biofilter must be increased so that the area of the biofilter is the same as if a 100foot length had been used. Hydraulic residence time should be no less than 5
minutes.
4. Select a vegetation cover suitable for the site.
5. Determine the peak flow rate to the biofilter during the 100-year 24-hour storm.
Using Manning’s Equation, find the depth of flow (typically n = 0.04 during the
100-year flow). The depth of the channel shall be one (1) foot (300 mm) deeper
than the depth of flow.
Construction and Maintenance Criteria (Biofiltration Swale)
1. Groomed biofilters planted in grasses shall be mowed during the summer to
promote growth and pollutant uptake.
2. Remove sediments during summer months when they build up to four (4) inches
at any spot, cover biofilter vegetation, or otherwise interfere with biofilter
operation. If the removal equipment leaves bare spots, reseed those spots.
3. Inspect biofilters periodically, especially after periods of heavy runoff. Remove
sediments, fertilize and reseed as necessary. Be careful to avoid introducing
fertilizer to receiving waters or groundwater.
4. Clean curb cuts when soil and vegetation buildup interfere with flow introduction.
5. Remove litter to keep biofilters free of external pollution.
53
Section 17.80.092.310 - Vegetative Filter Strip
A vegetative filter strip is a facility that is designed to provide stormwater quality
treatment of conventional pollutants but not nutrients. This BMP will not provide
stormwater quantity control. The primary use of vegetative filter strips will be along
roadways, parking lots and developments where sheet flow from the project will pass
through the filter strip before entering a conveyance system or a quantity control facility.
The vegetative filter strip is still in an interim phase of development. This BMP is
acceptable for use on any project that meets the General Criteria listed below; however,
the General Criteria may change in the near future as research projects and field tests
involving this BMP area completed.
General Criteria (Vegetative Filter Strip)
1. The width of a filter strip shall be 10 feet with a transverse slope between 1
percent and 15 percent.
2. Filter strips may be placed three (3) to four (4) feet from the edge of pavement, to
accommodate a vegetation free zone.
3. Once stormwater has been treated by a filter strip, it shall be collected and
conveyed to a stormwater quantity BMP.
4. Vegetative filter strips must not receive concentrated flow discharges.
5. Vegetative filter strips should not be used on slopes greater than 5 percent
because of the difficulty in maintaining the necessary sheet flow conditions.
6. Vegetative filter strips should be constructed after other portions of the project are
completed.
Design Procedure (Vegetative Filter Strip)
Provide an area next to the development to use as a filter strip. The strip should run the
length of the project to be treated and have a width of ten (10) feet with a slope between 1
and 15 percent. The flow from the development must enter the filter strip as a sheet flow.
Construction and Maintenance Criteria (Vegetative Filter Strip)
1. Construct filter strips after completion of the project.
2. Groomed filter strips planted in grasses shall be mowed during the summer to
promote growth.
3. Inspect filter strips periodically, especially after periods of heavy runoff. Remove
sediments and reseed as necessary.
54
4. Remove litter to keep filter strips free of external pollution and ensure sheet flow
through the filter strip.
55
Figure Sec. 17.80.092.320 - Biofiltration / Vegetated Filter Strip
56
Section 17.80.092.330 - Wet Pond
A wet pond is a facility that treats stormwater for water quality by utilizing a permanent
pool of water to remove conventional pollutants from runoff through sedimentation,
biological uptake, and plant filtration. A wet pond may also be designed to provide
stormwater quantity control through the use of a “live storage” area above the permanent
pool.
General Criteria (Wet Pond)
1. Wet ponds shall be multi-celled with at least two cells, and preferably three. The
cells should be approximately equal in size. The first cell should be 3 feet deep in
order to effectively trap coarser sediments and reduce turbulence which can
resuspend sediments. It should be easily accessible for maintenance purposes. The
berm dividing the pond into cells shall have a 5-foot minimum top width, a top
elevation set one (1) foot lower than the design water surface, maximum 3:1 side
slopes, and a quarry spall and gravel filter “window” between the cells.
2. All facilities shall be a minimum of 20 feet from any structure, right-of-way line,
and any vegetative buffer, and 100 feet from any septic tank/drainfield. All
facilities shall be a minimum of 50 feet from any steep (greater than 15 percent)
slope unless a geotechnical report has indicated that the steep slope will be stable.
3. Long, narrow, and irregularly shaped ponds are preferred, as these configurations
are less prone to short-circuiting and tend to maximize available treatment area.
The length-to-width ratio should be at least 3:1 and preferably 5:1. Irregularly
shaped ponds may perform more effectively and will have a more natural
appearance.
4. The pond bottom shall be level to facilitate sedimentation.
5. Interior side slopes up to the maximum water surface shall be no steeper than
3H:1V. Exterior side slopes shall be no steeper than 2H:1V. Slopes should be no
steeper than 4H:1V if they are to be mowed.
6. Pond walls may be retaining walls, provided that a fence is provided along the top
of the wall and that at least 25 percent of the pond perimeter will be a vegetated
soil slope of not greater than 3H:1V.
7. For berm embankments of 6 feet or less (including 1-foot freeboard), the
minimum top width shall be six (6) feet or as recommended by the engineer.
8. The toe of the exterior slope of pond berm embankment must be no closer than
five (5) feet from the right-of-way.
9. The berm embankment shall be constructed in accordance with Section 203.3(14)C Method C of WSDOT Standard Specifications.
57
10. A “key” must be excavated that is equal to 50 percent of the berm embankment
cross-sectional height and width (except on highly compacted till soils where the
“key” minimum depth can be reduced to one (1) foot of excavation into the till).
11. Anti-seepage collars must be placed on outflow pipes in berm embankments
impounding water greater than eight (8) feet in depth at the design water surface.
12. Exposed earth on the side slopes shall be sodded or seeded with the appropriate
seed mixture as soon as is practicable. Establishment of protective vegetative
cover shall be ensured with erosion protection and reseeded as necessary.
13. The inlet and outlet should be at opposite ends of the pond where feasible. If this
is not possible, then baffles can be installed to increase the flow path and water
residence time.
14. If a wet pond is used during construction, the pond should be cleaned before
becoming a permanent erosion control fixture.
15. Detention facility design must take into consideration overflow. Overflow may
result from higher intensity or longer duration storms than the design storm or
result from plugged orifices or inadequate storage due to sediment buildup in the
facility.
16. Design with maintenance in mind. Good maintenance will be crucial to
successful use of the impoundment. Hence, provisions to facilitate maintenance
operations must be built into the project when it is installed. Maintenance should
be a basic consideration in design and in determination of first cost.
Design Procedure (Wet Pond)
1. Calculate the runoff hydrograph for the water quality design storm.
2. The wet pond shall be designed with a permanent pool volume equal to the runoff
volume of the water quality design storm. It is not necessary to vegetate the
permanent pool, but establishment of a shallow marsh system can provide
additional pollutant removal capabilities.
3. If the wet pond is not to be used for stormwater quantity control, then design an
outlet such as a weir or an orifice that will pass the 100-year event through the
pond. Place the invert of the outlet at the same elevation as the top of the dead
storage volume. The flow leaving the wet pond will discharge either to a large
receiving body or to a quantity control BMP.
4. If the wet pond is to be used for quantity control, estimate the volume needed
above the dead storage volume. Base the estimate on the difference between the
25-year pre-project and 25-year post-project hydrographs.
58
5. Route the 2-year, 25-year and 100-year post-project storms through the pond, one
storm event at a time. An outlet device such as a weir or an orifice must be
designed to match each release rate. Orifice and weir equations can be used to
design an orifice or analyze a given outlet structure.
6. Continue to alter the volume until the allowable release rates are not exceeded.
7. Identify an overflow route to be used in the event that the pond capacity is
exceeded.
Construction and Maintenance Criteria (Wet Pond)
1. Roads in and around the pond shall be constructed according to standard WSDOT
practices and meet the following criteria: Maximum Grade: 15 percent to control
structure, 20 percent into pond. Provide 40-foot minimum outside radius on the
access road to the control structure and the turn around to the pond bottom. Fence
gates shall be provided for access roads at “straight” sections of road. Access
roads shall be 15 feet in width. Manhole and catch basin lids must be at either
edge of an access road or pad and be at least 3 feet from a property line.
2. Access shall be limited by a double-posted gate if a fence is required or by
bollards. Bollards shall consist of two fixed bollards on each side of the access
road and two removable bollards equally located between the fixed bollards.
3. Standing water removed during the maintenance operation may require disposal at
an approved discharge location. Maintenance of sediment forebays and attention
to sediment accumulation within the pond is extremely important. Sediment
deposition should be continually monitored in the basin. Significant
concentrations of heavy metal (e.g., lead, zinc, and cadmium) as well as some
organics such as pesticides, may be expected to accumulate at the bottom of these
treatment facilities. Testing of sediment especially near points of inflow, should
be conducted regularly to determine the leaching potential and level of
accumulation of hazardous material before disposal.
