Rich Branch Tributary to Muddy Branch
Stream Restoration
30% Concept Plan and Report
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Table of Contents
1.0
Project Background and Goals
.1
2.0
Restoration Assessment
6
3.0
30% Restoration Concept Plan
11
4.0
Hydrology and Hydraulic Analysis
15
4.1.
Hydrologic Analyses
15
4.2.
Hydraulics
19
5.0
References
25
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APPENDICES
APPENDIX A: Site Photographs
APPENDIX B: Cross Sections, Profiles, Bed Material, and Bank Erodibility Hazard Index (BEHI)
APPENDIX C: Stream Restoration 30% Plans
APPENDIX D: Hydrology Analyses
APPENDIX E: Hydraulic Analyses
APPENDIX F: Opinion of Probable Cost
1.0
Project Background and Goals
In an effort for Montgomery County to meet the requirements of the Municipal Separate Storm Sewer
Systems (MS4) permit, a partnership was formed to identify new storm water management pond
opportunities, existing stormwater pond retrofits, and stream restoration projects within the Great
Seneca and Muddy Branch Watersheds. The Flintsgrove Pond (50. 174) and 2,000 linear feet of stream
restoration above the pond (complaint site MBMB 309) was one of the projects identified in the
Watershed Study generated through the partnership. This reach was identified for restoration to reduce
channel and bank erosion that is encroaching on private properties and causing sediment deposition in
the Flints Grove Pond. Specifically, the project reach is located parallel and to the north of Rich Branch
Drive and extends from where the channel daylights from the stormdrain from Flints Grove Lane to
Flints Grove Pond. Downstream of the pond, the Unnamed Tributary flows in a westerly direction for
approximately 600 feet before joining Rich Branch, a third-order tributary of Muddy Branch. Figure 1
shows the location of the project reach.
Project Goals
The primary goals of the restoration are to 1) stabilize the Rich Branch tributary such that landowner
concerns regarding bank erosion and property loss are addressed and 2) reduce sedimentation in the
Flints Grove Pond. In the process of achieving these goals, the following should be considered:
1.
2.
3.
Improve the availability and complexity of aquatic habitat in areas disturbed by the
restoration.
Design in channel features that will enhance nutrient uptake and processing.
Minimize disturbance to existing mature trees.
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Figure 1. Site Location of the unnamed Tributary to
Muddy Branch Stream Restoration Project
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Figure 1. Site Location of the Rich Branch Tributary Stream Restoration Project.
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Watershed Description
Muddy Branch is a tributary to the Potomac River in the Middle Potomac Basin (02140202) and is
designated as Use I-P waters. The Middle Potomac Basin is comprised of roughly equal parts urban
(32%), agricultural (37%) and forested (30%) land uses (DNR 2012a). In the Muddy Branch watershed,
the development is concentrated in the headwaters and diffuses downstream where large tracts of
stream valley have been purchased as parkland by the County. The project reach is located within a
highly urbanized area of the Muddy Branch watershed, which is owned by the Homeowner’s
Association. The drainage area to the project reach is approximately 82.5 acres, of which approximately
20% is impervious cover. The runoff curve number for the area draining to the project reach is 70.
Based on review of available historic aerial imagery (1951 to present) on the Montgomery County Digital
Aerial Photo Image Server (Mongomery County 2012) and evidence in the field, the predominant land
use within the watershed prior to urbanization in the mid-1970s was pasture land.
Physiographic Province and Geology
The project area is located within the Piedmont Plateau physiographic province. This province has a
rolling and hilly topography underlain by igneous and metamorphic bedrock such as schist, gneiss, and
gabbro (MESS 2009). More specifically, the project site is within the Hampstead Upland District, of the
Harlord Plateaus and Gorges Region, in the Piedmont Upland Section of the Piedmont Plateau. The
lowest division of the physiographic province, the Hampstead Upland District, is characterized by hilly
uplands intersected by gorges with steep-walls. Important to note is that relatively close and to the east
of the project area is an area called Hunting Hill Area which has a concentration of mafic and ultramafic
bedrock and is less dissected and more gently sloped than the surrounding upland. However, based on
mapping and site observations it is not expected that the project area falls within the Hunting Hill Area
(MESS 2011).
The project area developed in rocks formerly referred to as the Wissahickon Formation which consists of
resistant metamorphic rocks. Since the 1968 Geologic Map of Maryland, the Wissahickon Formation had
been divided. According to Muller (1990), the project area falls with the formations: Prettyboy Schist
(pb), Unnamed Phyllite (py), Pleasant Grove Phyllite and Phyllitic Schist (pg). The rocks of the former
Wissahickon Formation are suspected to be approximately Upper Cambrian or Lower Ordovician in age,
not Precambrian as originally thought (MESS 2000).
