Member Agencies:
Alameda
Albany
Berkeley
Dublin
Emeryville
Fremont
Hayward
Livermore
Newark
Hydrograph Modification
Management Plan
Part A: General
Provisions For
Hydromodification
Management
Oakland
Piedmont
Pleasanton
Alameda Countywide
Clean Water Program
San Leandro
A Consortium of Local Agencies
Union City
Alameda County
Alameda County
Flood Control and
Water Conservation
District (District)
Zone 7 of the
District
Prepared by:
The Alameda County Public Works Agency
May 15, 2005 Final Submittal
HMP part A: General Provisions
ACCWP
Hydrograph Modification Management Plan
Alameda Countywide Clean Water Program
May 15, 2005
Contents
Part A: General Provisions For Hydromodification Management
1.
INTRODUCTION AND SCOPE ..............................................................................1
1.1 Context, purpose and scope of document .......................................................1
1.2 Regulatory requirement and development of the HMP ................................2
1.3 HMP Overall Organization ..............................................................................3
1.4 Relation of the HMP to other C.3 submittals and guidance .........................4
2.
ORGANIZATION AND USE OF THE HMP .........................................................7
2.1 Approach and terminology...............................................................................7
2.2 Overview of steps for determining Hydromodification Management
requirements for a project................................................................................8
2.3 Approval process .............................................................................................10
2.4 Proposed implementation timeline ................................................................10
2.5 Additional resources .......................................................................................10
3.
AREAS SUBJECT TO HYDROMODIFICATION
MANAGEMENT MEASURES...............................................................................12
3.1 General Assessment Approach ......................................................................12
3.2 ACCWP default susceptibility map...............................................................12
3.3 Modifications or alternatives to the default map .........................................14
4.
DESIGN REQUIREMENTS FOR ENHANCED HYDROMODIFICATION
MANAGEMENT......................................................................................................18
4.1 Flow Duration Control....................................................................................18
4.2 Bay Area Hydrology Model............................................................................20
4.3 Modifications or alternatives to default models ...........................................21
4.4 Alternatives to on-site Flow Duration Control .............................................21
4.5 Coordinating treatment and flood protection with Flow Duration design22
4.6 MEP standard..................................................................................................23
5.
REVIEW AND REPORTING PROCEDURES FOR MUNICIPALITIES........23
5.1 Review Process For Projects Incorporating HM Measures ........................23
5.2 Reporting..........................................................................................................24
6.
SPECIAL HYDROMODIFICATION MANAGEMENT PROVISIONS IN
SPECIFIC JURISDICTIONS OR WATERSHEDS.............................................25
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List of Tables
Table A1-1 Minimum and optional elements of the HMP ............................................................ 5
Table A1-2 Required submittals and other HMP milestones. Submittal dates required in
Provision C.3.f.vii are in boldface. Planned target dates are in italic face. ........................... 6
Table A2-1. Summary of default and alternative approaches for HM procedures...................... 11
Table A2-2. Summary of steps and sources of guidance for project proponents and municipal
staff to address hydromodification management and other stormwater management
requirements.......................................................................................................................... 11
List of Figures
Figure A3-1. Schematic view of portion of draft susceptibility map. ......................................... 16
Figure A3-2. Flow chart for determining whether Group 1 projects require enhanced
hydromodification management. Solid arrows indicate default procedure,
dotted arrows indicate alternative or optional procedure. ...................................... 17
Attachments
A1-1. NPDES Permit Provisions C.3.c, C.3.f and C.3.i
A2-1. Map of special flood control districts in Alameda County
A4-1. List of agencies responsible for well permitting and groundwater management in
Alameda County
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HMP part A: General Provisions
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Table of Contents: Part B: Design Guidance and Technical Reference (in separate volume)
1.
INTRODUCTION AND SCOPE ...................................................................................B-1
1.1 Context, purpose and scope of document.............................................................B-1
1.2 Overview of contents..............................................................................................B-1
2.
RATIONALE FOR HYDROMODIFICATION MANAGEMENT ...........................B-1
2.1 Why is hydromodification a concern?..................................................................B-1
2.2 ACCWP approach to assessment of potential impacts.......................................B-2
2.3 Planned refinements to default assessment for Western Alameda County ......B-3
2.4 Potential refinements to default assessment in Eastern Alameda County........B-5
2.5 Other potential refinements ..................................................................................B-5
3.
SELECTION OF HYDROMODIFICATION MANAGEMENT MEASURES........B-8
3.1 Categories of Hydromodification Management measures .................................B-8
3.2 General principles ..................................................................................................B-9
3.3 Design Guidance...................................................................................................B-10
4.
BAY AREA HYDROLOGY MODEL.........................................................................B-12
4.1 Overview ...............................................................................................................B-12
4.2 Calibrating and adapting the BAHM for Alameda County.............................B-13
4.3 Alternative input parameters or modeling approaches....................................B-15
4.4 Potential future refinements................................................................................B-15
5.
CHANNEL STABILITY ASSESSMENT ...................................................................B-17
5.1 Rationale and Overview ......................................................................................B-17
5.2 Template and generic scope ................................................................................B-18
5.3 Implementation of channel stability assessments ..............................................B-18
5.4 Limited assessment of channel resistance to erosion ........................................B-19
6.
FUTURE MODIFICATIONS AND HMP UPDATES...............................................B-22
6.1 Regional watershed planning and analysis........................................................B-22
6.2 Evaluation .............................................................................................................B-22
7.
REFERENCES ..............................................................................................................B-24
8.
GLOSSARY OF TERMS..............................................................................................B-26
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Table of Contents: Part C: Appendices (provided separately)
1. ASSESSMENT AND PLANNING REFERENCES
1.1
Hydromodification Susceptibility Draft map (EIP Associates, July 7, 2003)*
1.2
Hydromodification Potential Draft map (EIP Associates, July 14, 2003)*
1.3
Proposed Test of the approach for the ACCWP HMM Preliminary Map (Balance
Hydrologics, July 15, 2003)*
1.4
Progress memos of guidance for Channel Stability Assessments (GeoSyntec
Consultants, dates as noted )
• Generic Scope with outline and summary descriptions (August 13, 2004)
• Preliminary Issues and Concerns for Arroyo de La Laguna and tributaries
(August 10, 2004, updated)
• Activities for Defining Watershed and Development Characteristics (Oct. 13, 2004
• Methodology for Developing Existing Condition and Future Condition Hydrologic
Models (October 16, 2004)
• Geomorphic Field Data Requirements (October 11, 2004)
2. GUIDANCE FOR IMPLEMENTATION AND DESIGN
2.1
ACCWP C.3 Stormwater Handbook- Table of Contents and Executive Summary
(February , 2005)
2.2
Design Example Memorandum for Flow Duration Control (GeoSyntec Consultants,
May 25, 2004)
2.3
Project Summary and Draft scope for Bay Area Hydrology Model (May 3, 2005)
2.4
Draft comparison table of runoff management requirements
3. BACKGROUND DOCUMENTS (PREVIOUSLY SUBMITTED)
3.1
Literature Review Part A (February 13, 2004)*
3.2
Literature Review Part B (February 13, 2004)*
3.3
ACCWP HMP Workplan (February 13, 2004)*
*
Previously submitted completed documents in Part C are included in electronic version only
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ACRONYMS AND ABBREVIATIONS
ACCWP
ACFCWCD
ACPWA
ASCE
BASMAA
BMPs
CA
CalTrans
CASQA
Cfs
Alameda Countywide Clean Water Program (Program)
Alameda County Flood Control and Water Conservation District (District)
Alameda County Public Works Agency
American Society of Civil Engineers
Bay Area Stormwater Management Agencies Association
Best Management Practices
California
California Department of Transportation
California Stormwater Quality Association
Cubic Feet per Second
CWA
EPA
GIS
HM
HMP
MYP
NPDES
Program
RWQCB
SAS
Clean Water Act
United States Environmental Protection Agency
Geographic Information Systems
Hydromodification Management
Hydrograph Modification Management Plan
Multi-Year Plan for Monitoring and Assessment
National Pollutant Discharge Elimination System
Alameda Countywide Clean Water Program
California Regional Water Quality Control Board, S. F. Bay Region (Region 2)
Start at the Source (Bay Area Stormwater Management Agencies Association.
1999.)
Stormwater Quality Management Plan, July 2002-June 2008 (Plan)
Watershed Assessment and Monitoring Subcommittee
Watershed Assessment Unit
Zone 7 of ACFCWCD (Zone 7 Water Agency)
SQMP
WAMS
WAU
Zone 7
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1. INTRODUCTION AND SCOPE
1.1
Context, purpose and scope of document
This Hydrograph Modification Management Plan (HMP) was prepared by the Alameda
Countywide Clean Water Program (ACCWP, or "Program") in partial fulfillment of
requirements in its third five-year municipal stormwater discharge permit (Permit, RWQCB,
2003). It describes the proposed approach that will be used by ACCWP’s 17 member agencies
(Permittees) to manage increases in the frequency and duration of flow magnitude and runoff
volume when those increases
•
•
Result from increased impervious surface area at the sites of development and redevelopment
projects meeting the definition of Group 1 as described in provision C.3.c of ACCWP’s
NPDES permit, and
Are likely to cause increases in erosion of creek beds and banks, silt pollution or other
impacts to beneficial uses
Changes in the timing and volume of runoff from a site are known as “hydrograph modification”
or “hydromodification”. ACCWP’s HMP includes a simple (default) map-based approach for
determining which parts of the drainage network are susceptible to hydromodification impacts.