4. THIS SECTION MISSING
5. Remove litter to keep the pond free of external pollution.
6. Check the pond for damage caused by erosion and restabilize areas where there is
the potential for continued erosion.
7. Maintain pond shape and depth by removing accumulated sediments that exceed
10 percent of the designed pond depth.
59
Figure Sec. 17.80.092.340 - Detention / Wet Pond Schematic
60
Figure Sec. 17.80.092.350 - Section AA Detention / Wet Pond
61
Figure Sec. 17.80.092.360 - Detention / Wet Pond Detail
62
Sec. 17.80.092.370 - Nutrient Control Wet Pond
This BMP is similar to Sec.16.80.092.330 - Wet Pond, but also includes a shallow marsh
area which provides additional removal of nutrients. The shallow marsh area is contained
within the permanent pool of water. Stormwater quantity control can also be provided
through this BMP by adding a “live storage” area above the permanent pool. This BMP
should only be used when discharging to a documented nutrient sensitive water body.
General Criteria (Nutrient Control Wet Pond)
1. Wet ponds shall be multi-celled with at least two cells, and preferably three. The
cells should be approximately equal in size. The first cell should be easily
accessible for maintenance purposes. The berm dividing the pond into cells shall
have a 5-foot minimum top width, a top elevation set one-foot lower than the
design water surface, maximum 3:1 side slopes, and a quarry spall and gravel
filter “window” between the cells.
2. A portion of the pond shall be constructed as a shallow marsh area. Planting
wetland associated plants in the marsh area is not required but is beneficial and
recommended.
3. All facilities shall be a minimum of 20 feet from any structure, right-of-way line,
and any vegetative buffer required by the local government and 100 feet from any
septic tank/drainfield. All facilities shall be a minimum of 50 feet from any steep
(greater than 15 percent) slope unless a geotechnical report has indicated that the
steep slope will be stable.
4. Long, narrow and irregularly shaped ponds are preferred, as these configurations
are less prone to short-circuiting and tend to maximize available treatment area.
The length-to-width ratio should be at least 3:1 and preferably 5:1. Irregularly
shaped ponds may perform more effectively and will have a more natural
appearance.
5. The pond bottom shall be level to facilitate sedimentation.
6. Interior side slopes up to the maximum water surface shall be no steeper than
3H:1V. Exterior side slopes shall be no steeper than 2H:1V.
7. Pond walls may be retaining walls, provided that a fence is provided along the top
of the wall, and that at least 25 percent of the pond perimeter will be a vegetated
soil slope of not greater than 3H:1V.
8. For berm embankments of 6 feet or less (including 1-foot freeboard), the
minimum top width shall be 6 feet or as recommended by the engineer.
9. The toe of the exterior slope of pond berm embankment must be no closer than 5
feet from the right-of-way.
63
10. The berm embankment shall be constructed in accordance with Section 203.3(14)C Method C of WSDOT Standard Specifications.
11. A “key” must be excavated that is equal to 50 percent of the berm embankment
cross-sectional height and width (except on highly compacted till soils where the
“key” minimum depth can be reduced to 1-foot of excavation into the till).
12. Anti-seepage collars must be placed on outflow pipes in berm embankments
impounding water greater than 8 feet in depth at the design water surface.
13. Exposed earth on the side slopes shall be sodded or seeded with the appropriate
seed mixture as soon as is practicable. Establishment of protective vegetative
cover shall be ensured with erosion protection and reseeded as necessary.
14. The inlet and outlet should be at opposite ends of the pond where feasible. If this
is not possible, then baffles can be installed to increase the flow path and water
residence time.
15. If a nutrient control wet pond is used during construction, the pond should be
cleaned before becoming a permanent erosion control fixture.
16. Detention facility design must take into consideration overflow. Overflow may
result from higher intensity or longer duration storms than the design storm or
result from plugged orifices or inadequate storage due to sediment buildup in the
facility.
17. Design with maintenance in mind. Good maintenance will be crucial to successful
use of the impoundment. Hence, provisions to facilitate maintenance operations
must be built into the project when it is installed. Maintenance should be a basic
consideration in design and in determination of first cost.
Design Procedure (Nutrient Control Wet Pond)
1. Calculate the runoff hydrograph for the water quality design storm.
2. The nutrient control wet pond shall be designed with a permanent pool volume
equal to the runoff volume of the water quality design storm.
3. The pond surface area to depth relationship shall be such that 30 percent of the
pond area has a depth of 0 to 2 feet and the remainder of the pond has a depth of 2
to 6 feet.
4. If the wet pond is not to be used for stormwater quantity control, then design an
outlet such as a weir or an orifice that will pass the 100-year event through the
pond. Place the invert of the outlet at the same elevation as the top of the dead
64
storage volume. The flow leaving the wet pond will discharge either to a large
receiving body or to a quantity control BMP. The pond design is finished after
this step is completed.
5. If the wet pond is to be used for quantity control, estimate the volume needed
above the dead storage volume. Base the estimate on the difference between the
25-year pre-project and 25-year post-project hydrographs.
6. Route the 2-year, 25-year and 100-year post-project storms through the pond, one
storm event at a time. An outlet device such as a weir or an orifice must be
designed to match each release rate. Orifice and weir equations can be used to
design an orifice or analyze a given outlet structure.
7. Continue to alter the volume until the allowable release rates are not exceeded.
8. Identify an overflow route to be used in the event that the pond capacity is
exceeded.
Construction and Maintenance Criteria (Nutrient Control Wet Pond)
1. Roads in and around the pond shall be constructed according to standard WSDOT
practices and meet the following criteria: Maximum Grade: 15 percent to control
structure, 20 percent into pond. Provide 40-foot minimum outside radius on the
access road to the control structure and the turn around to the pond bottom. Fence
gates shall be provided for access roads at “straight” sections of road. Access
roads shall be 15 feet in width. Manhole and catch basin lids must be at either
edge of an access road or pad and be at least 3 feet from a property line.
2. Access shall be limited by a double-posted gate if a fence is required or by
bollards. Bollards shall consist of two fixed bollards on each side of the access
road and two removable bollards equally located between the fixed bollards.
3. If possible, standing water removed during the maintenance operation must be
disposed of to a sanitary sewer at an approved discharge location.
4. Maintenance of sediment forebays and attention to sediment accumulation within
the pond is extremely important. Sediment deposition should be continually
monitored in the basin. Significant concentrations of heavy metal (e.g., lead, zinc,
and cadmium) as well as some organics such as pesticides may be expected to
accumulate at the bottom of these treatment facilities. Testing of sediment
especially near points of inflow, should be conducted regularly to determine the
leaching potential and level of accumulation of hazardous material before
disposal.
5. Remove litter to keep the pond free of external pollution.
65
6. Check the pond for damage caused by erosion and restabilize areas where there is
the potential for continued erosion.
7. Maintain pond shape and depth by removing accumulated sediments that exceed
10 percent of the designed pond depth.
Sec. 17.80.092.380 Wet Vault/Tank
Wet vaults and tanks are underground storage facilities that treat stormwater for water
quality through the use of a permanent pool of water that acts as a settling basin. Wet
vaults and tanks can also provide stormwater quantity control by adding a live storage
volume above the permanent pool.
General Criteria (Wet Vault/Tank)
1. Wet vaults/tanks cannot provide the equivalent level of treatment accomplished
by wet ponds because neither biological uptake nor vegetative filtration are
available as pollutant removal mechanisms. Gravity-settling of suspended solids
is the primary removal mechanism but vaults/tanks are unlikely to be as effective
as open ponds in removing particulates because little or no soil layer exists in
which to permanently stabilize trapped sediments. Also being underground, vaults
and tanks are more difficult to inspect and maintain. Therefore, they should only e
used when more desirable BMP’s are not practicable.
2. Wet vaults/tanks shall be a minimum of 20 feet from any structure, property line,
and any septic tank/drainfield. All facilities shall be a minimum of 50 feet from
any steep slope, unless a geotechnical report has addressed the potential impact on
a steep slope.
3. The length-to-width ratio at the design surface area shall be no less than 3:1.
4. The vault shall be divided into two cells using a baffle, wit the first cell occupying
about 25 percent of the area. The top of the baffle wall must be coincident with
the depth of the permanent pool.
5. Design with maintenance in mind. Good maintenance will be crucial to successful
use of a vault/tank. Hence, provisions to facilitate maintenance operations must be
built into the project when it is installed. Maintenance must be a basic
consideration in design and cost.