Soils
According to the Natural Resources Conservation Service (NRCS) Web Soil Survey, the stream valley floor
is mapped as Baile silt loam, 0 to 3 percent slopes (GA), and the valley slopes are mapped as Glenelg silt
loam, 3 to 8 percent slopes (2B), Gleneg silt loam, 8 to 15 percent slopes (2C), and Brinklow-Blocktown
channery silt loams, 15 to 25 percent slopes (1GD). The Baile soil series in the GA soil map unit is a poorly
drained soil that has formed in local alluvium over residuum from mica schist and gneiss in footslope
landscape positions. The Glenelg soil series in the 2B and 2C soil map units is a well drained soil
3
weathered from micaceous schist residuum on uplands. Brinklow and Blocktown soil series in the 16D
soil map unit are well drained soils that have formed in residuum that weathered from phyllite and
schists. In contrast Brinklow is a moderately deep soil, approximately 20 to 40 inches to lithic bedrock,
while Blocktown is a shallow soil, approximately 10 to 20 inches to paralithic bedrock (Soil Survey Staff
2012). The channery modifier in the soil map unit name of 16D indicates the surface texture has
between 15 to 35 percent of channery rock fragments.
The hydric rating designation for soil map unit GA is hydric which means the Baile soil series that makes
up the soil map unit meets all the hydric soil the criteria. Soil map units 2B, 2C, and 1GD, all have a
‘partially hydric’ hydric rating designation. Partially hydric means that only a portion or component(s) of
the soil map unit meets the hydric soil criteria. In soil map units 2B, 2C, and 1GD, only 5 percent of each
of the soil map units meet the hydric soil criteria (Soil Survey Staff 2012). This hydric rating designation
as given by NRCS Soil Survey is for planning purposes only and onsite investigations are recommended
to determine the extent of hydric soils at the project site.
Forest Stands
Three (3) forest stands were identified onsite and are discussed below. All three forest stands are
priority retention forests because they contains streams, nontidal wetlands, their associated buffers,
and function as a habitat for wildlife.
Forest Stand 1 is a mid-successional tulip poplar! white pine / mixed deciduous forest and is bounded
by Rich Branch Drive to the south and east, WUS Si to the north, and Forest Stand 2 to the west. Stand 1
occupies approximately 1.34 acres within the project area. The stand appears to be in fair condition with
disturbance from residential land use directly to the south. Four (4) specimen trees were located within
the stand.
Forest Stand 2 is a late successional red maple / American beech forest and is bounded Flints Grove
Lane/Flints Grove Drive to the north, Dehaven Court to the east, and continues beyond the study area to
the south and southwest. The stand occupies approximately 1.72 acres within the project area and
appears to be in good condition. Seven (7) specimen trees were located within the stand.
Forest Stand 3 is a mid successional tulip poplar! mixed oak forest and is bounded by Flints Grove Lane
to the north, Rich Branch Drive to the south and east, and Forest Stand 1 to the west. This stand
occupies approximately 11.8 acres and appears to be in good condition despite disturbance from the
surrounding residential land use and the asphalt foot path which runs through the stand. One hundred
and three (103) specimen trees were located within the stand.
Wetlands
The field investigation performed during July 2012 located two (2) nontidal wetland systems, two (2)
perennial streams, and one (1) intermittent stream classified as “waters of the U.S” (WUS). Additionally,
one (1) ephemeral drainage channel was identified within the study corridor. Additional information
regarding the wetlands and waterways found is discussed below.
4
Nontidal Wetlands
Wetland WP1 is a palustrine, emergent wetland located north of an unnamed perennial stream (WUS
Si), south of Antigone Drive and west of Flint’s Grove Lane. Approximately 0.03 acres of this wetland is
within the project area. This emergent wetland is a small linear system which flows to WUS Si. The
Cowardin classification system indicated this wetland to be a palustrine, emergent, persistent, saturated
(PEM1B) wetland.
Wetland WP2 is palustrine, forested/emergent wetland located within the SWM facility, northwest of
Rich Branch Drive and east of Flints Grove Drive. This wetland system receives hydrology from WUS Si
and overland flow, and outlets in a southwesterly direction to WUS Si. Approximately 0.39 acres of this
wetland is within the project area. This is a man-made wetland caused by the accumulation of water
from WUS Si within the SWM facility. The Cowardin Classification system indicates this wetland to be a
palustrine, forested, broad-leaved deciduous / emergent persistent, saturated (PFO1/EM1E) wetland.