Projects meeting certain eligibility criteria, and from which runoff passes through these
susceptible areas, will be required to incorporate one or more Hydromodification Management
(HM) measures in their design. The HMP also describes procedures for detailed assessment of
the potential impacts of development-related hydrograph changes in a specific section of a
watercourse.
For particular classes of projects, the HMP describes a default design standard which requires
evaluation of runoff under both pre-project and post-project conditions for the full range of
rainfall events simulated through continuous modeling tools being developed to support
implementation of HM by ACCWP and other stormwater programs. HM measures will be
designed to ensure that the volumes and durations of post-project runoff from the site match the
characteristics of pre-project runoff.
The HMP provides the rationale for these requirements and summarizes technical projects and
guidance development currently in progress to support HMP implementation. It also describes
proposed mechanisms for incorporating future technical refinements and alternatives to the
default requirements. Implementation of the provisions in the HMP will begin 90 days after
RWQCB approval of ACCWP’s Final HMP, which is expected to occur no earlier than July
2005.
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1.2
Page A2 of 25
Regulatory requirement and development of the HMP
Provision C.3 of the Permit requires member agencies to continue implementing performance
standards for new development and redevelopment controls that are described in the ACCWP
Stormwater Quality Management Plan (SQMP or Plan), and also specifies enhanced
requirements to address stormwater-generated problems to the maximum extent practicable.
These enhanced requirements are applicable to projects meeting the definition of Group 1 as
described in Provision C.3.c.i. Starting August 15, 2006 additional projects in Group 2
(described in C.3.c.ii; see Attachment A1-1) will also be required to meet requirements for
numerical design of treatment measures, but enhanced requirements for HM will not apply to
these Group 2 projects. Part A2.2 below provides additional description of applicable projects.
Provision C.3.f of the Permit specifies the enhanced requirements for limiting “the increase of
peak stormwater runoff rates”. Provision C.3.f.i states that member agencies will
manage increases in peak runoff flow and increased runoff volume, for all Group 1
Projects where such increased flow and/or volume is likely to cause increased erosion of
creek beds and banks, silt pollutant generation, or other impacts to beneficial uses. Such
management shall be through implementation of a Hydrograph Modification
Management Plan (HMP). The HMP, once approved by the Regional Board, shall be
implemented so that post-project runoff shall not exceed estimated pre-project rates
and/or durations, where the increased stormwater discharge rates and/or durations will
result in increased potential for erosion or other significant adverse impacts to beneficial
uses, attributable to changes in the amount and timing of runoff. The term duration in
this Provision is defined as the period that flows are above a threshold that causes
significant sediment transport and may cause excessive erosion damage to creeks and
streams.
Provision C.3.f.iv lists the minimum elements that must be included in the HMP proposal, and
Provisions C.3.f.iii, C.3.f.v and C.3.f.vi describe possible ways that the HMP may address these
requirements. Table A1-1 summarizes relevant permit language from these provisions and
identifies where these elements are found in the HMP.
Provision C.3.f. v.1 requires the ACCWP member agencies as a group to complete the HMP
according to a schedule of required submittals. These submittals and other milestones of HMP
development are described in Table A1-2. ACCWP submitted its Draft HMP on November 15,
2004. Comments provided by the RWQCB to Program staff have been incorporated in this Final
Submittal HMP document. All regulatory requirements referenced in this document are based
on the third Permit as issued on February 19, 2003. Specific requirements and numbering of
Provisions may be superseded or revised by a reissued Group permit covering ACCWP and
other Bay Area stormwater programs, currently in development.
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This document was prepared by the Clean Water Division of the Alameda County Public Works
Agency, which provides Program management and administration under the terms of the
ACCWP Memorandum of Understanding. Program staff for development of the HMP were:
James Scanlin, Program Manager
Arleen Feng, Project Manager
Oversight and direction for project development was provided by the ACCWP Management
Committee and participants in the ACCWP C.3 Work Group:
Hank Ackerman, Alameda County
Danny Akagi, City of Berkeley
Saied Aminian, City of Dublin
Diamera Bach, Alameda County
Karen Borrmann, Alameda County
Lesley Estes, City of Oakland
Soren Fajeau, City of Newark
Arleen Feng, ACCWP
Nathan French, City of Berkeley
Darren Greenwood, City of Livermore
Loren Jensen, City of Berkeley
Diana Keena, City of Emeryville
Mary Lim, Zone 7 Water Agency
Pamela Lung, City of Livermore
Abbas Masjedi, City of Pleasanton
Melissa Morton, City of Dublin
Nicole Narver, City of Fremont
Dije Ndreu, City of Hayward
Mona Olmsted, Zone 7 Water Agency
Donna Rolle, Alameda County
Jim Scanlin, ACCWP
Peter Schultze-Allen, City of Emeryville
Jeff Tang, Zone 7 Water Agency
Courtney Vasquez, City of Union City
Technical support and assistance was provided by the following ACCWP technical consultants
and sub consultants:
Applied Marine Sciences (Jay Johnson, Paul Salop)
AQUA TERRA Consultants (Douglas Beyerlein, Joe Brascher, Tony Donigian and Robert
Dusenbury)
Balance Hydrologics (Scott Brown, Barry Hecht and Stacey Porter)
EIP Associates (Matthew Huisman and Jim Steele)
GeoSyntec Consultants (Peter Mangarella and Gary Palhegyi)
URS Corporation (Terry Cooke and Alexei Dufour)
1.3
HMP Overall Organization
The ACCWP HMP consists of 3 parts. Part A, General Provisions, provides an overview of
the proposed implementation activities by ACCWP member agencies to incorporate measures
for Hydromodification Management (HM) in development and redevelopment projects. It
describes ACCWP’s general approach for the HMP and summarizes the proposed
implementation provisions in a format that is accessible to municipal staff and a range of
audiences. The HMP focuses on enhanced HM requirements that may be applicable to Group 1
projects, but also includes some proposed implementation activities that may apply to all
projects.
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Part B contains background and further documentation for HMP implementation. Part B
reviews basic concepts and describes guidance that will be provided to member agencies and
project proponents for selection and design of HM measures. It also provides details of the
proposed approach for assessment and design protocols, and description of future work products
that will be developed to support implementation of the HMP.
Part C, Appendices, includes examples and descriptions of additional guidance and reference
materials that ACCWP will provide to municipal staff, project proponents and reviewers. It also
incorporates by reference previous ACCWP submittals related to Provision C.3.f. These
previous submittals will be included in the Appendices to the Final HMP, along with any
specialized ACCWP guidance documents not attached to Part A.
1.4
Relation of the HMP to other C.3 submittals and guidance
Provision C.3 of the Permit includes enhanced provisions for addressing many aspects of new
development and significant redevelopment projects. Many of these enhanced provisions have
been implemented before submittal of the Final HMP, particularly those relating to design of
stormwater treatment measures. ACCWP has developed a C.3 Stormwater Handbook (C.3
Handbook) to guide staff of municipal member agencies in implementing all C.3 permit
provisions relating to new and significant redevelopment. Appendix C2.1 shows the Table of
Contents and Executive Summary for the C.3 Handbook version distributed to member agencies
in February 2005. Section 4 of the C.3 Handbook describes municipal practices for HM, based
on Part A of the HMP. General considerations for HM implementation are also incorporated in
Sections 2 (Changes To The Development Review Process) and 3 (Selecting Stormwater Control
Measures for Development Projects) of the C.3 Handbook. All Handbook sections, including
Section 6 (operation and Maintenance Verification Program) and 7 (Data Collection and
Reporting) will be further updated following RWQCB approval of the Final HMP.
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Table A1-1 Minimum and optional elements of the HMP
Permit
Provision
Description of Element
(italic font = Permit language; plain = proposed ACCWP
approach)
Required elements
Location in
HMP
C.3.f.iv.1
C.3.f.iv
C.3.f.iv.2
Appendices
C3.1, C3.2
C.3.f.iv.3
C.3.f.iv.4
C.3.f.iv.5
A review of pertinent literature;
A protocol to evaluate potential hydrograph change impacts to
downstream watercourses from proposed projects;
a) Default: draft map and technical memo (submitted Feb. 15, 2004
as appendix to Literature Review Part A)
b) Optional: channel stability assessment template
An identification of the rainfall event below which these standards and
management requirements apply, or range of rainfall events to which these
requirements apply;
a) Default: continuous simulation of rainfall time series, minimum
20 year record in Bay Area Hydrology Model
b) Optional: alternative local rainfall record with similar statistical
representation of maximum events
A description of how the Permittees will incorporate these requirements
into their local approval processes, or the equivalent;
Guidance on management practices and measures to address identified
impacts.
Part A3, App.
C1.1
Part B5, C1.4
Part A4, B4
Part B4, C2.1
Parts A2.2,
A5, C2.1
Part B3
Optional elements
C.3.f.iii
C.3.f.vi.2
C.3.f.vi.3
C.3.f.vi.4
C.3.f.vi.5
C.3.f.vi.6
Identify conditions under which some increases in runoff may not have a
potential for increased erosion or other impacts to beneficial uses.