Design Procedure (Wet Vault/Tank)
1. Route water quality design storm through the basin to develop a hydrograph.
2. The permanent pool for water quality treatment shall be sized to equal the total
volume of the water quality design storm.
66
3. If the wet vault is to be used for quantity control, estimate the volume needed
based on the difference between the 25-year pre-project and 25-year post-project
hydrographs.
4. Route the 2-year, 25-year and 100-year post-project storms through the vault, one
storm event at a time. An outlet device such as a weir or an orifice must be
designed to match each release rate. Orifice and weir equations can be used to
design an orifice or analyze a given outlet structure.
5. Continue to alter the volume until the allowable release rates are not exceeded.
6. An overflow outlet must be provided in the event the vault/tank capacity is
exceeded. This overflow outlet should discharge to the same location as the main
outlet.
Construction and Maintenance Criteria (Wet Vaults/Tanks)
Materials
1. Vaults Minimum 3,000 psi structural reinforced concrete. All construction joints
must be provided with water stops.
2. Tank Pipe material, joints, and protective treatment for tanks shall be in
accordance with WSDOT/APWA Standard Specifications Section 9.05, and
AASHTO designations as noted below:
•
Corrugated iron or steel pipe and pipe arch, Treatment 1 through 6.
•
Aluminized Type 2 corrugated steel pipe and pipe arch (meets AASHTO
designations M274 and M36).
•
Steep spiral rib pipe, Treatment 1 through 6.
•
Aluminum spiral rib pipe. Corrugated aluminum pipe and pipe arch.
•
Reinforced concrete pipe.
Structural Stability
1. Vaults: All vaults shall meet structural requirements for overburden support and
HS-20 traffic loading. Cast-in-place wall sections shall be designed as retaining
walls. Vaults shall be placed on native material with suitable bedding. Vaults shall
not be allowed in fill slopes unless analyzed in a geotechnical report for stability
and construction practices.
67
2. Tanks All tanks shall meet structural requirements for overburden support and
traffic loading, if appropriate. HS-20 live loads must be accommodated for tanks
lying under roadways or parking areas. Metal tank end plates must be designed
for structural stability at maximum hydrostatic loading conditions. Flat end plates
generally require thicker gauge material than the pipe and/or require reinforcing
ribs. Tanks shall be placed on native material with a suitable bedding. Tanks shall
not be allowed in fill slopes.
3. Buoyancy (Tanks): In moderately pervious soils where seasonal groundwater
may induce flotation, buoyancy tendencies must be balanced by ballasting with
backfill, providing concrete anchors, increasing the total weight, or by providing
subsurface drains to permanently lower the groundwater table. Calculations must
be submitted which demonstrate stability.
68
Figure Sec. 17.80.092.390 - Typical Detention Tank
69
SECTION 17.80.094
EROSION CONTROL GUIDELINES
70
SEC. 17.80.094 - EROSION CONTROL GUIDELINES.
Sec. 17.80.094.010 - Introduction
Each year tons of sediment are washed and blown from construction sites into local
streams, rivers and lakes. It is a major source of pollution to these water bodies. Eroded
materials also clog streets, storm drains, culverts and stream channels and cause private
property damage. The degradation of wildlife habitat and water quality and the burden
placed on taxpayers for cleanup could be largely avoided through implementation of
adequate erosion control practices.
This handbook introduces plan submittal requirements and measures for construction site
erosion control. All construction activities, public and private, which involve disturbance
of the land surface are covered by the requirements in this handbook. The handbook was
developed to address erosion control requirements for the Longview-Kelso Urban Area.
Erosion control measures are required for construction areas where the ground surface
will be disturbed with clearing, grading, fills, excavations and other construction
activities.
An important factor when developing construction and erosion control plans is:
construction practices which minimize the amount of disturbed land area and avoid or
minimize work on steep slopes are encouraged. Such practices can provide the following
positive results:
•
Less chance of soil washing off the site.
•
The number and size of erosion control measures required will be minimized.
•
The costs of maintaining erosion control facilities are minimized.
•
As much top soil as possible is retained on the site, making revegetation and
landscaping easier to establish.
Sec. 17.80.094.020 - Erosion Control Plan Submittal Requirements
Planning considerations and submittal requirements for erosion control plans for various
types of construction projects are presented below.
Approval of a construction erosion control plan by the jurisdiction does not relieve the
applicant's responsibility to ensure that erosion control measures are constructed and
maintained to contain sediment at the construction site.
71
Sec. 17.80.094.030 - Considerations in Planning for Erosion Control
Following are recommended steps and check lists to use in the development and
implementation of an acceptable erosion control plan. This information will provide the
necessary tools to gain jurisdictional approval of construction activities for all types of
construction sites and developments.
STEP 1: Identify Site Characteristics:
EXISTING:
• topography/contours
• existing drainage patterns and existing drainage systems on and immediately up and
downstream of site site
• soils, as necessary
• wetlands and sensitive areas, creeks, and other identified areas of concern
FUTURE:
• future site contours
• future site drainage system type and location
• future impervious areas
STEP 2: Lay Out Preconstruction Plan and Proposed Base Measures:
•
•
•
•
•
determine construction timing and sequence
establish primary site access point(s) for construction traffic
lay out limits of clearing and construction activities establish
base protection measures •
entrance
establish maintenance procedures for erosion control measures
STEP 3: Measures During Construction:
•
•
•
•
continue establishment of site interior base measures as site clearing and other site
disturbances occur, including stockpile protection and sediment filters along slope
contours
determine if construction may occur during wet season (November 1 through April
30) establish and schedule wet weather measures including cover measures over
exposed soils
establish maintenance procedures for erosion control measures STEP 4: Post
Construction Measures
establish stabilized cover or other measures before removing base erosion control
measures and as approved by permitting agency
72
Sec. 17.80.094.040 - Permit Application Requirements: Construction on Existing
Lots of Record
Construction on existing lots of record and construction of additions to existing single
family homes and duplexes which will cause ground surface disturbance in excess of
5000 square feet of area have the following requirements for construction period erosion
control.
Erosion control methods are as designated in Table 17.80.094.100 in Section
17.80.094.090.
Submit with application for permit the following information:
• completed Erosion Control Information form from the end of this section (or other
form as supplied by the local jurisdiction),
• construction schedule information, as required, including:
- construction start and completion dates,
- dates when erosion control measures will be in place,
- timing of site clearing & grading, fills placement, excavations,
- projected date of removal of erosion control measures (after landscaping is
established or after establishment of a healthy grass stand or other vegetation).
The following may also be required by the permitting jurisdiction:
• a site plan showing locations of the various required erosion control measures; or
• a typed sheet stating locations of the various required erosion control measures (can
be submitted as part of the construction schedule, above).
Sec. 17.80.094.050 - Other Private Developments Construction
Construction on private property, other than those sites covered in Section 17.80.094.040
above, which will cause ground surface disturbance have the following requirements for
erosion control. Recommended erosion control measures are as designated in Tables
17.80.094.110 and Sec. 17.80.094.120 in Section 17.80.094.090.
Submit with construction plans for subdivision approval, grading, building, or erosion
control permit the following information:
• completed Erosion Control Information form from the end of this section (or other
form as supplied by the local jurisdiction),
• construction schedule with the following information:
- construction start and completion dates,
- dates when erosion control measures will be in place,
- timing of site clearing & grading, fills placement,
excavations,
- projected date of removal of erosion control measures (after landscaping is
established or after establishment of a healthy grass stand or other vegetation).
73
Submit also with construction plans:
An erosion control plan drawing on a site plan showing:
- locations, types and applicable dimensions of erosion control measures,
- applicable details of erosion control measures showing full dimensions and
construction information,
- existing and proposed ground contours,
- locations and sizes of existing and proposed drainage pipes and channels (labeled
as such and with arrows indicating flow direction),
- site entrances/exits,
- applicable standard erosion control notes (given at the end of this section), with
additions or changes as required,
- other notes including references to timing of placement and removal of erosion
control measures, and erosion measure specifications such that types and
quantities of materials necessary for the installation of the erosion control
measures are fully detailed.
If the site erosion control plan includes sediment traps or ponds, the applicant shall also
submit calculations used for determining trap or pond sizing. Because of particular site
conditions or preferences, the applicant may desire in certain cases to use different
erosion control measures than are recommended in Tables 17.80.094.110 or Sec.
16.80.094.120. In such cases, the applicant must submit calculations or other supporting
information used to determine the sizing and layout of the submitted erosion control plan.
Design criteria and use of the Soil Conservation Service Universal Soil Loss Equation for
the purpose of sizing sediment traps or ponds, and proposed new erosion control
measures are described at the end of this Section.