Nontidal Waters of the U.S.
The un-named tributary to Rich Branch (WUS Si) is an approximately 9.5-foot wide nontidal, perennial
stream which enters the project area at a culvert southwest of Flints Grove Lane and conveys flow
southwest through the SWM facility and continues beyond the limits of the project area. Approximately
1,620 linear feet (LF) of this stream is within the project area. Upstream of the SWM facility this stream
has undergone severe bank erosion with major undercutting, vertical banks, undermined tree roots, and
exposed soils. The Cowardin Classification for this system is riverine, lower perennial, unconsolidated
bottom, cobble-gravel/sand (R2UB1/2).
WUS S2 is an approximately 5-foot wide intermittent stream that is located northwest of Rich Branch
Drive, along the left bank of WUS Si. Approximately 65 LF of this stream is within the project area. The
Cowardin Classification for this system is riverine, intermittent, streambed, cobble gravel (R4SB3).
WUS S3 is an approximately 3.5-foot wide nontidal, perennial stream located northwest of Rich Branch
Drive, along the left bank of WUS Si, east of WUS S2. This perennial stream receives hydrology from
groundwater seepage and conveys flow for approximately 79 LF to its confluence with WUS 51. The
Cowardin Classification for this system is riverine, lower perennial, unconsolidated bottom, sand
(R2UB2).
WUS 54 is an approximately 3-foot wide ephemeral channel which enters the project area at a culvert
south of Dehaven Court, west of Antigone Drive, east of Flints Grove Drive, and north of WUS Si. This
channel conveys flow southwest approximately 380 LF to its confluence with Wetland WP2. The
Cowardin Classification system is not applicable due to the ephemeral nature of the channel.
Water Resources
The unnamed tributary to Rich Branch (NHD Reachcode 02070008001321) is a first order perennial
tributary of Rich Branch, which flows to Seneca Creek before draining to the Potomac River. Seneca
Creek is impaired for ammonia, chlorides, and total suspended solids (TSS) and the Potomac River has a
total maximum daily load (TMDL) for sediment (EPA, 2013).
FEMA Floodplain
The project site is outside of the mapped FEMA 100-year flood zone. A Special Flood Hazard Area
(SFHA), an area subject to inundation by the 1% annual chance flood (100-year flood), was denoted
along Rich Branch, approximately 450 feet downstream of the downstream-most portion of the project
site (FEMA, 2012).
2.0
Restoration Assessment
Field Assessment
The Rich Branch Tributary was assessed to document the channel profile, cross section, bed materials
and bank erosion hazard index (BEHI) in late March/early April 2012. The methods for the assessment
were based upon the NRCS Part 654 National Engineering Handbook (NRCS 2007) and adapted using
best professional judgment to reflect both the study goals and site conditions. These data were used to
describe the contemporary channel conditions and classify the channel in accordance with the Rosgen
Stream Classification System (Rosgen 1994). Representative photographs of the project reach area
presented in Appendix A, and the results of the field assessment are presented in Appendix B.
Profile
A continuous channel profile was measured from the stormdrain outfall at the headwater (sta. 0) to
where the stream channel transitions into a wetland feature in Flints Grove Pond (sta. 1404). The profile
documents relative elevations along the project length and facet slopes along the channel thalweg. Top
of bank and or bar features were also documented periodically, and bankfull elevations were recorded
where discernible.
Cross Section
To document representative channel conditions, channel cross sections were measured at
representative locations (typically riffles) throughout the project reach. A total of five cross sections
were measured to characterize channel morphology within the project reach. Each of the cross sections
was monumented with a 2-foot rebar and cap.
Bed Materials
Bed materials were measured using the pebble counting procedure developed by Wolman in 1954
(Bunte and Abt 2001). At each representative riffle cross section, a pebble count consisting of a
minimum of 100 particles was conducted. Results were used to characterize the grain-size distribution
of the surface of the channel bed.
Bank Erosion Hazard Index
Stream bank erosion was documented throughout the project reach using the BEHI procedure (Rosgen
2001). Representative BEHI scores were computed at 10 representative banks within the channel to
calibrate the field crew. Once calibrated, the field crew visually scored the banks along the entire project
reach according to the calibrated measurements. The results characterize the spatial distribution and
cumulative extent of bank erosion in the project area.