Reduced controls or no controls on peak stormwater runoff discharge
rates and/or durations may be appropriate in those cases, subject to the
conditions in the HMP.
Channel stability assessments, for engineered channels or supported by
analysis of low potential for future increases in impervious surface area in
the watershed
Evaluation of stream form and condition, including slope, discharge,
vegetation, underlying geology, and other information, as appropriate
HMP management measures may include
• measures to minimize impervious surfaces and directly connected
impervious area
• stormwater detention, retention, and infiltration
• measures …to allow expected changes in stream channel cross
sections, stream vegetation, and discharge rates, velocities, and/or
durations without adverse impacts to stream beneficial uses
A mechanism for pre- vs. post-project assessment to determine the
effectiveness of the HMP and to allow amendment of the HMP, as
appropriate
ACCWP HMP Part A_5-15-05.doc
Part B5, C1.4
Part B5, C1.4
Part B3,
Appendix
C3.2
Part B6
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Table A1-2 Required submittals and other HMP milestones. Submittal dates required in
Provision C.3.f.vii are in boldface. Planned target dates are in italic face.
Deliverables
ACCWP submittal
or milestone
Technical Memorandum and HMP areas map for
pilot assessment of areas where hydromodification
management is required (Balance Hydrologics and
EIP Associates 2003)
Discussion of regional concerns with East County
stakeholders, ACFCWCD and Berkeley/Albany area
Detailed workplan and schedule
Literature review, expanding on basic HMP
literature review prepared by SCVURPPP:
Part A - Background & Assessment Strategy
Part B - Planning and Design of Hydromodification
Management Measures
Demonstration of Western Washington Hydrology
Model for sizing on-site detention in Design
Example #2
Scope of work and workplan for developing template
and guidance for channel stability assessment studies
in engineered channels
Draft HMP
RWQCB staff comments on Draft HMP
Report and guidance for applying channel stability
template to refine default HMP map and model
parameters
Review of Draft
by C.3 Work Group
Submitted as appendix to
Literature Review Part A
Program staff meetings
with separate regions
Submitted to RWQCB*
Submitted to RWQCB*
September 2003
Presentations to
C.3 Work Group,
ACFCWCD and Zone 7
Review & approval by C.3
Work Group
June 2004
Bay Area Hydrology Model
-Detailed workplan for adaptation to Bay Area
-Test version of software package
-Release software and guidance
Final HMP
Review drawings and substrate of engineered
channels for verification of default assumptions
Revised version of default map, incorporating
updated channel information and highly developed
watersheds
Additional guidance on selection of HM measures,
coordination with treatment and flood design
Hydromodification management incorporated in
ACCWP C.3 Stormwater Handbook
HMP Implementation for applicable projects:
-Site planning and hydrologic source controls
-Enhanced HM and Flow Duration design
Date
Feb. 15, 2004
SeptemberNovember 2003
Feb. 15, 2004
Feb. 15, 2004
September 2004
Submitted to RWQCB*
Meeting with staff
Review of drafts by
C.3 Work Group,
ACFCWCD and Zone 7
Finalize report
Nov. 15, 2004
January 12, 2005
January 2005 to
May 2005
Approved by C.3 WG
For internal review
Public release
Submit to RWQCB*
Review of report by
C.3WG, ACFCWCD
Final review by C.3WG,
ACFCWCD
December 2004
September 2005
November 2005
May 15, 2005
September 2005
Review & approval by C.3
Work Group
Updated Handbook
September 2005
Upon RWQCB approval
July 2005**
October 2005**
June 2005
September 2005
October 2005
*Review and approval by ACCWP’s C.3 Work Group and Management Committee prior to submittal to RWQCB.
**Based on assumed July 2005 RWQCB approval date for ACCWP HMP.
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2. ORGANIZATION AND USE OF THE HMP
2.1
Approach and terminology
Hydromodification management conceptually incorporates three main processes:
a) Assessment of the need for management measures to prevent or minimize adverse impacts
from hydromodification caused by development and redevelopment
b) Implementation of appropriate management measures
c) Evaluation of management and actions in order to adaptively improve management strategies
Each of these processes can be implemented at multiple scales and levels of complexity.
However, actual physical response of a stream to urbanization is a complex process affected by
many factors including watershed characteristics and past land-use history. Comprehensive
watershed assessment and planning, while important for effective long-term management of
hydromodification impacts, is primarily addressed in other provisions of ACCWP’s Permit; see
ACCWP (2004) for discussion and examples. The main objectives of the Hydromodification
Management provisions in the HMP are:
•
•
Minimize additional hydromodification impacts from new development and significant
redevelopment projects, particularly those in Group 1 and Group 2.
When feasible, contribute to improved understanding of hydromodification processes and
suggest future adaptive improvements to the procedures in the current HMP.
To facilitate implementation of HM for projects on varying scales, ACCWP proposes two levels
of procedures for projects to meet assessment and implementation requirements, through either
• A simple default procedure, generally based on available information which may be less
precise in characterizing individual sites and watersheds
• One or more alternative procedures that are generally more detailed or require more planning
and coordination beyond the project site.
Table A2-1 summarizes the default and alternative options for addressing the two main
requirements discussed in the HMP: assessment of susceptibility and design of HM measures.
Resources being developed to support the default options are:
•
•
Hydromodification Susceptibility Map (Appendix C1.1)
The “Bay Area Hydrology Model” (BAHM), a software tool to automate sizing calculations
for detention facilities that address the Flow Duration Control standard. The BAHM also can
calculate the volume required for volume-based treatment facilities according to the
minimum hydraulic sizing criteria in Permit Provisions C.3.d.i.1 and C.3.d.i.2.
Alternative options are discussed mainly in Part B, along with additional background on
hydromodification and stream response. A Glossary is also included in Part B.
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2.2
Overview of steps for determining Hydromodification Management requirements
for a project
Applicable Projects
The enhanced runoff management requirements of Provision C.3 apply to new and significant
redevelopment projects in Group 1, which is defined in Provision C.3.c.i to include public or
private projects in three general categories:
1. Commercial, industrial, or residential developments that create one acre (43,560 square
feet) or more of impervious surface, including roof area, streets and sidewalks.
2. Streets, roads, highways, and freeways that are under the Permittees’ jurisdiction and that
create one acre (43,560 square feet) or more of new impervious surface.
3. Significant Redevelopment projects, in which addition or replacement of a combined
total 43,560 sq ft or more of impervious surface is made on a previously developed site.
Areas of previously existing impervious surface that are retained as part of the final
project, but not replaced as a result of the project, are not counted in determining whether
a project belongs to Group 1, although C.3 requirements for enhanced treatment and/or
HM may apply to these areas.
Provision C.3.c.i allows certain types of projects to be excluded from the above definitions (see
Attachment A-1-1 for full description). Projects are also excluded from Group 1 if:
•
•
a privately-sponsored development application has been deemed complete by a Permittee by
February 15, 2005 (i.e. a complete application has been submitted and if Planning
Commission approval has been granted, no changes are proposed after that date for which
Planning Commission re-approval is required); or
with respect to public projects, for which funding has been committed and for which
construction began by February 15, 2005.
On August 15, 2006 the definition of Group 1 Projects will change only for application of
numeric sizing of treatment measure, to include Group 2 which “is in all ways the same as the
Group 1 Project definition above, except that the size threshold of impervious area for new and
Significant Redevelopment projects is reduced from one acre (43,560 sq ft) of impervious
surface to 10,000 square feet” (Provision C.3.c.ii). In the future, ACCWP or the RWQCB may
propose changes in the definition of projects to which enhanced HM requirements may apply,
based on findings of a HMP Review Report prepared according to the guidelines in Part B6 of
the HMP.
Part A3 describes the procedure for determining if a project in Group 1 is required to incorporate
enhanced HM requirements in its design. Group 1 projects that do not require enhanced HM,
and all non-Group 1 projects, are still required to include site planning and design measures to
prevent and minimize impacts as continuing implementation of Performance Standards,
described in Provision C.3.a of the Permit.
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Design requirements
For projects subject to enhanced HM requirements, Part A4 describes the procedures for
designing appropriate HM measures. For many projects this will require incorporation of one or
more Flow Duration Control facilities for control of the increased post-project runoff. The
default Point of Compliance at which post-project flow should be compared to pre-project flow
is where runoff leaves the project site. In this default approach, there is no need for detailed
modeling or analysis of watershed drainage conditions beyond the project site. In some areas
where an appropriately designed regional facility is available the permitting agency may allow
the regional facility to serve as a cumulative Point of Compliance for multiple projects as
described in Part A6.
Selection of HM measures should be integrated with selection of stormwater treatment measures.
Individual ACCWP member agencies may have additional requirements specific to local
watersheds for other aspects of runoff management such as flood protection, stream restoration
or maintenance of local storm drain networks. Part A6 is reserved for special provisions or
requirements that may apply to specific jurisdictions or watersheds
Table A2-2 shows the general sequence of steps for integrating HM and other stormwater
requirements into project planning and design. The strategy for designing a project to meet HM
requirements has three main aspects:
•
•
•
Site planning to minimize runoff effects by minimizing impervious surfaces and their
connection to the runoff conveyance system, and preserving natural drainage patterns.
Many techniques and design features that are distributed throughout the site to reduce
hydrograph change, or “hydrologic source controls”, can also serve as source controls to
reduce stormwater pollution from the site.