Sec. 17.80.094.060 - Private Construction in Public Rights-of-Way
Private construction in public rights-of-way has the same erosion control plan submittal
requirements as noted in Section 17.80.094.050 above.
An exception is construction of private utilities and similar localized construction or
maintenance activities. Such construction must meet noted erosion control measures in
Table 17.80.094.120 for utilities construction and stockpiles as applicable. For such
construction, the applicant need only submit the Erosion Control Information form given
at the end of this section as required, unless different erosion measures than indicated in
Table 17.80.09.120 are desired, in which case an erosion control site plan drawing must
be submitted per Section 17.80.094.050 above.
Sec. 176.80.094.070 - Public Works Construction
Public Works construction projects have the same erosion control plan design
requirements as note above for private construction in public rights-of-way. Erosion
74
control plan submittal/review requirements are per the appropriate jurisdiction's existing
public works design review process.
75
SECTION 17.80.094
RECOMMENDED EROSION CONTROL MEASURES
76
RECOMMENDED EROSION CONTROL MEASURES
Sec. 17.80.094.080 -Introduction and General Plan Requirements
Recommended construction period erosion control measures are detailed in this chapter.
The methods and measures outlined in this handbook are recommended for use in the
development of appropriate erosion control plan permit submittals as outlined in Section
17.80.094.090.
Erosion control measures are required during all construction and site disturbance activity
and until permanent site ground covers are in place. As further detailed in Sections
17.80.094.090 and 17.80.094.100, certain base erosion control measures are required for
construction sites at all times of the year. Also, additional cover or filtration measures
are required during the wet weather season (November 1 through April 30).
Erosion control measures utilized for any construction site must be designed to achieve a
disturbed area erosion loss of no more than 1 ton per acre per year, based on the Soil
Conservation Service (SCS) Universal Soil Loss Equation. The erosion control measures
outlined in the matrices in Section 17.80.094.090 are designed to achieve a one ton per
acre per year goal. If, based on particular site conditions, different types or combinations
of erosion control measures are desired for a construction site, the SCS Universal Soil
Loss Equation or other approved supporting methodology/information must be used to
design the erosion control system, and the design calculations must be submitted to the
appropriate jurisdiction's permit review section with the proposed erosion control plan. A
brief summary of the use of the Universal Soil Loss Equation is presented at the end of
this section.
The designer should keep in mind when laying out an erosion control plan that the
purpose of the plan is to minimize erosion and sedimentation from disturbed ground
surfaces. Minimizing the area of clearing and grading, phasing of construction, and use of
other methods to reduce the amount of land area disturbed will provide the greatest
erosion control benefits.
The site owner/developer is responsible for seeing that erosion control measures are
installed, maintained and working as designed. An approved erosion control plan does
not waive the owner/developer's responsibility for ensuring that erosion control is
achieved. If an installed erosion control system does not adequately contain sediment on
site, then the erosion measures must be field adjusted as necessary by the applicant, and
as approved by the permitting jurisdiction.
Sec. 17.80.094.090 - Erosion Control Matrices
Tables 17.80.094.100 through Sec. 17.80.094.130 are matrices presenting recommended
erosion control measures for various site and construction types.
77
Table 17.80.094.100 is a matrix summarizing recommended erosion controls for single
family residential and duplex construction activities on single lots of record.
Table 17.80.094.120 summarizes recommended erosion control measures for larger
construction sites including commercial, industrial and subdivision development and
construction.
Table 17.80.094.130 is a matrix presenting recommended erosion controls for small,
linear utilities construction and ditches/swales.
Erosion control measures are divided into two categories:
•
Base measures which are required for construction sites at all times while there is
disturbed, unstabilized ground surface on the site, and
•
Supplementary wet weather measures which are required between November 1 and
April 30 in addition to the base measures.
Base measures are indicated on Tables 17.80.094.100 through 17.80.094.130 as primary
recommended base measures and alternate measures. Wet weather measures are
indicated on Tables 17.80.094.100 through 17.80.094.130 as primary recommended
measures and alternate measures.
Each erosion control measure presented in the matrices is presented in further detail with
design, construction and maintenance criteria in Section 17.80.094.130. Applicable
sections are referenced in parenthesis after each erosion control measure presented in the
matrices in said Tables.
78
Table 17.80.094.100 - EROSION CONTROL MATRIX
SINGLE FAMILY/DUPLEX RESIDENTIAL
BASE MEASURES
1. Gravel construction entrance
(Sec. 17.80.094.140)
SINGLE FAMILY /
DUPLEX RES.
Slope<2%
Slope>2%
Base
STOCK PILES
Base
2. Sediment fence/barrier @ toe of
disturbed area or stockpile
(Sec. 17.80.094.170 & .190)
Base
Measure
3. Sidewalk subgrade gravel barrier
(site slopes to street @ <5% grade)
(alternate to #2,)
(Sec. 17.80.094.220)
Alternate to
#2
4. Undisturbed buffer @ toe of
Disturbed areas (alternate to #2.)
(site slopes<10%)
(Sec. 17.80.094.230)
Alternate to
#2
Base Measure
WET WEATHER MEASURES**
5. 6-mil plastic sheet cover
(Sec. 17.80.094.270)
Supplemental
Wet Weather Measure
(Nov. 1 - April 30)
6. 2"-min. straw mulch cover
(Sec. 17.80.094.250)
Alternate Wet
Weather Measure
POST CONSTRUCTION
7. Reestablish ground cover or
landscape prior to removing erosion
measures
(Sec.
17.80.094.240)
Base
Measure
** - or other method acceptable to jurisdictional authority
79
Table 17.80.094.110 - EROSION CONTROL MATRIX
COMMERCIAL, SUBDIVISION & LARGE SITE CONSTRUCTION
SITE SLOPE
BASE MEASURES
STOCK
<2%
<10%
<15%
<20%
<30%
<50%
>50%
Base
Measure
Base
Measure
Base
Measure
Base
Measure
Base
Measure
Base
Measure
Base
Measure
2. Sediment fence / barrier @ toe of
disturbed area or stockpile
(Sec. 17.80.094.170 & .190)
Base
Measure
Base
Measure
Base
Measure
Base
Measure
Base
Measure
Base
Measure
3. Undisturbed buffer @ toe of
disturbed area
(Sec. 17.80.094.230)
Altern.
to #2
4. Sediment fence installed on
Contours (spacing)
(Sec. 17.80.094.170)
Base
Measure
(300')
Base
Measure
(150')
Base
Measure
(100')
Base
Measure
(50')
Base
Measure
(25')
Base
Measure
(25')
Optional
Base
Measure
Optional
Base
Measure
Optional
Base
Measure
Optional
Base
Measure
Optional
Base
Measure
Optional
Base
Measure
Suppl.
WWM
Suppl.
WWM
Suppl.
WWM
Suppl.
WWM
Suppl.
WWM
7. 2"-min. straw mulch cover
(Sec. 17.80.094.250)
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
8. Erosion blankets with anchors
(Sec. 17.80.094.260)
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
9. 6-mil plastic sheet cover
(Sec. 17.80.094.270)
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
Suppl.
WWM
10. Sediment trap or pond
(Sec. 17.80.094.280)
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
Altern.
WWM
Base
Measure
Base
Measure
Base
Measure
Base
Measure
Base
Measure
1. Gravel construction entrance
(Sec. 17.80.094.140)
5. Temp. interceptor dikes / swale
around active work areas
(Sec.
17.80.094.300)
Optional
Base
Measure
PILES
Base
Measure
WET WEATHER MEASURES**
6. Establish grass
17.80.094.240)
(Sec.
Altern.
WWM
Suppl.
WWM
POST CONSTRUCTION
11. Reestablish ground cover or
landscape prior to removing erosion
measures
(Sec. 17.80.094.240)
Base
Measure
Base
Measure
** - or other method acceptable to jurisdictional authority
80
Table 17.80.094.120 - EROSION CONTROL MATRIX
UTILITIES CONSTRUCTION AND
STOCK PILES/DITCHES/SWALES PROTECTION
BASE MEASURES
UTILITIES CONSTRUCTION
Catch Basin
Ditch
Drainage
Drainage
1. Sediment fence or barrier @ toe
of disturbed area or stock pile
DITCHES/
SWALES
Base Measure
2. Check dams
(Sec. 17.80.094.260 & .190)
3. Storm drain inlet protection
barrier
(Sec. 17.80.094.320)
STOCK PILES
Base
Measure
Base Measure
Base
Measure
WET WEATHER MEASURES**
4. Establish grass
(Sec. 17.80.094.240 & .270)
Suppl. Wet Weather
Measure
(Nov. 1 - April 30)
5. 6-mil plastic sheet cover
(Sec. 17.80.094.270)
Suppl. Wet Weather
Measure
(Nov. 1 - April 30)
6. 2"-min. straw mulch cover
(Sec. 17.80.094.250)
Alt. Wet Weather
Measure
7. Erosion blanket with anchors
(Sec. 17.80.094.260)
Alt. Wet Weather
Measure
Alt. Wet Weather
Measure
POST CONSTRUCTION
7. Reestablish ground cover or
landscape prior to removing erosion
measures
(Sec.