Findings
Overall the project reach is an entrenched channel with 4-to 6-foot high banks. The stream is generally
confined by the stream valley walls and has a low sinuosity (<1.2). While the stream channel can be
broken into four distinct reaches, overall these reaches exhibit similar geometric characteristics and
active processes. Reaches 1 and 2, the upstream-most reaches, each represent a single evolutionary
sequence of incising, widening and restabilizing with characteristic Rosgen “6”, “F” and “C” geometries,
respectively. In contrast, Reaches 3 and 4, are less dynamic and are tending toward stability. A brief
summary of each of the reaches follows and Figure 2 provides an overview of the reach condition, the
locations of the reaches, cross sections and other channel features.
Reach I (Proposed Geometry Station 0÷00 to 3÷00)
Reach 1 extends from the stormdrain outfall to proposed geometry station 3+00 where the streamflow
plunges over a 5.45-foot high knickpoint in the channel. The upper part of this reach is incising from
station 0+00 to 1+50, lacks a floodplain, and has a characteristic v-shaped cross section with 6-foot high
banks on both sides (Rosgen “64c” channel, see Cross section 1 in Appendix B). The channel bed is
dominated by gravel and cobble (D50=21 mm, DM=110 mm) that is adequate to resist the bankfull flows.
However, it should be noted that without a functional floodplain, the stress on the channel bed
continues to increase proportional to the channel flow above bankfull. This additional stress erodes the
bed and causes the channel incision.
Moving downstream from station 1+50 to 3+00, the channel cross section opens up, and a narrow
floodplain bench has formed. This part of the channel is more characteristic of a Rosgen “C4b” channel
with a floodplain bench that is between 1 and 1.5 feet above the stream bed and the high bank
decreases to 4 feet (see Cross section 2 in Appendix B). The formation of the bench feature downstream
of an incising reach is characteristic of a system that has progressing through an evolutionary sequence
of degrading, widening, restabilizing. The channel bed is still dominated by gravel and cobble (D50 = 24
mm, D~=82 mm) is this portion of the reach.
7
The overall slope of Reach 1 is 1.4% excluding the headcut and riprap apron. The slope facets in the
upper part of the reach are generally steeper than the lower segment, which is consistent with the
deposition and recovery of the lower reach.
Reach 2 (Proposed Geometry Station 3÷00 to 9÷50)
Reach 2 extends from the headcut defining Reach 1 downstream to the top of a bedrock knickpoint near
station 9+50. Overall Reach 2 is very similar to Reach 1 except that the widening and incising below the
channel knickpoint is much more severe and the channel banks are rapidly eroding and failing (see Cross
section 3 in Appendix B). The channel section within Reach 2 remains entrenched, generally lacks a
floodplain, with much of it characteristic of a Rosgen “F” geometry. This “F” type geometry is more
indicative of a widening channel, which is likely the result of the significant sediment supply from
actively migrating headcut upstream. The channel bed is dominated by gravel and cobble (D50=31 mm,
DM=86 mm). Similar to Reach 1, this bed material will erode and lead to progressive channel incision as
flows exceed bankfull.
The overall slope of Reach 2 is 1.8% excluding the headcut. Several debris jams create locally flatter or
steeper segments, but in general the slope facets in the upper part of the reach are steeper than the
lower segment. This sequence of diminishing bed slope with downstream distance is similar to that
observed in Reach 1.
Of note in Reach 2 is the remnant of what is presumed to be a makeshift concrete block dam at station
4+25 for watering livestock. Failure of this dam is likely what triggered some of the instability in the
reach, due to headcutting through the unconsolidated sediment deposits behind the dam.
Reach 3 (Proposed Geometry Station 9÷50 to 11+25)
Reach 3 originates at a bedrock knickpoint and extends downstream to approximately proposed
geometry station 11+25 where the stream valley becomes less confined and the channel is hydraulically
influenced by the stormwater pond. The overall channel is moderately entrenched with a u-shaped
geometry (see Cross section 4 in Appendix B), much of it consistent with a Rosgen “B” channel. Reach 3
is the steepest reach at 3.3%, and is highly confined by the valley walls. The reach does experience some
localized erosion, but is fairly resilient due to the presence of boulders and bedrock. The channel bed is
dominated by gravel and cobble (D50=28 mm, DM=81 mm). With the exception of some high erosion
rates on the steep unprotected banks of the valley wall, the reach appeared stable.