Engineering design of runoff facilities to mitigate potential flow increases from the
remaining runoff before it is discharged from the site, when these increases would otherwise
contribute to one or more of three potential impact types: a) flooding, b) reduced water
quality and c) impacts to streams from hydromodification.
Incorporating administrative and operational requirements including operational runoff
management measures, operations and maintenance agreements to ensure continuing
performance of HM measures as intended by the designer.
Depending on the design strategy and site constraints, it may be appropriate or necessary for onsite measures to consider flood peak issues as well as manage ordinary HM impacts. In some
cases Step 7 may involve coordinating multiple design requirements for individual detention
facilities, which is discussed further in Part A4. Attachment A2-1 shows the jurisdictions of the
ACFCWCD and Zone 7 to assist coordination of flood protection requirements. Parts B and C
of the HMP include additional guidance on selection and design of HM measures. The project
proponent is responsible for confirming all physical conditions and jurisdictional requirements
that may be applicable to the project site.
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2.3
Approval process
ACCWP member agencies will incorporate the provisions of the final HMP in their process for
reviewing and approving new development and redevelopment projects. Incorporation of HM in
this process will be added to and coordinated with the changes being implemented to address
other C.3 provisions, which are described in Section 2 of the ACCWP C3 Stormwater
Handbook. Part A5 provides guidelines for submittal and review of documents to demonstrate a
project’s incorporation of enhanced HM measures.
Requirements for ongoing verification of operations and maintenance of treatment measures as
described in Provision C. 3.e will also apply to detention or storage facilities designed to address
the Flow Duration Control standard, and these facilities will be identified as Flow Control
facilities in databases and records maintained by the member agencies.
2.4
Proposed implementation timeline
C.3.f states that implementation of the HMP will begin after RWQCB approval of the final
HMP, which will be submitted by May 15, 2005. ACCWP proposes a 2-stage implementation
process:
•
•
Upon receiving RWQCB approval of the final HMP, consideration of the site planning and
hydrologic source control aspects of HM will be incorporated in review of applicable
projects. For these projects, municipal record-keeping that is required by other C.3
provisions for Group 1 projects will also identify those source control measures implemented
for treatment purposes which also are intended to contribute to reduction of post-project
flows.
Implementation of the enhanced HM requirements for Flow Duration Control using the
default map and BAHM will be implemented for Group 1 projects that are still in review (i.e.
not “deemed complete” starting 90 days after the RWQCB approves the final HMP. This is
based on the estimated interval between expected RWQCB approval date and the projected
November 2005 date for completing development and testing of the BAHM software and
preparation of guidance for users of the software, as outlined in Part B4. Member agencies
have discretion to require that FDC considerations be included on significant projects before
that date, using the methods described in the Design Example in Appendix 2.2 and other
guidance to be provided in the final HMP.
2.5
Additional resources
Parts B and C provide additional technical references and updates on continuing development of
guidance materials.
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Table A2-1. Summary of default and alternative approaches for HM procedures
Procedure
Assess potential to cause
hydromodification impacts
Implement HM measures
Default method
Default susceptibility
map
Design on-site measures
with Bay Area
Hydrology Model
Optional or alternative method
Updated default map or Channel
stability assessment, where applicable
Regional flow duration control;
in-channel measures to allow
increased flows.
Table A2-2. Summary of steps and sources of guidance for project proponents and municipal
staff to address hydromodification management and other stormwater management requirements.
Step
Information needs
Guidance
(existing or planned)
1. Gather site information
2. Preliminary development
layout
3. Determine applicable
requirements
Location, existing site conditions
Local planning and zoning
restrictions, sensitive areas, etc.
Checklist for proponents
Site design examples; other
guidelines
Impervious area estimate to
determine if project is in Group 1;
local drainage area, special
requirements or exemptions
specific to local jurisdiction
Site topography and existing
drainage pattern, general soil type,
preliminary development plan
C3 Handbook; Impervious
surface form; Default
Susceptibility Map;
Checklist for project
proponents, other guidance
Start at the Source and site
design examples; IPM/
landscaping guidance and
checklists
Preliminary site plan, special
considerations affecting design
(soils, slope, water table height,
local flooding or safety issue, etc.)
CASQA Handbooks; C.3
Handbook; Design examples
Rainfall records adjusted for site,
landcover and soils data (packaged
with Bay Area Hydrological
Model design tool
Design storm event derived from
rainfall records (data included in
BAHM)
Confirm Flow Duration curve
matching for pre-and post-project
conditions (incorporated in
BAHM)
CASQA Handbooks; Bay
Area Hydrology Model,
BAHM user manual, other
guidance
Guidance by Flood Control
agency or other appropriate
reference
BAHM user manual; Design
examples; guidance by
ACFCWCD or Zone 7
Locations and types of structural
treatment or HM measures
C.3. Handbook; model
Operations & Maintenance
agreement
4. Incorporate site design
and hydrologic source
control measures in
preliminary site plan
5. Select preliminary layout
of measures/facilities for
flood control, treatment,
and/or flow control
6. Test initial design of
treatment measures and flow
control facilities
7. Identify detention volume
for flood control
8. Check performance of
system to meet all design
requirements; adjust design
and details as needed
9. Check operations and
maintenance requirements
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3. AREAS SUBJECT TO HYDROMODIFICATION MANAGEMENT MEASURES
3.1
General Assessment Approach
The potential for a project to cause increased hydromodification impacts depends on both the
degree of change in runoff patterns at the project site and on the susceptibility of the downstream
drainage network to negative impact from hydrograph change. ACCWP’s proposed default
approach to assessing the need for HM measures uses a susceptibility map developed from
available GIS information. Balance Hydrologics (2003) developed a protocol for using specific
attributes of channels and topography to predict portions of the drainage network to impacts
from future hydromodification. The GIS data is supplemented by “tags” for known areas of bank
instability, sensitive habitat or special projects where localized concerns may override default
classification of a watercourse. This protocol was used to develop the pilot default susceptibility
map in Appendix C1.1. A portion of the proposed susceptibility map is shown schematically in
Figure A3-1. The inventory of GIS data developed through this pilot map also will support the
Alameda County version of the Bay Area Hydrology Model described in Section 4.2.
The flow chart in Figure A3-2 describes the process for determining where enhanced HM
measures are required for Group 1 projects. The solid arrows indicate the possible pathways in
the default procedure based primarily on the default susceptibility map. Alternative or optional
procedures are indicated by dotted arrows. Use of these alternatives by project proponents is
subject to the discretion of the permitting agency based on local requirements and conditions in
the watershed, using guidelines to be provided in Part B of the final HMP.
3.2
ACCWP default susceptibility map
The default susceptibility map identifies areas of Alameda County where watercourses may be
susceptible to hydromodification impacts. Group 1 projects located in these areas must meet the
enhanced HM provisions in Part A4 unless the permitting agency accepts an alternative
assessment that meets the guidelines of the HMP. Projects located in these areas should be
encouraged to include hydrologic source control measures for hydromodification management if
they are likely to cause hydrograph changes, even if not required to meet the enhanced HM
requirements for Group 1 projects.
Increases in the proportion of directly connected impervious area on a project site can also
produce adverse hydromodification impacts even if overall project imperviousness does not
increase, or increases less than the minimum amount that would qualify a project as belonging to
Group 1. For projects with substantial increase in the area of directly connected impervious
surface, evaluation of expected changes in post-project flow may be required.
Details of the rationale and methods for classifications in the default map were developed for the
pilot mapping project (see Appendix 1.1, 1.2 and 1.3). Features of the default map include:
•
Hilly areas, where high slopes occur on or in the vicinity of the project, require HM measures
for all applicable projects, because of high potential for increased erosion due to concentrated
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runoff and increased velocity. These impacts may occur to parts of the existing drainage
network that are not represented by perennial creek channels. However, enhanced HM
provisions do not apply to urban areas in the hills that are drained completely through
existing underground storm drain systems.
•
Low-lying areas near the historical margin of San Francisco Bay are exempt from HM
requirements, where these are generally low-gradient depositional areas with low risk of
erosion from hydromodification due to urbanization. Since these areas are generally at the
base of watersheds with extensive existing impervious area, historical channel alterations
have produced straightened and widened channels that have been designed to carry increased
flows, frequently with bed and bank surfaces continuously hardened with concrete or
sackcrete,. In the region of tidal influence, short segments of engineered earthen channels
(less than 500 feet long with trapezoidal cross-section) can also assumed exempt from
erosional susceptibility if located downstream of a concrete channel of similar or greater
length and comparable cross-sectional dimensions. the occurrence and status of such
segments will be reviewed with channel management agencies as part of GIS data
development for the map. Exempting areas draining to these channels may also address
flood prevention concerns that require rapid drainage of local runoff to minimize the
potential for increasing flood peaks.
•
The need for HM measures in intermediate areas on the Bay Plain (western Alameda
County) depends on two factors: the characteristics of the channels carrying runoff from the
project site, and the existing level of urbanization. In the watershed. If all downstream
watercourses are fully hardened (for example, in enclosed culverts or with channel bed and
banks continuously concrete-lined) as far as the boundary of the near-tidal exemption zone,
on-site HM measures are not required.