17.80.094.240)
Base
Measure
Base
Measure
Base Measure
** - or other method acceptable to jurisdictional authority
81
Sec. 17.80.094.130 - Recommended Design Criteria
The following sections provide design, construction and maintenance criteria and
recommendations for the erosion control measures listed in the matrices in section Sec.
17.80.094.090.
Sec. 17.80.094.140 - Gravel Construction Entrances
Purpose
To reduce the amount of mud, dirt, rocks, etc, transported onto roads by motor vehicles
or storm water runoff by constructing a stabilized pad of gravel at entrances/exits to
construction sites.
Conditions Where Practice Applies
At any construction site where traffic will be leaving the site and moving directly onto
public roads, other paved areas, or other approved access points.
Design Criteria/Specifications
•
See Figure Sec. 17.80.094.150 for details.
•
Material should be clean pit run or 3/4"-minus gravel (or larger, as needed).
•
The gravel pad shall be at least 8 inches thick and 50 feet in length. Width shall be
the full width of the vehicle ingress and egress area. (A 20-foot minimum pad length
may be acceptable as approved for single family and duplex residential construction
sites.)
•
Use subgrade reinforcement geotextile under gravel pads for all but construction of a
single family/duplex residence on existing lots of record.
•
Additional gravel may have to be added periodically to maintain proper function of
the pad.
•
Additional Measures.
If the gravel pad does not adequately remove dirt and mud from vehicle wheels such that
mud and dirt tracking is evident off site, additional measures must be taken. Such
measures may include washing off wheels before vehicles leave the site or other
construction techniques/work operations modifications.
Wheel washing should be done on the gravel pad or in an approved wheel wash structure
located onsite, adjacent to and on the site interior side of the gravel pad. Wash water
shall be drained through a silt-trapping structure prior to leaving the construction site.
See Figure Sec. 17.80.094.160 for details of a typical wheel wash structure.
82
Another additional measure is to construct gravel filter berms across on-site traffic wheel
paths to capture and retain sediment. Berms shall be 1 foot high with 3:1 side slopes,
constructed of 3/4 to 3-inch well-graded or crushed rock with less than 5 percent fines.
Berms must be inspected regularly and accumulated sediment removed and rock added or
replaced as needed. Berms should be spaced as follows:
- every 300 feet on slopes less than 5 percent,
- every 200 feet on slopes between 5 and 10 percent,
- every 100 feet on slopes greater than 10 percent.
83
Figure Sec. 17.80.094.150 - Gravel Construction Entrance
84
Figure Sec. 17.80.094.160 - Wheel Wash
85
Sec. 17.80.094.170 - Temporary Sediment Fences
Purpose
To reduce the transport of sediment from a construction site by providing a temporary
physical barrier to sediment and reducing runoff velocities.
Conditions Where Practice Applies
•
Down slope of disturbed areas where runoff occurs as sheet runoff.
•
At the toe of soil stockpiles.
•
At intervals as indicated on Table 17.80.094.180 along the contours of large disturbed
areas.
•
At grade breaks exceeding 20%.
•
Following discharge from a sediment trap or pond.
•
Sediment fences shall not be installed across streams.
Design Criteria/Specifications
•
See Figure Sec. 17.80.094.180 for details.
•
Maximum sheet or overland flow path length to sediment fence is per Table
17.80.094.180.
•
Selection of filter fabric tensile and bursting strength characteristics depends on the
support fence. Fabric attached to chain-link fence need not possess the same strength
as one attached to a fence of 6- by 6-inch reinforcing wire or one attached only to
posts. Selection is thus based on standard engineering principles. Synthetic filter
fabric shall contain ultraviolet ray inhibitors and stabilizers to provide a minimum of
6 months of expected usable construction life at a temperature range of 0°F to 120°F.
•
Unless otherwise waived by the jurisdiction, when standard strength filter fabric is
used, a wire support fence shall be fastened securely to the upslope side of the posts
using heavy-duty wire staples at least 1 inch long, tie wire or hog rings. The wire
shall extend into the trench a minimum of 4 inches and shall not extend more than 36
inches above the original ground surface.
•
When extra-strength filter fabric and closer post spacing are used, the wire mesh
support fence may be eliminated. In such a case, the filter fabric is stapled or wired
directly to the posts with all other provisions of the above standard note for standard
strength filter fabric applying.
86
Figure Sec. 17.80.094.180 - Sediment Fence
87
•
All materials to be in good physical condition to provide proper sediment retention.
•
Where practical the filter fabric shall be purchased in a continuous roll cut to the
length of the barrier to avoid use of joints. When joints are necessary, filter cloth
shall be spliced together only at a support post, with a minimum 6-inch overlap, and
both ends securely fastened to the post.
•
Sediment fences shall be inspected by applicant/contractor immediately after each
rainfall and at least daily during prolonged rainfall. Any required repairs shall be
made immediately.
•
At no time shall more than a one foot depth of sediment be allowed to accumulate
behind a sediment fence. Sediment should be removed or regraded into slopes, and
the sediment fences repaired and re-established as needed.
Sec. 17.80.094.190 - Straw Bale Sediment Barrier
Purpose
To reduce the transport of sediment from a construction site by providing a temporary
physical barrier to sediment and reducing runoff velocities. Also may be used to divert
runoff around active work areas or into sediment filtration/sedimentation areas.
Conditions Where Practice Applies
• May be substituted for temporary sediment fence for installations of less than 200
lineal feet and for single family or duplex residential construction activities on
existing lots of record, as approved.
• At toe of soils stock piles.
• In existing, undisturbed drainage ditches/swales used to convey drainage through
disturbed areas of construction site (as allowed by the jurisdiction). Note: see Sec.
17.80.094.360 "Check Dams" for use of straw bales and other methods to protect new
and disturbed swales and ditches.
• Note: see Section 17.80.094.300 "Temporary Interceptor Dikes and Swales" for use of
straw bales as flow interceptor dikes.
Design Criteria/Specifications
• See Figures Sec. 17.80.094.200 and Sec. 17.80.09.210 for details of straw bale
sediment barriers.
• Straw bales shall be standard 40 to 60 pound rectangular bales of cereal grain or seed
straw.
88
• Stakes shall be wood of size as shown on Figures Sec. 17.80.094.200 & .210 and
driven through bales and into ground to a minimum depth of 12 inches.
• Straw bales shall be keyed into existing ground 4 to 6 inches. Straw Bale Sediment
Barriers may be left in place or used as mulch after completion of site work if
approved by the jurisdiction.
• At no time shall more than a one foot depth of sediment be allowed to accumulate
behind straw bale sediment barriers. Sediment should be removed or regraded into
the slope, or new lines of barriers installed uphill of sediment-laden barriers.
89
Figure Sec. 17.80.094.200 - Straw Bale Sediment Barrier
90
Figure Sec. 17.80.094.210 - Straw Bale Sediment Barrier
91
Sec. 17.80.094.220 - Sidewalk Subgrade Gravel Barrier
Purpose
To reduce the transport of sediment from a construction site by using the sidewalk
subgrade gravel as a temporary filter for sediment-laden runoff.
Conditions Where Practice Applies
Single family/duplex residential construction sites, where the site slopes to a street with
planned but unconstructed sidewalks, and site slopes are less than 5 percent.
Design Criteria/Specifications
Sidewalk subgrade gravel must be in place during the entire construction period, from the
time of initial site clearing/grading through establishment of permanent site cover. If the
sidewalk concrete is to be poured prior to be established of permanent site cover,
approved sediment barriers must be installed prior to pouring concrete.
Sidewalk subgrade gravel must have a minimum 4-inch depth and a 4-foot width.
If the sidewalk subgrade gravel does not provide an effective filter such that sediment is
leaving the construction site, additional measures must be applied. These may include
replacement of gravel or installation of sediment barriers.
Subgrade gravel may not meet the local jurisdiction's specifications for sidewalk concrete
placement if too much sediment has infiltrated the rock. The permittee must therefore
weigh the benefits of eliminating sediment barriers versus the possibility that the
subgrade gravel may be rejected by the local jurisdiction.
Sec. 17.80.094.230 -Undisturbed Buffers
Purpose
To provide a natural vegetated buffer area for filtering erosion from construction areas, as
an alternate in certain cases or supplemental measure to sediment barriers.