Reach 4 (Proposed Geometry Station 11.25 to 12÷25)
Reach 4 extends from Reach 3 downstream to where the channel loses definition to the wetland formed
by the stormwater pond at approximate profile station 12+25. This has formed a broad (25 foot)
floodplain of sand and gravel and becomes very sinuous compared to the upper reaches. The channel
slope decreases to 1% and flattens decreases as it enters the pond. The channel geometry in Reach 4 is
largely consistent with a Rosgen “C” channel type. A riffle cross section was not taken in this reach, since
riffles were aligned perpendicular to the valley slope such that surveying would not capture the valley
8
wall and a representative cross-sectional geometry. Instead, Appendix B includes a pool cross section
taken to illustrate the broader relationship between the channel and valley wall through the reach. Bed
material in Reach 4 is finer than the upper reaches, and dominated by sand and gravel (D50=18 mm,
D~=51 mm). Overall this reach appears stable with the exception of some moderate erosion along the
outer meanders. Relative to upstream reaches, the pattern of decreasing slope and bed material size is
consistent the long-term influence of the pond on in-channel processes. Floodflows reaching the pond
decelerate and deposit bedload and suspended load.
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FIgure 2. Stream Assessment
Locations of Reach Breaks, Cross Sections, Profiles and BEHI Results
Figure 2. Restoration Assessment Locations of Reach Breaks, Cross Sections, Profiles and BEHI Results
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3.0
30% Restoration Concept Plan
For the Rich Branch Tributary, an effective channel restoration is proposed, which incorporates the
following key elements;
1.
Improve the availability and complexity of aquatic habitat in areas disturbed by the restoration.
a.
Stabilization of channel headcuts that progressively degrade the project reach.
b. Stabilization of the toe of vulnerable banks to minimize further bank erosion and
channel widening.
2.
3.
Design in channel features that will enhance nutrient uptake and processing
a.
Raising the channel invert such that a functional floodplain effectively reduces channel
shear stress.
b.
Enhance or mitigate existing stormwater outfall protection to maintain stability.
Minimize disturbance of existing mature trees.
a.
Protection of existing trees that reinforce banks and slow channel widening and
migration.
A detailed topographic and tree survey was completed by Century Engineering in November, 2012. The
survey allowed detailed analysis of design constraints such as existing infrastructure and utilities,
topography, trees, and property ownership. The proposed restoration design has been developed to a
30% conceptual level to facilitate assessment of the approach in satisfying the project goals and gaining
County and public approval. It is anticipated that specific components of the design will be refined
during the 60% design phase as additional analyses are undertaken, and upon receipt of feedback from
DEP and the homeowners association (HOA) during the public input process including the public
meeting.
Three alternative design approaches were evaluated in preparation of the concept plan; (1) re
connecting the channel to the existing floodplain, (2) removing legacy sediments and creating a new
floodplain at a lower elevation, and (3) stream bank stabilization in place. Alternative (2) would provide
two of the three key design elements, but would unnecessarily disturb the existing forest canopy and
result in a large number of trees lost. Alternative (3) could potentially minimize impacts to the forest,
depending on access, but would not address the other key design elements. The steepness of the reach
and narrow valley also make the third alternative difficult to achieve for a sustainable period.
Alternative (1), to re-connect the stream to the existing floodplain using in stream structures, was
decided as the best approach.
In the 30% design plans, cobble riffles and rock steps are placed along the proposed stream channel.
These elevated riffles will act to enhance hyporheic flows, dissipate hydraulic energy, provide grade
11
control, and attenuate storm flows. The proposed channel bed elevations as shown on the profile are
based on: 1) conveying frequent, small peak flows below adjacent geomorphic surfaces (floodplain and
terrace features) across a series of riffles, and 2) reintroducing less frequent, larger peak flows to the
adjacent geomorphic surfaces. The proposed channel cross section is designed to convey up to 50 cubic
feet per second (cfs). The 50 cfs design discharge is based on both observed bankfull indicators and
regional bankfull curve estimates (Figure 3). Higher flows will extend beyond the lateral limits of the
proposed channel banks, spreading out onto the adjacent terraces.
After careful consideration, the following recommendations were applied to specific problems identified
within the following segments, to the proposed stream restoration design.
Station 0+00 to 3+00 Install a series of cobble riffles to raise and armor the channel invert and form
pools that will provide variable habitat and dissipate energy from the stormdrain as it transitions to the
natural channel. Because the invert of the stormdrain is well below the top of bank, full connection to
the floodplain is not possible, but this approach will reduce the stress on the channel banks allowing
them to recover and is adaptable to avoid trees.
—
Station 3+00 to 5+00 Install a series of rock steps and cobble riffles to eliminate the 5-foot headcut
and spread the drop in grade over a longer distance. This reach also has few trees, especially on the left
bank so a more stable alignment and floodplain can be graded as needed.
—
Station 5+00 to 6+00 Construct a series of rock steps and grade out the existing berm to provide stable
tie-in to the stormwater outfall.