Provision C.3.f.ii states that enhanced HM requirements do not apply to “infill projects in
highly developed watersheds, where the potential for single-project and/or cumulative
impacts is minimal.” For purposes of this determination, highly developed watersheds are
defined as those which have at least 90% or the total area covered with existing urban land
uses and an average existing imperviousness of at least 65% as determined using the
ACCWP landcover dataset developed from remote sensing satellite imagery (described in
Buchan (2003). This GIS dataset for the initial default map lacks the resolution to
automatically classify most Alameda County watersheds in relation to the 65% threshold for
imperviousness. Review of detailed watershed studies by ACFCWCD or other agencies
may provide data for mapping the imperviousnees of some highly developed watersheds
before the start of HMP implementation; the results of future studies may also be
incorporated in map refinements as described in Part A3.3 below..
In some cases downstream watercourses have been extensively channelized and engineered
to convey flood peaks even though not fully hardened; these channels may have potential to
be
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stable under projected flow increases. More detailed analyses may also be made to of
watershed imperviousness. In these cases, special supplemental assessments may provide a
basis for modifying the HM requirement either on a project specific basis, for a watershed or
for groups of watersheds with similar characteristics; these are described in Part A3.3.
•
The default classification of channel type or characterization as a highly developed
watershed may be overridden by additional mapped “tags” based on special information
about a watercourse’s condition or potential beneficial uses that is not captured in the
available GIS data. Examples may include channel reaches with localized areas of bank
instability, fish passage or other habitat values, or sites of existing or planned restoration
projects.
•
All areas in eastern Alameda County require default application of HM measures based on
known stability problems in the Arroyo de la Laguna channel and the potential or existing
habitat concerns in the Alameda Creek main stem. These watersheds together form a large,
complex system and further assessment and planning activities may be suggested to develop
improved recommendations for HM in this region of the County (see Section B6). The
default susceptibility map shows the extension of these watersheds beyond the County
boundaries for illustration purposes only.
3.3
Modifications or alternatives to the default map
Alternative assessments of the susceptibility classification for particular watersheds or portions
of watersheds may be based on three categories of available information described below, with
associated procedures for documentation and reporting:
a. Refinements to the default map based on more accurate and updated information:
• Field-verified mapping of the extent of hardened channel that is assumed resistant to
hydromodification impacts.
• Confirmation of the low susceptibility to hydromodification impacts of low-gradient earthen
engineered channels in near-tidal areas.
• Improved calculations of overall watershed or catchment impervious area, based on analysis
of as-built plans or high-resolution photographs.
The improved data may be submitted by project proponents on a project-specific basis, or
compiled and analyzed by the Program or member agencies. Proposed modifications to the
default map based on these data will be documented for review by the Program and the
District or Zone 7 as applicable; if accepted as a basis for modifying the default map, the
modifications will be identified in ACCWP annual reports
b. Recommendations of special studies of channel stability and resistance to erosion, conducted
according to guidelines and template described in Section B5. Special studies should also
incorporate analysis of the potential for future increases in hydromodification in specific
watersheds, by reviewing land use and planning data to compare existing and maximum
future development conditions (including land cover type, and percentage and distribution of
impervious area).
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Special channel studies should be documented in a report with content and format equivalent
to the template, including any recommendations or justification for area-specific
modifications to the default map. Copies of the report should be provided to the permitting
member agency, Program staff and the District or Zone 7 where applicable. Key findings or
recommendations that have been accepted from special channel studies will be summarized
in the ACCWP annual reports, and also in the HMP Review Report described in Part B,
Section 6.2
c. The existence of an approved Equivalent Limitation Protocol based on implementation of
regional or watershed-based management measures that provide equivalent protection
against hydromodification impacts for the subject area (see section 4.4 below).
Documentation for an ELP would typically include at least one of the following:
• Special channel stability studies evaluating how existing or proposed channel restoration
activities will increase resistance of the channel to hydromodification impacts from increases
in runoff due to anticipated development in the watershed.
• Design plans and calculations for proposed regional detention facilities, with description of
how facility performance is expected to meet or exceed Flow Duration design criteria for the
area drained.
A proposed Equivalent Limitation Protocol must be submitted for Water Board approval as a
modification to the HMP as described in Part B section 6.
At the discretion of the permitting agency, proponents of individual projects may be allowed to
submit requests for modification of the default requirement supported by one or more of the
above types of documentation. Project proponents are in all cases responsible for verifying the
location and condition of relevant downstream watercourses for proposed projects.
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Figure A3-1. Schematic view of portion of hydromodification susceptibility map. Projects in
the gray (near-tidal) zone would be exempt from Hydromodification Management (HM)
provisions. Enhanced HM would be required for projects in the pink zone (in vicinity of high
slopes) and potentially required for projects in the white (Bay Plain) zone depending on
characteristics of downstream channels and watershed development.
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Is the project in
Group 1
Page A17 of 25
Is the project
in a low-gradient
Bayside zone? (dark gray
on default map)
OR in a highly developed
watershed?
(not shown)
Yes
No
Does the project
increase site impervious
area, or direct connection
of existing impervious
area?
No
No
Yes
ƒ ACCWP provisions for
enhanced hydromodification
management are not
required for the project.
ƒ Promote the incorporation of
site design and hydrologic
source control measures to
minimize runoff and
hydrograph change
ƒ Runoff detention for flood
protection may also be
required
ƒ Local provisions for stream
protection may also apply
Yes
Are watercourses
downstream from project fully
resistant? (green lines
on default map)
No
Is project in region
of general high slope?
(pink on default
map)
Yes
No
Yes
Proceed with Flow
Duration design using
site-specific
modifications
(off-site alternatives
may reduce need for
on-site measures)
Yes
•
The project must incorporate enhanced
hydromodification management design.
•
Permitting agency may allow special assessments, which may
take into account available regional or in-stream alternatives
The type of special
assessment that is allowed
may vary depending on
location, project type and
other site factors
Assessment indicates no
future hydromodification
impacts in watershed?
No, or not
applicable
Yes
Special Assessment
justifies modification
of default design
criteria?
No, or not
applicable
Proceed with
default Flow
Duration design of
on-site measures
Figure A3-2. Flow chart for determining whether Group 1 projects require enhanced
hydromodification management. Solid arrows indicate default procedure; dotted arrows indicate
alternative or optional procedure.
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4. DESIGN REQUIREMENTS FOR ENHANCED HYDROMODIFICATION
MANAGEMENT
4.1
Flow Duration Control
Measures to mitigate hydromodification impacts can be grouped into three types:
•
•
•
Site planning and hydrologic source control measures which are generally distributed
throughout a project site
End-of-pipe measures that manage excess runoff from the site after hydrologic source control
measures are applied.
In-stream or restorative measures that modify susceptible watercourses to withstand
projected increases in runoff without increasing impact to beneficial uses.
Part B3 discusses specific measures in more detail. For Group 1 projects the default requirement
is to use a combination of hydrologic source control and on-site end-of-pipe measures to meet a
Flow Duration standard for post-project runoff. Projects that are not in Group 1 should
incorporate site planning and hydrologic source control measures, but are not required to meet
the Flow Duration standard.
Flow Duration Control (FDC) differs from traditional “design storm” approaches to design of
detention facilities for flood control or water quality treatment. Instead of specifying static
holding times for one or a few discrete events, the Flow Duration standard is to manage runoff
discharge over the full range of runoff flow levels predicted through continuous hydrologic
modeling based on a long-term precipitation record.
Background concepts
Stormwater management facilities are often designed to maintain peak flow rates at their predevelopment levels to prevent increases in the frequency of flooding due to new development.
However facilities that control peak flow rates usually allow the cumulative duration of elevated
flows to increase over time, which can still cause increased erosion of the downstream system.
Protecting these stream systems from hydromodification requires management measures that
control the duration of a range of geomorphically significant flows – those capable of moving
sediment and eroding stream bank material. These processes are most affected by small, frequent
storms that become much more frequent when development occurs in the watershed. In many
parts of North America, observations that peak flow controls alone are not adequate for erosion
control have initiated revisions to stormwater requirements to incorporate a flow duration
standard, where pre- and post-development flow duration curves must be matched according to
specified criteria.
Flow Duration Control requires that the increase in surface runoff created by the installation of
impervious surfaces be retained on-site with gradual discharge either to groundwater through
infiltration, reduction by evapotranspiration, and/or discharge to the downstream watercourse at
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a level less than the critical flow for bed and bank mobility of the stream. Critical flow or Qc is
the lower threshold of in-stream flows that contribute to sediment erosion and sediment transport
or effective work; the duration of channel flows below Qc may be increased indefinitely without
significant contribution to hydromodification impacts. GeoSyntec et. al. (2003) incorporated
considerations of effective work in a study to support HMP development in Santa Clara County.
Based on the initial Silver-Thompson Creek study and subsequent testing with two additional
watersheds, the maximum low-flow discharge leaving a project site is defined as Qcp in the
SCVURPPP HMP Report (SCVURPPP 2005). This report recommends that a default sitespecific Qcp be estimated as 10% of the pre-project 2 year flow, as identified through the flowfrequency analysis and modeling performed as part of Flow Duration design.