Conditions Where Practice Applies
An undisturbed buffer may be used as approved as an alternate to a sediment barrier at
the toe of the site slopes if the buffer meets the following criteria:
•
the buffer is an undisturbed grassy area or covered with other approved dense
vegetation,
92
•
the buffer is downhill and in the drainage path of the construction/disturbed area,
•
there are no concentrated flows from the disturbed site entering the buffer,
•
the buffer area is owned by the applicant or approved for such use in writing by the
owner,
•
slopes in the buffer and its drainage area are less than 10 percent,
•
the grassed buffer area impacted by the potential disturbed area runoff is at least equal
in area to the uphill construction/disturbed area draining to it.
Sec. 17.80.094.240 - Establishing Temporary Grasses and Permanent Vegetative
Cover
Purpose
To reduce erosion and sedimentation by stabilizing exposed soils with vegetation and
mulching.
Conditions Where Practice Applies
•
Ground surfaces exposed during the wet season (November 1 through April 30).
•
Areas which will not be subjected to heavy wear by on-going construction traffic.
•
Exposed ground surfaces at end of construction period (permanent cover must be
established prior to removal of any erosion control measures).
•
Temporary or permanent stabilization of new or disturbed ditches or swales.
Design Criteria/Specifications: Temporary Erosion Control Grasses
•
Temporary grass cover measures must be fully established by November 1 or other
cover measures will have to be implemented until adequate grass coverage is
achieved. To establish an adequate grass stand for controlling erosion by November
1, it is recommended that seeding and mulching occur by October 1.
•
Hydromulch shall be applied with grass seed at a rate of 2000 lb./acre. On slopes
steeper than 10%, hydroseed and mulch shall be applied with a bonding agent
(tackifier). Application rate and methodology to be per seed supplier
recommendations.
•
Dry, loose, weed-free straw used as mulch shall be applied at double the hydromulch
application requirement (4000 lb./acre). Anchor straw by working in by hand or with
equipment (rollers, cleat tracks, etc.).
93
•
Mulch shall be spread uniformly immediately following seeding.
•
Soil Preparation - Top soil should be prepared according to landscape plans, if
available, or recommendations of grass seed supplier. It is recommended that slopes
be roughened before seeding by 'track-walking', (driving a crawling tractor up and
down slopes to leave a pattern of cleat imprints parallel to slope contours) or other
method to provide more stable sites for seeds to rest.
•
Seeding - Recommended erosion control grass seed mixes are as follows. Similar
mixes designed to achieve erosion control may be substituted if approved by
jurisdiction.
1. Dwarf Grass Mix (low height, low maintenance):
Dwarf Perennial Ryegrass, 80% by weight
Creeping Red Fescue, 20% by weight
Application rate: 100 pounds minimum per acre
2. Standard Height Grass Mix
Annual Ryegrass, 40% by weight
Turf-type Fescue, 60% by weight
Application rate: 100 pounds minimum per acre
•
Fertilization for grass seed - As per supplier's recommendations. Development areas
within 50 feet of water bodies and wetlands must use a non-phosphorus fertilizer.
•
Netting and Anchors, as needed - For disturbed areas on slopes and in ditches/swales,
biodegradable netting or jute is desirable and may be used instead of bonding agents
to provide a stable area for seeding. Netting should be anchored per manufacturer's
recommendations.
•
Watering - Seeding shall be supplied with adequate moisture to establish grass.
Supply water as needed, especially in abnormally hot or dry weather or on adverse
sites. Water application rates should be controlled to provide adequate moisture
without causing runoff.
•
Re-seeding - Areas which fail to establish grass cover adequate to prevent erosion
shall be re-seeded as soon as such areas are identified, and all appropriate measures
taken to establish adequate cover.
Design Criteria/Specifications: Permanent Vegetative Groundcover
At the end of site construction, paving, approved permanent site landscaping or
establishment of a healthy stand of grass (or alternative vegetation as approved) must
occur prior to removal of site erosion control measures.
94
Sec. 17.80.094.250 - Straw Mulch
Purpose
To reduce erosion by providing a protective cover over disturbed bare or reseeded soils.
Also can be used to enhance success of seeding/ revegetation.
Conditions Where Practice Applies
•
As a cover on ground surfaces and stockpiles exposed during the wet season
(November 1 through April 30).
•
As a mulch to enhance vegetation establishment in areas that have been seeded.
Design Criteria/Specifications
•
Loose, weed-free straw mulch shall be applied at a rate of no less than 4000 pounds
(2 tons) per acre, and shall have a minimum depth in-place of two (2) inches.
•
Mulch must be stabilized in place by hand or machine punching the straw into the
soil, spraying it with a tacking agent, or covering it with an erosion blanket. See Sec.
17.80.094.260 "Erosion Blankets" for appropriate design criteria for such coverings.
Sec. 17.80.094.260 - Erosion Blankets
Purpose
To provide immediate protection and physical stabilization of disturbed soils. Typically
used when vegetative cover cannot be achieved due to soils, slopes or time of year. Can
be used to enhance success of seeding, planting and/or sodding.
Conditions Where Practice Applies
•
On areas of steep slopes (greater than 50%) and areas of moderate slopes that are
prone to erosion.
•
As a cover on ground surfaces exposed during the wet season (November 1 through
April 30).
•
As supplemental aid to seed and/or mulch treatment on slopes or in ditches or swales.
Design Criteria/Specifications
•
Erosion blankets may be used on level areas and on slopes up to 1:1. Where soil is
highly erodible, netting shall only be used in conjunction with an organic mulch such
95
as straw or wood fiber. The blanket must be applied so that it is in complete contact
with the soil; if it is not, erosion will occur beneath it. Erosion blankets shall be
securely anchored to the slope per manufacturer's recommendations.
•
Deformed plastic filament matting such as Enkamat (and other erosion control
blankets as approved) may be used for stream velocity protection and other special
applications when approved by the jurisdiction.
Sec. 17.80.094.270 - Plastic Sheet Covering
Purpose
To provide immediate erosion protection to slopes and disturbed areas when vegetative
cover cannot be achieved due to soils, slopes or time of year. To provide erosion
protection on soils, spoils, and other erodible stockpiles.
Conditions Where Practice Applies
•
Disturbed areas which require immediate erosion protection.
•
On areas of steep slopes (greater than 50%) and areas of moderate slopes that are
prone to erosion.
•
On ground surfaces and stockpiles exposed during wet weather season (November 1
through April 30).
•
As a temporary measure to provide erosion protection and assist in germination on
areas seeded between November 1 and March 31.
Design Criteria/Specifications
•
Plastic sheeting shall be polyethylene and have a minimum thickness of 6 mil.
•
Covering shall be installed and maintained tightly in place by using sandbags or tires
on ropes with a maximum 10 foot grid spacing in all directions. All seams shall be
taped or weighted down full length and there shall be at least a 12-inch overlap of all
seams. For seams parallel to the slope contour, the uphill sheet shall overlap the
downhill sheet. No runoff shall be allowed to run under the plastic covering.
•
Drainage from areas covered by plastic sheeting shall be controlled such that no
discharge occurs directly onto uncontrolled, disturbed areas of the construction site.
•
Clear plastic sheeting may be installed on areas seeded between November 1 to
March 31 to provide a greenhouse-type environment, and remain until vegetation is
firmly established.
96
Sec. 17.80.094.280 - Sediment Traps & Ponds
Purpose
To collect and store sediment eroded from exposed ground surfaces, disturbed during the
construction period, prior to establishment of permanent vegetation and drainage
facilities.
Conditions Where Practice Applies
•
Downhill of areas with exposed soils during the wet season (November 1 through
April 30).
•
Sediment Traps: where the tributary drainage area is 3 acres or less (but not
including single family and duplex residences constructed singly, on existing lots of
record), and slopes are less than 50%.
•
Sediment Ponds: where the tributary drainage area is 10 acres or less and slopes are
less than 50%.
Design Criteria/Specifications
Temporary interceptor dikes or swales may be constructed to divert runoff to sediment
traps or ponds.
A. Sediment Traps
The sediment trap may be formed completely by excavation or by construction of a
compacted embankment. It shall have a sediment storage depth not to exceed 1.5
feet, topped by a 2 foot deep settlement zone. Sediment trap side slopes shall be 3:1
or flatter. The outlet of the trap should be a weir/spillway, providing a minimum one
(1) foot overflow depth between the spillway and embankment. A filter fabric fence
or similar filter must be constructed to filter runoff from the trap prior to discharge
from the construction site.
•
See Figure Sec. 17.80.094.290 for details.
•
Calculate the required sediment storage volume using the SCS Universal Soil
Loss Equation as described in the attached section and assuming a minimum one
year sediment accumulation period for design purposes. To convert tons of
sediment as calculated to cubic feet, multiply by 0.05 tons per cubic foot.