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Station 6+00 to 9+75 Install cobble riffles at key locations to reinforce and backwater areas of existing
bank erosion or high erosion potential. These riffles support a full reconnection to the existing or
constructed floodplain benches. Toe protection is required depending on the bank constraints.
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Station 9+75 to 10+25 Enhance the natural bedrock outcrop by redirecting flow from the valley walls
by re-aligning the channel meander upstream and downstream.
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Station 10+25 to 12+25 Re-align the channel using long cobble riffle structures to provide a stable
profile and enhance flow diversity through the tie-in to the downstream pond.
—
The design cross section and hydraulic calculations are provided below. The main channel section was
designed to convey 50 cfs, which closely resembles bankfull indicators in the field, and the 2-year
recurrence interval discharge as discussed in Section 4.1. Overbank and channel velocities were checked
during the 10-year event as well. A cobble-sized riffle substrate material is proposed to ensure stability
during the 10-year event.
Template Design Spreadsheet
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The anticipated benefits of the proposed approach include the following:
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Stabilize eroding channel side slopes, and associated tree loss, currently a sediment source, by
reducing energy within the channel as well as providing grade control to ensure long term
stability of the system
Preserve valuable overstory trees through precision construction access, minimized excavation,
and choice fill placement.
0
Recharge groundwater through retaining stormwater runoff in the floodplain including
depressional features and wetland areas to facilitate infiltration. Raise the invert (bottom) of the
stream channel to increase the elevation at which groundwater can enter the bottom and sides
of the stream channel. An incised channel reduces the local groundwater elevation by allowing
groundwater to enter the channel at a lower elevation, i.e., the incised (down cut) channel
invert.
Extend the duration of baseflow in the restored stream as a result of storing stormwater runoff
as additional groundwater, which ‘leaks’ into the stream channel during drier periods
4.0
Hydrology and Hydraulic Analysis
Hydrological analyses were performed for the subwatershed areas draining to the top of the stream
restoration site to the 66-inch CMP storm water outfall below Flints Grove Lane, the 18-inch RCP
storm water outfall draining Rich Branch Drive, and direct drainage to the stream valley. The
analyses were performed in order to inform the Rich Branch Tributary stream restoration design,
and to properly model the hydraulics of the system.
In addition, hydraulic analyses were performed on the stream reach to be restored from below
Flints Grove Lane to the Flints Grove storm water management pond. The hydraulic analyses were
completed in order to assess the proposed restoration design in terms of future velocity, shear
stress, and 100-year water surface elevation (100-year WSEL).
Presented below are the results of the hydrologic and hydraulic analyses based on the 30% concept
design for the Rich Branch Tributary stream restoration and assumed WSEL for proposed conditions
associated with the retrofit of the Rich Branch Stormwater pond. Based on the preliminary
modeling results presented below and in Appendix D, the 100-year WSEL increases, but the extent is
not expected to extend beyond the limits of the HOA property for the proposed stream condition.
4.1.
Hydrologic Analyses
The hydrologic analyses were performed in concert with Task A additional hydrologic analyses for
the drainage area storm water management ponds including Site 5 Flints Grove and Site 3 Fox Hills
North. As shown in Figure 4, the Flints Grove management pond captures runoff from the primarily
residential watershed upstream of the Rich Branch Tributary project area. As mentioned earlier, a
retrofit of the Flints Grove Pond to enhance the water quality treatment is currently being designed
under a separate task.
Figure 4 illustrates the drainage area delineations for the different subwatersheds analyzed for this
study.
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Drainage areas to Rich Branch Tributary
Figure 4. Hydrologic delineation for the Rich Branch Tributary site.
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TR-20 Method
The NRCS SCS TR-20 method is a synthetic hydrograph method, which is based on a nondimensional unit hydrograph and hypothetical storm patterns to derive direct runoff through NRCS
Runoff Curve Numbers. The basic assumption is that storm frequencies directly correspond with
those of flood flows.
The watershed characteristics that are required by the computer program include drainage area,
NRCS Runoff Curve Number (RCN), time of concentration (Tc), Antecedent Runoff Condition (ARC),
and rainfall depths for the 24-hour durations. The antecedent runoff condition was assumed to be 2
as described in TR-20. This value represents the average moisture condition when flooding occurs.