Application of Flow Duration Control to Project Areas
The flow duration approach involves a continuous model that applies a time series of at least 20
years of rainfall records through a watershed area or project site to generate a simulated
stormwater runoff record based on two sets of inputs, one representing future development and
the other representing pre-project conditions. The 20 year precipitation record is necessary to
capture the range of runoff events that are cumulatively responsible for most of the erosion and
sediment transport in the watershed, primarily those that would recur at intervals of 10 years or
less in the pre-project condition. This approach looks at the cumulative frequency distribution of
flow durations and sizes under pre-project conditions, with a design objective to preserve that
distribution under post-project flows through a combination of site design, infiltration and
detention.
Typically the increase in surface runoff volume created between the pre-project and post-project
development conditions is routed through a flow duration control basin or other structure that
diverts and detains a certain portion of the increased runoff. The flow duration basin is designed
conceptually to have two pools: a low flow pool and a high flow pool. The low flow pool is
designed to capture and retain small to moderate size storms, the initial portions of larger storms,
and dry weather flows. The high flow pool is designed to store and release higher flows to
maintain, to the extent possible, the pre-project runoff durations. In practice the two pools are
usually integrated into a single detention basin, tank or vault that works as a unit with a
specialized outlet structure.
Specialized discharge control structures are required to provide variable discharge from the
pools depending on the volume (height) of detained water. Typically these may consist of
orifices of differing sizes and positions, and/or notched weirs, which may be nested. A special
low-flow orifice from the bottom allows continuous discharge below the project critical flow rate
Qcp. Appendix C2.2 provides a design example for a flow duration facility. Adequate
maintenance of the low-flow orifice or notch is critical to proper performance and Flow Duration
facilities are subject to requirements for reporting and Operations and Maintenance Verification
similar to those for numerically sized treatment facilities. The outlet may be in a protective
enclosure to reduce risk of clogging. Further guidance is discussed in Part B3 and will be
developed in conjunction with final guidance for the BAHM.
Separate flow duration and infiltration basins may be needed for water quality reasons or a bypass pipeline could carry the excess runoff to a safe discharge location, if feasible. Combining
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flow duration and infiltration into a single facility reduces the overall land requirements for
stormwater management. The flow duration basin can also serve as a water quality treatment
facility and can be designed to treat dry and wet weather flows using a combination of extended
detention and natural treatment processes. Dry weather “nuisance flows” will also be captured
and infiltrated in the basin.
4.2
Bay Area Hydrology Model
To facilitate the simulation modeling aspect of FDC for project proponents and their engineers,
ACCWP is working with other Bay Area stormwater programs to develop a Bay Area Hydrology
Model software interface which will be based on Version 2.1 or later of the Western Washington
Hydrology Model (WWHM) developed by AQUA TERRA Consultants for the State of
Washington Department of Ecology (WDOE). The WWHM was specifically developed to help
engineers design facilities to meet a Flow Duration Control standard for development projects in
19 counties covered by the Western Washington Stormwater Manual (WDOE 2003). The present
version incorporates comments and suggestions on Version 1 gathered through WDOEsponsored workshops for engineers and public solicitation. Version 3 of the WWHM is in the
planning stages; upgraded capabilities can be incorporated in future versions of the BAHM.
The BAHM will be provided for downloading from the Web along with county-specific data,
similar to the distribution mechanism for Western Washington. The proposed BAHM package
will include:
•
•
•
•
•
•
Databases to automatically identify default rainfall conditions for a project location selected
within the County boundary
An interface for developing a schematic drainage model of the project site, with forms for
entering areas of land use or impervious surface for multiple subbasins.
Continuous simulation of pre-project and post-project runoff from the site using actual longterm rainfall records
A design module for sizing a FDC detention facility and discharge structure to meet the Flow
Duration standard for matching post-project and pre-project duration-frequency curves.
Options to size facilities for volume-based treatment, and incorporate runoff reductions
attributable to some common hydrologic source control measures.
Standardized output report files that can be saved in Word format, and include all
information about data inputs, model runs, facility design, and summary of the hydrological
statistics showing the compliance of post-project flow duration curves with the Flow
Duration standard.
The Flow Duration standard for the WWHM is based on sequential evaluation of each flowduration bin in the flow-duration histogram; decision rules for determining compliance allow
post-project duration for a limited number of bins to reach 110% of pre-project durations, since
other flow bins will have decreased durations. Current WWHM compliance restricts post-project
flows between Qcp and the pre-project 2-year peak flow to zero (0) exceedance of the preproject flow-duration curve, with up to 10% exceedance allowed at individual points along the
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curve above the pre-project 2-year peak flow. Evaluation of WWHM assumptions and default
parameters and recommendations for Bay Area defaults will be incorporated in the workplan for
BAHM adaptation, described in Part B4.
4.3
Modifications or alternatives to default models
The BAHM incorporates an HSPF model engine (see Appendix C2.3). At the discretion of the
permitting agency, project proponents may propose other models with similar ability to simulate
hydrologic parameters for a mixture of pervious and impervious surfaces. GeoSyntec used the
US Army Corps of Engineers Hydrologic Engineering Center-Hydrologic Modeling System
(HEC-HMS) in the demonstration examples prepared for SCVURPPP and ACCWP, and some
jurisdictions in southern California have used SWMM.
The BAHM will allow users to use alternative values to the defaults set for model parameters
such as runoff coefficients for land uses, permeability of various soil types, or critical low-flow
Qcp. A User’s manual will be developed for the BAHM that will identify the basis for default
equations and parameters. Use of non-default parameters will be identified in the BAHM output
report and may require supporting documentation such as:
•
•
•
Field data that refines the default datasets in the BAHM, or identifies site specific conditions.
Special studies such as channel stability assessments, refined rainfall analyses, etc.
Comprehensive watershed modeling that may be conducted by other agencies
For large projects in which the drainage network cannot be adequately characterized in the grid
size provided by the BAHM schematic interface, the project area may need to be broken into
subcatchments and each of these modeled separately for facility design and Flow Duration
compliance. The limiting project size will depend partly on whether the BAHM is based on the
WWHM Version 2.1 or the Version 3 currently in development. BAHM guidance will provide
guidelines for determining when this approach is needed.
As an alternative, the permitting agency has the discretion to accept continuous simulation
modeling of runoff from the subject watershed under pre-project baseline conditions, as well as
scenarios for post-project conditions or for cumulative added development. Evaluation of
hydromodification potential and the design of HM measures should use channel template outline
with added the effective work analysis as developed in GeoSyntec et. al. (2003).
4.4
Alternatives to on-site Flow Duration Control
Alternatives may be available that can fulfill the HM requirements for one or more projects.
These can include:
•
Existing regional detention facilities planned or in progress, which are located between the
project site and any downstream susceptible reaches or watercourses. The project design
should include analysis of runoff for the maximum future development condition in the
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•
Page A22 of 25
catchment area that includes the project, with Flow Duration Control of discharge to
downstream watercourses.
In-stream modifications that stabilize or restore the channel to enable it to handle increased
flows from recent and future development. Where possible, less intrusive, environmentally
sensitive techniques such as revegetation or bioengineering stabilization should be used.
As noted in Part B3, permitting processes and timelines for regional facilities and in-stream
modifications are different from those for ordinary development and redevelopment projects.
These measures are best applied in watersheds where streams that have already been
significantly impacted by urbanization. On-site default measures are preferred for projects that
drain to streams that largely retain natural characteristics and physical processes.
4.5
Coordinating treatment and flood protection with Flow Duration design
The table in Appendix C2.4 summarizes the concerns and jurisdictions involved in different
aspects of runoff management. The Program and member agencies will develop guidance
following this outline, but the project proponent is responsible for coordinating multiple
requirements by different agencies. Considerations include:
•
•
•
•
Hydrologic source control or stormwater treatment measures on a project site can
significantly reduce the detention volume (and land area) required for a downstream Flow
Duration Control facility for HM, especially when the project site in located on sandy soils
with high pre-project infiltration rates.
Flow Duration Control facilities located upstream of stand-alone treatment measures can
significantly reduce the treatment volume required for measures that are sized by volumebased criteria.
Detention requirements for volume-based treatment can be partially met by a facility
designed to meet Flow Duration requirements. The BAHM may provide added design tools
to calculate average residence times for estimation of treatment potential. When the facility
is a wet pond, additional treatment volume can be designed as “dead storage” below the
bottom depth specified for the Flow Control pond volume.
Flood detention requirements are usually based on a design storm event approach which
differs from the continuous simulation approach used in design for Flow Duration Control.
In general the system must be checked separately for flood and Flow Duration requirements.
Flood control requirements may apply regardless of whether a project is classified as
belonging to Group 1 for treatment or HM purposes.
Additional design considerations
ACCWP prepared a Vector Control Plan (ACCWP, 2004a) to address the requirement of Permit
Provision C.3.e.iv. This plan includes design guidance for treatment measures to prevent the
production of potential disease vectors, particularly mosquitoes. These guidelines recommend
that extended detention basins, which include most Flow Duration facilities, be designed to drain
water completely within 72 hours after a storm event. If simulation modeling indicates that
drawdown times will frequently exceed 72 hours, the designer should explore alternative design
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configurations to increase infiltration or otherwise reduce typcial detention times. Consulation
with state or local vector control agencies should also be considered. Additional design
guidelines from the Vector Control Plan are summarized in Attachment 3-6 of the C.3
Handbook.
Provision C.3.i describes limitations on the use of infiltration measures for stormwater treatment
in order to protect groundwater (Attachment A1-1), which may constrain the design and
selection of drainage systems and measures for both treatment and HM. The permit contains
minimum provisions for groundwater protection and project proponents should verify any
additional requirements or recommendations with local groundwater management agencies (see
Attachment A4-1).