•
Determine the bottom surface area of the sediment trap using the calculated
sediment volume and the maximum 1.5 foot depth and 3:1 side slope
requirements.
97
Figure Sec. 17.80.094.290 - Sediment Trap
98
•
Determine the total trap dimensions by adding an additional 2 feet of depth for
settling volume (before overtopping of spillway) above the sediment storage
volume, while not exceeding 3:1 side slopes.
•
A 3:1 ratio of trap length to width is desirable. Length is defined as the average
distance from the inlet to the outlet of the trap.
B. Sediment Ponds
A sediment pond may be formed by partial excavation and/or by construction of a
compacted embankment. It may have one or more inflow points carrying polluted
runoff. Baffles to spread the flow throughout the pond should be included. A
securely anchored riser pipe is the recommended principal discharge mechanism,
with an emergency overflow spillway. The riser pipe should be perforated and
covered with filter fabric and gravel 'cone' for filtration; or solid with a l" diameter
dewatering hole and perforated drain pipe. Outlet protection shall be provided to
reduce erosion at the pipe outlet. A filter fabric fence or similar filter must be
constructed to filter runoff from the pond prior to discharge from the construction
site.
•
The sediment pond shall have a sediment storage depth no greater than 3 feet,
topped by a 2-foot (minimum) to 4-foot (maximum) deep settlement zone and an
additional 1 foot minimum of freeboard. The pond side slopes shall be 3:1 or
flatter.
•
See Figure Sec. 17.80.094.300 for details.
•
The sediment storage volume is determined in the same manner as mentioned
above for sediment traps. The pond riser pipe and outlet pipe shall be sized to
carry the 10-year design storm (or as otherwise required by the jurisdiction).
•
A 3:1 ratio between the pond length and width is desirable. Length is defined as
the average distance from the inlet to the outlet of the trap. Use baffles in the
pond to help prevent short-circuiting and to increase the effective pond length
where site conditions prohibit constructing a pond with a direct 3:1 length to
width ratio.
99
Figure Sec. 17.80.094.300 - Sediment Pond
100
Sec. 17.80.094.300 - Temporary Interceptor Dikes and Swales
Purpose
To intercept storm runoff from drainage areas above unprotected slopes and direct to a
stabilized outlet. To intercept storm runoff from a disturbed site and direct it to a
sediment trap or pond.
Conditions Where Practice Applies
Where the volume and velocity of runoff from disturbed slopes must be reduced. When
an interceptor dike or swale is placed above a disturbed slope, it reduces the volume of
water reaching the disturbed area by intercepting runoff from above.
Where sediment traps or ponds are to be used. Interceptor dikes and swales can be used
to direct site runoff to a sediment trap or pond.
Design Criteria/Specifications
Intercepted runoff must be directed to a stabilized area such that no erosion occurs due to
the additional water and velocity, or to a sediment pond or trap.
See Figure Sec. 17.80.094.310 for details. Straw Bales may also be used to intercept
runoff. See Sec. 17.80.094.190 "Straw Bale Sediment Barrier" for installation criteria
and specifications.
Interceptor dikes and swales shall be stabilized with approved cover before put into use.
Such cover may include grass, rock or erosion blankets.
Spacing between interceptor dikes and swales along slope contours is as follows:
Slope Spacing
< 5%
300 feet
5-10% 200 feet
10-40% 100 feet
•
Minimize construction traffic over dikes and swales.
•
The upslope side of interceptor dikes shall provide positive drainage to the dike
outlet. Provide energy dissipation measures as necessary to minimize erosion at dike
outlet.
•
Grades for drainage parallel to interceptor dikes shall be between 0.5 and 1.0 percent.
•
Maximum grade of interceptor swales shall be 5 percent, and provide positive
drainage to outlet.
101
•
Outlets shall lead to sediment trap/pond when runoff from disturbed areas is
intercepted by dikes/swales. Outlets shall be stabilized to prevent erosion.
•
Temporary dikes and swales shall be graded out at the completion of construction,
when permanent vegetation has been established.
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Figure Sec. 17.80.094.310 - Interceptor Swales and Dikes
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Sec. 17.80.094.320 - Storm Drain Inlet Protection
Purpose
To prevent sediment from entering storm drain systems prior to permanent stabilization
of disturbed areas.
Conditions Where Practice Applies
•
Where interior site storm drain inlets are operational before permanent stabilization of
the disturbed drainage area, as approved by jurisdiction.
•
Adjacent to and immediately downhill of utility type construction in existing paved
areas with catch basin drainage.
•
In public right-of-way areas for use during approved flushing operations.
Design Criteria/Specifications
•
Design criteria and specifications for three recommended alternative methods of
storm drain inlet protection are presented on Figures Sec. 17.80.094.330, .340, &
.350.
•
Berms may be required to direct drainage to flow through the filters and prevent
bypassing of the inlets.
•
At no time shall more than a one (1) foot depth of sediment be allowed to accumulate
against storm drain inlet protection measures. Sediment must be removed and inlet
protection measures restored as needed to maintain their sediment trapping and
filtering capability.
104
Figure Sec. 17.80.094.330 - Filter Fabric Inlet Barrier
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Figure Sec. 17.80.094.340 - Block and Gravel Inlet Barrier
106
Figure Sec. 17.80.094.350 - Gravel and Wire Mesh Inlet Barrier
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Sec. 17.80.094.360 - Check Dams
Purpose
To reduce the velocity of concentrated flows, reducing erosion of the swale or ditch, and
providing for sedimentation of suspended soil particles.
Conditions Where Practice Applies
•
In new or disturbed ditches and swales to reduce velocities and erosion.
•
In interior site ditches or swales conveying runoff from disturbed areas (other base
and cover measures still required in addition to the check dams for disturbed drainage
areas).
•
No check dams may be placed in streams, without local jurisdiction and state
agencies approval as required.
Design Criteria/Specifications
•
See Figure Sec. 17.80.094.370 for details.
•
Check dams shall be constructed of either rock or logs. Check dams may also be
constructed of straw bales or other materials as approved. (See Sec. 17.80.094.190
"Straw Bale Sediment Barrier" for approved uses and design criteria for straw bales.)
•
Construct a 1-foot deep sump immediately upstream of check dams for storage of
settled sediment.
•
Check dams shall be spaced such that the toe of the upstream dam is at the same
elevation as the top of the next downstream dam.
•
Rock check dams shall be constructed of rock spalls, 4"-minus. The rock must be
placed by hand or mechanical placement (no dumping of rock to form dam) to
achieve complete coverage of the ditch or swale and to ensure that the center of the
dam is lower than the edges.
•
Log check dams shall be constructed of 4- to 6- inch diameter logs. The logs shall be
embedded into the soil at least 18 inches.
•
Check dams shall be checked for sediment accumulation after each significant
rainfall. Sediment shall be removed before filling sump.
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Figure Sec. 17.80.094.370 - Check Dams
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SECTION 17.80.096
FORMS AND RESOURCES
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Sec. 17.80.096.010 - STANDARD FORMS
GRADING/EROSION CONTROL INFORMATION
GENERAL CONTRACTOR NAME/ADDRESS/PHONE:
CASEFILE NO.:
PERMIT NO.
APPLICANT NAME/ADDRESS/PHONE:
EXCAVATION CONTRACTOR NAME/ADDRESS/PHONE:
OWNER NAME/ADDRESS/PHONE:
SITE/JOB CONTACT:
24 HR/AFTER HOURS EMERGENCY CONTACT PERSON. TITLE, TELEPHONE:
PROPERTY DESCRIPTION:
STREET ADDRESS AND CROSS STREET:
LEGAL DESCRIPTION:
TAX LOT NO.:
¼ SECTION:
DISTURBED WORK AREA/ACRES:
SITE SIZE/ACRES:
LOCATION & ADDRESS WHERE SPOILS LEAVING SITE WILL BE TAKEN:
(PERMIT MAY BE REQUIRED)
SITE RUNOFF DRAINS TO (CIRCLE ONE):
CATCH BASIN
DITCH
PIPE
CREEK
PRIVATE PROPERTY
EROSION/SEDIMENTATION CONTROL (ESC) MEASURES
MINIMUM ESC REQUIREMENTS
MINIMUM ESC REQUIREMENTS
DURING CONSTRUCTION:
FOLLOWING CONSTRUCTION:
SEDIMENTATION FACILITIES
STABILIZE EXPOSED SURFACE
STABILIZED CONSTRUCTION ENTRANCE
REMOVE AND RESTORE TEMPORARY ESC
PERIMETER RUNOFF CONTROL
FACILITIES
CLEARING AND GRADING RESTRICTIONS
CLEAN AND REMOVE SILT AND DEBRIS
COVER PRACTICES
ENSURE OPER. OF PERM.FACILITIES
CONSTRUCTION SEQUENCE
OTHER:
OTHER:
PLAN FOR EROSION CONTROL PREPARED AND SUBMITTED IN ACCORDANCE WITH EROSION
CONTROL GUIDELINES. EROSION CONTROL PLAN DRAWING. AS REQUIRED. HAS PLAN
CONSTRUCTION NOTES COMPLETE, INCLUDING EMERGENCY PHONE NUMBER,
SCHEDULE/STAGING FOR INSTALLATION AND REMOVAL OF EROSION CONTROL MEASURES,
ANDAPPLICABLE STANDARD NOTES.