NOAA Atlas 14 rainfall depth and rainfall distribution values were used in the TR-20 model. NOAA
Atlas 14 non-dimensional unit hydrography was utilized in the TR-20 model to be consistent with
rainfall depths and recommendations in the Hydrology Panel Report. The hydrologic analyses used
the TR-20 methodology within the computer aided design system HydroCAD. HydroCAD uses the
same computational procedures ofTR-20, but also maintains a complete database for the
watershed and drainage system. Reach routing was also including in the model, to route the
discharge from the upstream outfall (DA1) to P011*2; and to route the combined discharge from both
outfalls (DA1 and DA2) to the pond (P011*3). The storage-indication plus translation method was
used for reach routing, which accounts for internal reach storage and the effects of travel time on
the peak and volume of discharge.
Existing Land Use and Runoff Curve Numbers
For this study, Biohabitats used Montgomery County LiDAR, Maryland Department of Planning 2002
Land Use, and NRCS soils mapping data to delineate the subwatersheds and analyze potential
runoff. The land use data and hydrologic soil groups for each of the subsheds are shown in Table 1.
The majority of the site was in B-and D- hydrologic soil groups. There were no A-type hydrologic
soils. Medium Density Residential was the dominant land use.
Table 1. Land use data and hydrologic soil groups for the Rich Branch Tributary Site
DA1: Flints Grove Curve number calculation
Land Cover
Quantity
% of DA
CN
Other pervious (turf) -B soils (ac)
36.8
76%
61
Total impervious area (ac)
11.8
24%
98
Total drainage area (ac)
48.7
Weighted curve number
70
Table 1 (Con’t). Land use data and hydrologic soil groups for the Rich Branch Tributary Site
DA2: Rich Branch Curve number calculation
Land Cover
Quantity
% of DA
CN
Other pervious (turf) B soils (ac)
3.3
66%
61
Total impervious area (ac)
1.7
34%
98
Total drainage area (ac)
5.0
-
Weighted curve number
74
DA3: Direct Drainage Curve number calculation
Land Cover
Quantity
% of DA
CN
Woods D soils (ac)
2.8
29%
77
Woods B soils (ac)
4.8
49%
55
Other pervious (turf) B soils (ac)
1.2
12%
61
Total impervious area (ac)
1.0
10%
98
Total drainage area (ac)
9.8
-
-
-
Weighted curve number
66
Rainfall Data
NOAA Altas 14 rainfall data were utilized for the project, assuming a 24-hr duration and Type II
distribution. The following rainfall depths were used for the hydrologic analysis.
Table 2: Rainfall Depths used in the HydroCAD model
Return Interval
Rainfall Depth (in)
1-year
2.6
2-year
3.2
10-year
5.1
100-year
7.2
Time of Concentration
Time of concentration is the time required for runoff to travel from the hydraulically most distant
part of the drainage area to a point of investigation in the watershed. The aerial photo, topography,
and storm drainage data were utilized to compute the time of concentration from flow paths. Three
segments of the flow path, including overland flow, shallow concentrated flow, and open channel
flow, contribute to the total travel time. Those three segmental travel times along the flow path are
summed to produce the time of concentration for the drainage area. The upper reach of the
watershed begins as overland flow, which was generally taken as the first 100-feet of flow path. The
flow then transitions into shallow concentrated flow in low points and street gutters. When shallow
concentrated flows reach a pipe or the defined channel, it was treated as open channel flow. The
18
total computed time of concentration to P01 1, 2, and 3 are approximately 23.7, 6.1, and 12.1
minutes, respectively. The time of concentration computations are included in Appendix D.
Hydrologic Analysis Results
The subwatershed data was entered into HydroCAD in order to determine the hydrology of the
study areas at the 66-inch CMP storm water outfall below Flints Grove Lane (DA1), the 18-inch RCP
storm water outfall draining Rich Branch Drive (DA2), and direct drainage to the stream valley (DA3).
Results of the hydrologic analysis are summarized in Table 3. Included in the summary are the direct
runoff and drainage area from individual subsheds, and the cumulative peak discharges and
drainage areas to the mainstem of the stream. Despite significant contributing discharges from the
Rich Branch and Direct Drainage areas, due to the effects of timing of the hydrographs and reach
routing, the Flints Grove Outfall had the largest contributions to peak discharge. The output report
from HydroCAD is included in Appendix D.
Table 3. Hydrologic results from the HydroCAD model of the Rich Branch Site
Contributing Discharge Combined Discharge
Location
Event
from DA (cfs)
at P01 (cfs)
.
Flints Grove Outfall,
DA1, P0141
Rich Branch Outfall,
DA2, P0142
Direct Drainage,
DA3, P0143
4.2.