4.6
MEP standard
Where on-site enhanced HM measures are not practicable for achieving Flow Duration Control
for an applicable project, and regional or in-stream alternatives are not available, the project
shall comply with HMP requirements through the use of appropriate site design, source control
and treatment control measures having flow control benefits to the maximum extent practicable.
As a guideline, a permitting agency may determine that full implementation of enhanced HM
may be considered impracticable if a project proponent demonstrates that the combined
construction cost of both the required numerically sized treatment measures and Flow Control
measures exceeds 2% of the project construction cost, excluding land costs. For this purpose,
costs of control measures should only include direct costs of providing the treatment or flow
control function and not other normal site enhancements such as landscaping, or grading that
would be required for other purposes. Costs for meeting flood protection requirements may only
be included to the extent that the facilities are being used to meet HM requirements.
5. REVIEW AND REPORTING PROCEDURES FOR MUNICIPALITIES
5.1
Review Process For Projects Incorporating HM Measures
ACCWP member agencies will incorporate the provisions of the final HMP in their processes for
review and approval of new development and redevelopment projects. Guidance for municipal
staff will include a supplement to Attachment 2-1 of the C.3 Handbook, which summarizes
major changes required by other C.3 provisions to the review process for private projects. The
table supplement will describe the additional HM requirements and list model documents and
other implementation tools for each stage of the review process, starting with a pre-submittal
meeting and continuing through plan check and project approval to post-construction operation
and maintenance. Checklists for project proponents and reviewers will also be modified to
incorporate HM requirements.
Supporting guidance for the BAHM will include checklists of submittal requirements and for
review of output reports. Municipal agencies may be assisted in review of by other agencies or
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outside parties through agreements or service contracts. ACCWP and other stormwater
programs may develop additional design examples for Flow Duration facilities.
Member agencies may accept alternative assessment or design procedures for Group 1 projects
to meet the enhanced HM requirements. Pre-submittal meetings are recommended for project
proponents that intend to submit applications using alternative procedures. Consultation with
other agencies such as ACFCWCD or Zone 7 is also encouraged. Guidance for the channel
stability assessment template will include guidelines for checking data inputs. ACCWP may
also assist in general review of format and inputs of special studies.
5.2
Reporting.
Group 1 projects for new development and redevelopment which include facilities designed for
Flow Duration Control will be subject to requirements for verifying Operation and Maintenance
similar to those described in the C3 Handbook for treatment measures. These requirements will
be included in the Operation and Maintenance Agreement associated with the conditions of
approval for applicable projects. Section 4 of the C3 Stormwater Handbook will be updated to
provide guidelines for reporting and tracking these projects. Information collected for applicable
projects will include:
•
•
•
•
•
Project information also required under permit provision C.3.n, such as location, type and
impervious area.
Drainage area or areas from which runoff is managed by enhanced HM measures.
The number, size and type of measures for site planning and enhanced HM that were
incorporated in the project design.
Whether default or alternative approaches were used to support design of HM measures, and
the extent of any modifications to the default map.
Any special local watershed management requirements that were applied to the project,
including alternative regional facilities or in-stream measures.
The Program will coordinate compilation of this information for inclusion in a programmatic
review of HMP implementation to be included in ACCWP Annual Reports. The first review will
begin two to three years after the HMP implementation date, and reviews will be repeated at
intervals of two to three years thereafter. These reviews will also identify key findings of any
special studies that were conducted to provide alternative analyses of watersheds where HM
measures have been implemented. The Program may also incorporate this information in more
detailed HMP review reports as described in Part B6 and subject to authorization by the ACCWP
Management Committee.
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6. SPECIAL HYDROMODIFICATION MANAGEMENT PROVISIONS IN SPECIFIC
JURISDICTIONS OR WATERSHEDS
Individual member agencies may have special policies or ordinances for stream protection
applicable in all or part of their jurisdictions. Project proponents are responsible for working
with municipal staff to identify any special local provisions may encourage or affect specific
forms of HM implementation Examples of such provisions include:
• Watershed-based land-use planning measures, such as stream buffers, which may be
incorporated in local General Plans, zoning codes or watercourse ordinances.
• Special permitting provisions for project design and review of projects on streamside
properties, such as those required by Oakland’s Creek Protection Ordinance.
• Specific plans for regional HM measures or in-stream restoration projects, such as the
restoration plan for Lower Codornices Creek.
• Any Equivalent Limitation Protocols that may be proposed in accordance with Provision
C.3.f.vii as additional alternatives to the default HM requirements in specific jurisdictions or
watersheds. The process for submitting proposals for Equivalent Limitation Protocols will
be similar to that described in Part B6 for HMP modifications.
ACCWP HMP Part A_5-15-05.doc
Alameda Countywide Clean Water Program
Hydrograph Modification Management Plan
Part A: General Provisions For Hydromodification Management
Attachments
Attachment A1-1
Excerpts from NPDES PERMIT NO. CAS0029831, Provisions C.3.c, C.3.f and C.3.i
ORDER R2-2003-0021, issued February 19, 2003
c. Applicable Projects – New and Redevelopment Project Categories
New development and significant redevelopment projects that are subject to Provision C.3
are grouped into two categories based on project size. While all projects regardless of size
should consider incorporating appropriate source control and site design measures that
minimize stormwater pollutant discharges to the maximum extent practicable, new and
redevelopment projects that do not fall into Group 1 or Group 2 are not subject to the
requirements of Provision C.3. Provision C.3 shall also not apply to projects for which a
privately-sponsored development application has been deemed complete by a Permittee or,
with respect to public projects, for which funding has been committed and for which
construction is scheduled by February 15, 2005.
i.
Group 1 Projects
Permittees shall require Group 1 Projects to implement appropriate source control and
site design measures and to design and implement stormwater treatment measures, to
reduce the discharge of stormwater pollutants to the maximum extent practicable.
Implementation of this requirement shall begin February 15, 2005. Group 1 Projects
consist of all public and private projects in the following categories:
1.
Commercial, industrial, or residential developments that create one acre (43,560
square feet) or more of impervious surface, including roof area, streets and
sidewalks. This category includes any development of any type on public or
private land, which falls under the planning and building authority of the
Permittees, where one acre or more of new impervious surface, collectively over
the entire project site, will be created.
Construction of one single-family home, which is not part of a larger common
plan of development, with the incorporation of appropriate pollutant source
control and design measures, and using landscaping to appropriately treat runoff
from roof and house-associated impervious surfaces (e.g., runoff from roofs,
patios, driveways, sidewalks, and similar surfaces), would be in substantial
compliance with Provision C.3.
2. Streets, roads, highways, and freeways that are under the Permittees’ jurisdiction
and that create one acre (43,560 square feet) or more of new impervious surface.
This category includes any newly constructed paved surface used primarily for
the transportation of automobiles, trucks, motorcycles, and other motorized
vehicles. Excluded from this category are sidewalks, bicycle lanes, trails, bridge
accessories, guardrails, and landscape features.
1
ACCWP Hydrograph Modification Management Plan
Part A: General Provisions For Hydromodification Management
Attachment A1-1
Excerpts from NPDES permit
3. Significant Redevelopment projects. This category is defined as a project on a
previously developed site that results in addition or replacement, which combined
total 43,560 sq ft or more of impervious surface on such an already developed site
("Significant Redevelopment"). Where a Significant Redevelopment project
results in an increase of, or replacement of, more than fifty percent of the
impervious surface of a previously existing development, and the existing
development was not subject to stormwater treatment measures, the entire project
must be included in the treatment measure design. Conversely, where a
Significant Redevelopment project results in an increase of, or replacement of,
less than fifty percent of the impervious surface of a previously existing
development, and the existing development was not subject to stormwater
treatment measures, only that affected portion must be included in treatment
measure design. Excluded from this category are interior remodels and routine
maintenance or repair. Excluded routine maintenance and repair includes roof or
exterior surface replacement, pavement resurfacing, repaving and road pavement
structural section rehabilitation, within the existing footprint, and any other
reconstruction work within a public street or road right-of-way where both sides
of that right-of-way are developed.
ii. Group 2 Projects
The Group 2 Project definition is in all ways the same as the Group 1 Project definition
above, except that the size threshold of impervious area for new and Significant
Redevelopment projects is reduced from one acre (43,560 sq ft) of impervious surface to
10,000 square feet. Permittees shall require Group 2 Projects to implement appropriate
source control and site design measures and to design and implement appropriate
stormwater treatment measures to reduce stormwater pollution to the maximum extent
practicable. Projects consisting of one single family home not part of a larger common
plan of development are excluded from the Group 2 Project definition, and therefore
excluded from the requirement to implement appropriate stormwater treatment measures.