I HAVE READ AND WILL COMPLY WITH THE ABOVE AND WILL CONSTRUCT AND MAINTAIN
ESC MEASURES AS NECESSARY TO CONTAIN SEDIMENT ON THE CONSTRUCTION SITE.
OWNER SIGNATURE
DATE RECEIVED:
APPLICANT SIGNATURE
OFFICIAL USE ONLY
DATE APPROVED:
ACCEPTED BY:
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Sec. 17.80.096.020 - SCS Universal Soil Loss Equation
Erosion control measures utilized for any construction site must be designed to achieve a
disturbed area erosion loss of no more than 1 ton per acre per year, based on the Soil
Conservation Service (SCS) Universal Soil Loss Equation. The erosion control measures
outlined in the matrices in section Sec. 17.80.094.090 are designed to achieve the 1 ton
per acre per year goal. If, based on particular site conditions, different types or
combinations of erosion control measures are desired for a construction site, the SCS
Universal Soil Loss Equation or other approved supporting methodology/information
must be used to design the erosion control system, and the design calculations must be
submitted to the appropriate jurisdiction's permit review section with the proposed
erosion control plan.
A brief summary of the use of the Universal Soil Loss Equation follows. For more
detailed information, Agriculture Handbook No. 537, "Predicting Rainfall Erosion
Losses, A Guide to Conservation Planning" (USDA, 12/78) is recommended.
The Universal Soil Loss Equation is:
A = R*K*LS*CV*PR
where:
A is the computed soil loss per unit area, generally in tons per acre per year.
R is the rainfall and runoff factor. (SCS office)
K is the soil erodibility factor and can be found in County SCS Soil Survey manuals, in
the table of Physical and Chemical Properties of Soils.
LS is the slope-length factor. LS can be determined for a site using the known slope
length and percent slope of a site.
CV is the cover and management factor. Use 1.0 for a condition of no ground cover
during construction.
PR is the erosion control practice factor. Use 1.0 for a condition of bare slopes. Use 0.9
to represent a condition of trackwalking up and down slopes.
To use the equation to determine the necessary measures to reduce the soil loss to 1 ton
per acre per year, first determine the loss expected from a cleared site without erosion
control measures (only calculate erosion losses for cleared and disturbed areas). Then
various factors can be manipulated to represent different types and combinations of added
erosion control measures and soil losses recalculated until the desired result is achieved.
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Factors of the equation can be adjusted as follows to represent erosion control measures.
LS can be reduced by reducing slope lengths. This can be achieved by intercepting and
re-routing flows uphill of the disturbed area and thus removing their erosive potential
entirely from the site, or by adding interceptor dikes or swales in the disturbed area to
direct flows from all or part of the area to a sediment trap or pond.
CV can be reduced by using cover practices such as seeding and mulching, erosion
control blankets or plastic sheeting.
PR can be reduced by such measures as grooving or stair stepping steep slopes.
(Note: design criteria for the erosion control methods noted above are given in Section
17.80.094.130)
The best means of reducing total tonnage of erosion from a site as well as reducing the
amount of erosion control measures required is to minimize the ground area that is
cleared and disturbed at any given time.
Sec. 17.80.096.030 - Recommended standard notes for erosion control plans.
Approval of this erosion/sedimentation control (ESC) plan does not constitute an
approval of permanent road or drainage design (e.g. size and location of roads, pipes,
restrictors, channels, retention facilities, utilities, etc.).
The implementation of these ESC plans and the construction, maintenance, replacement,
and upgrading of these ESC facilities is the responsibility of the applicant/contractor until
all construction is completed and approved and vegetation/landscaping is established.
The boundaries of the clearing limits shown on this plan shall be clearly flagged in the
field prior to construction. During the construction period, no disturbance beyond the
flagged clearing limits shall be permitted. The flagging shall be maintained by the
applicant/contractor for the duration of construction.
The ESC facilities shown on this plan must be constructed in conjunction with all
clearing and grading activities, and in such a manner as to insure that sediment and
sediment laden water do not enter the drainage system, roadways, or violate applicable
water standards.
The ESC facilities shown on this plan are the minimum requirements for anticipated site
conditions. During the construction period, these ESC facilities shall be upgraded as
needed for unexpected storm events and to ensure that sediment and sediment-laden
water do not leave the site.
The ESC facilities shall be inspected daily by the applicant/contractor and maintained as
necessary to ensure their continued functioning.
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The ESC facilities on inactive sites shall be inspected and maintained a minimum of once
a month or within the 48 hours following a storm event.
At no time shall more than one foot of sediment be allowed to accumulate within a
trapped catch basin. All catch basins and conveyance lines shall be cleaned prior to
paving. The cleaning operation shall not flush sediment-laden water into the downstream
system.
Stabilized construction entrances shall be installed at the beginning of construction and
maintained for the duration of the project. Additional measures may be required to insure
that all paved areas are kept clean for the duration of the project.
Sec. 17.80.096.040 - Standard Notes for Sediment Fences:
The filter fabric shall be purchased in a continuous roll cut to the length of the barrier to
avoid use of joints. When joints are necessary, filter cloth shall be spliced together only
at a support post, with a minimum 6-inch overlap, and both ends securely fastened to the
post.
The filter fabric fence shall be installed to follow the contours where feasible. The fence
posts shall be spaced a maximum of 8 feet apart and driven securely into the ground a
minimum of 24 inches.
When standard strength filter fabric is used, a wire support fence shall be fastened
securely to the upslope side of the posts using heavy-duty wire staples at least 1 inch
long, tie wire or hog rings. The wire shall extend into the trench a minimum of 4 inches
and shall not extend more than 36 inches above the original ground surface.
The standard strength filter fabric shall be stapled or wired to the fence, and 12 inches of
the fabric shall be extended into the trench. The fabric shall not extend more than 36
inches above the original ground surface. Filter fabric shall not be stapled to existing
trees.
When extra-strength filter fabric and closer post spacing are used, the wire mesh support
fence may be eliminated. In such a case, the filter fabric is stapled or wired directly to
the posts with all other provisions of the above standard note for standard strength filter
fabric applying.
Sediment fences shall be removed when they have served their useful purpose, but not
before the upslope area has been permanently stabilized.
Sediment fences shall be inspected by applicant/contractor immediately after each rainfall
and at least daily during prolonged rainfall. Any required repairs shall be made
immediately.
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Sec. 17.80.096.050 Informational Resources
City of Portland, Bureau of Environmental Services and Washington County, Department
of Land Use and Transportation. "Erosion Control Plans Technical Guidance
Handbook", November 1989; revised April 1990, January 1991.
King County, Washington, Surface Water Management Division, Department of Public
Works. January 1990. "Surface Water Design Manual".
Washington State Department of Transportation, January 1998. "Highway Runoff
Manual".
Washington State Department of Transportation, January 1998. "Hydraulics Manual".
Oregon Administrative Rules. 1989. OAR 340-41-455.
Oregon Department of Transportation, State Highway Division. 1984. "Standard
Specifications for Highway Construction".
Oregon Department of Transportation, State Highway Division. "Hydraulics Manual".
U.S. Department of Agriculture, Science & Education Administration. December 1978.
“Predicting Rainfall Erosion Losses, A Guide to Conservation Planning”. Agriculture
Handbook No. 537.
USDA Soil Conservation Service and Washington County Soil and Water Conservation
District. April 1981. "Urban Conservation Guide for Washington County, Oregon",
parts A and B.
Sec. 17.80.098 - Violations; penalties
1. It is a Class I Civil Infraction for any person to break, damage, destroy, uncover,
deface or tamper with any structure or facility which is a part of the stormwater
runoff and erosion control system.
2. It is a Class I Civil Infraction for any person who is responsible to do so, to fail to
maintain stormwater runoff and/or erosion control facilities and structures as required
by this chapter and by the Guidelines. Each calendar day that a violation occurs
constitutes a separate offense. In addition, the City may institute injunctive,
mandamus, or other appropriate action or proceedings for the enforcement of this
chapter.
115