Implementation of this requirement shall begin by August 15, 2006, at which time the
definition of Group 1 Projects is changed to include all Group 2 Projects.
iii. Proposal for Alternative Group 2 Project Definition
The Program and/or any Permittee may propose, for approval by the Regional Board, an
Alternative Group 2 Project definition, with the goal that any such alternative definition
aim to ensure that the maximum created impervious surface area is treated for the
minimum number of projects subject to Permittee review. Any such proposal shall
contain supporting information about the Permittees' development patterns, and sizes and
numbers of proposed projects for several years, that demonstrates that the proposed
definition would be substantially as effective as the Group 2 Project definition in
Provision C.3.c.ii. Proposals may include differentiating projects subject to the
Alternative Group 2 Project definition by land use, by focusing solely on the techniques
recommended by Start at the Source for documented low pollutant loading land uses,
and/or by optimum use of landscape areas required by Permittees under existing codes as
treatment measures. Proposals may be submitted anytime, with the understanding that
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Part A: General Provisions For Hydromodification Management
Attachment A1-1
Excerpts from NPDES permit
the Group 2 Project definition, as described in Provision C.3.c.ii will be upheld as the
default in the absence of an approved Alternative Group 2 Project definition.
________________________________________________
f. Limitation on Increase of Peak Stormwater Runoff Discharge Rates
i. The Permittees shall manage increases in peak runoff flow and increased runoff volume,
for all Group 1 Projects where such increased flow and/or volume is likely to cause
increased erosion of creek beds and banks, silt pollutant generation, or other impacts to
beneficial uses. Such management shall be through implementation of a Hydrograph
Modification Management Plan (HMP). The HMP, once approved by the Regional
Board, shall be implemented so that post-project runoff shall not exceed estimated preproject rates and/or durations, where the increased stormwater discharge rates and/or
durations will result in increased potential for erosion or other significant adverse
impacts to beneficial uses, attributable to changes in the amount and timing of runoff.
The term duration in this Provision is defined as the period that flows are above a
threshold that causes significant sediment transport and may cause excessive erosion
damage to creeks and streams.
ii. Provision C.3.f.i does not apply to new development and significant redevelopment
projects where the project discharges stormwater runoff into creeks or storm drains where
the potential for erosion or other impacts to beneficial uses, is minimal. Such situations
may include discharges into creeks that are concrete-lined or significantly hardened (e.g.,
with rip-rap, sackrete, etc.) downstream to their outfall in San Francisco Bay,
underground storm drains discharging to the Bay, and construction of infill projects in
highly developed watersheds, where the potential for single-project and/or cumulative
impacts is minimal. Guidelines for identification of such situations shall be included as a
part of the HMP. However, plans to restore a creek reach may re-introduce the
applicability of HMP controls, and would need to be addressed in the HMP.
iii. The HMP may identify conditions under which some increases in runoff may not have a
potential for increased erosion or other impacts to beneficial uses. Reduced controls or
no controls on peak stormwater runoff discharge rates and/or durations may be
appropriate in those cases, subject to the conditions in the HMP. In the absence of
information demonstrating that changes in post-development runoff discharge rates and
durations will not result in increased potential for erosion or other adverse impacts to
beneficial uses, the HMP requirements shall apply.
iv. The HMP proposal, at a minimum, shall include:
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Part A: General Provisions For Hydromodification Management
Attachment A1-1
Excerpts from NPDES permit
1.
2.
A review of pertinent literature;
A protocol to evaluate potential hydrograph change impacts to downstream
watercourses from proposed projects;
3. An identification of the rainfall event below which these standards and
management requirements apply, or range of rainfall events to which these
requirements apply;
4. A description of how the Permittees will incorporate these requirements into
their local approval processes, or the equivalent; and,
5. Guidance on management practices and measures to address identified impacts.
The Permittees may prioritize which individual watersheds the HMP would initially
apply to, if it were demonstrated in the HMP that such prioritization is appropriate.
The Permittees may work appropriately with the Santa Clara Valley Urban Runoff
Pollution Prevention Program and/or other Bay Area stormwater programs as part of
completing these requirements. For example, the Permittees may wish to expand on the
literature review being completed by the Santa Clara Valley Urban Runoff Program
under its permit, rather than authoring their own literature review from scratch. While
such cooperation is encouraged, it shall not be grounds for delaying compliance beyond
the schedule set forth herein.
v. The identified maximum rainfall event or rainfall event range may be different for
specific watersheds, streams, or stream reaches. Individual Permittees may utilize the
protocol to determine a site- or area-specific rainfall event or event range standard.
vi. The HMP’s evaluation protocols, management measures, and other information may
include the following:
1.
2.
3.
4.
5.
6.
7.
HMPpartA_attachments
Evaluation of the cumulative impacts of urbanization of a watershed on
stormwater discharge and stream morphology in the watershed;
Evaluation of stream form and condition, including slope, discharge, vegetation,
underlying geology, and other information, as appropriate;
Implementation of measures to minimize impervious surfaces and directly
connected impervious area in new development and redevelopment projects;
Implementation of measures including stormwater detention, retention, and
infiltration;
Implementation of land use planning measures (e.g., stream buffers and stream
restoration activities, including restoration-in-advance of floodplains so that
floodplains will be able to handle the anticipated increased flows, revegetation,
use of less-impacting facilities at the point(s) of discharge, etc.) to allow expected
changes in stream channel cross sections, stream vegetation, and discharge rates,
velocities, and/or durations without adverse impacts to stream beneficial uses;
A mechanism for pre- vs. post-project assessment to determine the effectiveness
of the HMP and to allow amendment of the HMP, as appropriate; and,
Other measures, as appropriate.
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Part A: General Provisions For Hydromodification Management
Attachment A1-1
Excerpts from NPDES permit
vii. Equivalent limitation of peak flow impacts: The Permittees may develop an
equivalent limitation protocol, as part of the HMP, to address impacts from changes in
the volumes, velocities, and/or durations of peak flows through measures other than
control of those volumes and/or durations. The protocol may allow increases in peak flow
and/or durations, subject to the implementation of specified design, source control, and/or
treatment control measures and land planning practices that take into account expected
stream change (e.g., increases in the cross-sectional area of stream channel) resulting
from changes in discharge rates and/or durations, while maintaining or improving
beneficial uses of waters.
viii. The Permittees as a group shall complete the HMP according to the schedule below. All
required documents shall be submitted for approval by the Executive Officer, based on
the criteria set forth in this Order, except the HMP, which shall be submitted for approval
by the Regional Board. Development and implementation status shall be reported in the
Permittees’ Annual Reports, which shall also provide a summary of projects
incorporating measures to address this Provision and the measures used.
1.
2.
3.
4.
5.
February 15, 2004: Submit a detailed workplan and schedule for completion of
the literature review, development of a protocol to identify an appropriate limiting
storm, development of guidance materials, and other required information;
February 15, 2004: Submit literature review;
November 15, 2004: Submit a draft HMP, including the analysis that identifies
the appropriate limiting storm and the identified limiting storm event(s) or event
range(s);
May 15, 2005: Submit the HMP for Regional Board approval; and,
Upon approval by the Regional Board, implement the approved HMP, which
shall include the requirements of this Provision. Prior to approval of the HMP by
the Regional Board, the early implementation of measures likely to be included in
the HMP shall be encouraged by the Permittees.
_________________________________________________
i. Limitations on Use of Infiltration Treatment Measures - Infiltration and Groundwater
Protection
In order to protect groundwater from pollutants that may be present in urban runoff,
treatment measures that function primarily as infiltration devices (such as infiltration basins
and infiltration trenches not deeper than their maximum width) shall meet, at a minimum,
the following conditions:
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Part A: General Provisions For Hydromodification Management
Attachment A1-1
Excerpts from NPDES permit
i. Pollution prevention and source control measures shall be implemented at a level
appropriate to protect groundwater quality at sites where infiltration devices are to be
used;
ii. Use of infiltration devices shall not cause or contribute to degradation of groundwater
water quality objectives;
iii. Infiltration devices shall be adequately maintained to maximize pollutant removal
capabilities;
iv. The vertical distance from the base of any infiltration device to the seasonal high
groundwater mark shall be at least 10 feet. Note that some locations within the
Permittees’ jurisdiction are characterized by highly porous soils and/or a high
groundwater table; in these areas treatment measure approvals should be subject to a
higher level of analysis (e.g., considering the potential for pollutants such as on-site
chemical use, the level of pretreatment to be achieved, and similar factors);
v. Unless stormwater is first treated by a means other than infiltration, infiltration devices
shall not be recommended as treatment measures for areas of industrial or light industrial
activity; areas subject to high vehicular traffic (25,000 or greater average daily traffic on
main roadway or 15,000 or more average daily traffic on any intersecting roadway);
automotive repair shops; car washes; fleet storage areas (bus, truck, etc.); nurseries; and
other high threat to water quality land uses and activities as designated by each
Permittee; and,
vi. Infiltration devices shall be located a minimum of 100 feet horizontally from any water
supply wells.
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Part A: General Provisions For Hydromodification Management
Attachment A2-1
Map of special Flood Control Districts in Alameda County
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Part A: General Provisions For Hydromodification Management
Attachment A4-1
List of agencies responsible for well permitting and groundwater management in Alameda
County
Agency
Jurisdiction
Alameda County Public Works
Agency
Web Page: www.acgov.org/pwa/wells
Cities of Alameda, Albany, Emeryville, Hayward,
Oakland, Piedmont, and San Leandro; unincorporated
communities west of the hills including Castro Valley,
San Lorenzo, Ashland
Alameda County Water District
Web Page: www.acwd.org
Fremont, Newark, Union City
Zone 7 Water Agency
Web Page: www.zone7water.com
Cities of Dublin, Livermore, and Pleasanton;
unincorporated communities in eastern Alameda
County.
City of Berkeley
Berkeley
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