1-year
19.4
19.4
2-year
10-year
35.8
100.7
35.8
100.7
100-year
1-year
184.6
5.5
184.6
19.7
2-year
10-year
8.9
21.1
35.2
102.1
100-year
1-year
35.6
3.6
186.8
20.2
2-year
10-year
7.5
24.0
37.0
107.4
100-year
45.8
198.7
Hydraulics
The main objective of the hydraulics analysis is to evaluate instabilities in the existing conditions and
to predict the hydraulic conditions of the proposed stream restoration for Rich Branch Tributary.
Hydraulic computations were performed for the existing and proposed conditions for the design
discharge, 1, 2, 10, and 100-year storm events. The hydraulic analyses were developed for the Rich
Branch Tributary using the Corps of Engineers Computer Program, HEC-RAS Version 4.1, dated
January 2010.
Methodology and In put Data
Data used to develop the HEC-RAS model include cross-sections, Manning’s n values, and boundary
conditions. Before developing the HEC-RAS model, field visits were made to observe and
photograph the conditions of the stream channel, overbanks, and the existing structures, to
visualize high flow scenarios, and to determine flow distribution, effective and ineffective flow
areas.
Prior to the computation, the selection of the flow regime needs to be made since the boundary
conditions to be imposed are directly related to the flow regime. The hydraulic model was run using
a mixed super- and subcritical flow regime. Mixed flow computations were performed in this study
due to the steepness of the reach and presence of significant areas of supercritical flow.
Cross Section Data
The HEC-RAS cross sections were generated from detailed field survey data. Twenty-six cross
sections were cut from the existing topographic survey using AutoCAD. Cross sections were located
both in order to assess the existing conditions and the proposed structure locations in the concept
design. The cross sections were cut at the top of each proposed riffle or rock step, and upstream
and downstream of the bedrock outcrop. The details of each cross section in elevation view are also
included in Appendix E. All cross sections are oriented from left to right, looking downstream.
Boundary Conditions
Upstream and downstream boundary conditions were based on the normal depth at the existing
bed slope. Future modeling efforts will use a downstream boundary conditions based on the
modeling results for the Flints Grove Pond, with known water surface elevations for the proposed
grading. The results of the hydrologic model were used to inform the HEC-RAS hydraulic model of
the site. Since the peak discharges did not vary significantly along the Rich Branch Tributary, only
the maximum discharges at P01113 were used in the analysis, which should yield conservative water
surface elevations, velocities, and shear stresses.
Manning’s Roughness Coefficients
Manning’s ‘n’ values were estimated based on the geomorphic cross sections, substrate material,
and visual approximation of in-stream obstructions, vegetation, and cross sectional variability. Instream and floodplain roughness values were calculated according to the, “Guide for Selecting
Manning’s Roughness Coefficients for Natural Channels and Floodplains.” (Arcement and Schneider
1989). The resulting roughness values for the HEC-RAS model are shown in Table 4. Reach breaks
were based on the results of the geomorphologic survey. In addition, for stability of the model
during mixed flow regime, the roughness of very steep sections at the head cuts and bedrock
outcrop, the instream manning’s n values were raised until subcritical flow occurred, to simulate the
extra roughness contributions of the hydraulic jump and turbulence predicted in these locations.
Table 4. Manning’s n Roughness values used in the HEC-RAS model of the Rich Branch Site
Reach
HEC-RAS Station
Manning’s n
1
1315.63
1148.85
0.041
1
1070.34
1041.53
0.049
2
1016.79 flD472.37
0.066
3
417.02
[In 199.33
0.063
4
116.58 liD 28.66
0.040
F[urdJ[IJ]]]
1315.63 [ID 28.66
0.075
Modeling for the Proposed Condition
The HEC-RAS model for the proposed condition was developed by modifying the existing condition
model with the proposed stream restoration grading as shown on the 30% Concept Design plans.
For the proposed channel, the Manning’s ‘n’ values were changed to 0.035 based on the proposed
riffle design. Manning’s ‘n’ values were kept the same as the existing condition model, where there
is no proposed improvement.
Summary of HEC-RAS Results
The results for the concept design were principally used to estimate the resulting flood condition of
the proposed design and the expected velocities and shear stresses based on the concept plan, as
summarized in Tables 5-9. The maximum and average shear stress was reduced for the 2-yr event.
The maximum and average velocity increased, most likely due to the shallower flow caused by the
riffle structures and more supercritical flow condition, indicated by the increase in Froude number
and smaller flow area. A similar pattern was observed for the 10-yr event (Tables 7 & 8).
While subsequent design submittals will include an analysis of the stream stability, a preliminary
assessment was performed by comparing the calculated velocity and shear stress to the permissible
velocity and shear stress values for the respective channel materials reported by Fischenich (2001).
Per Fischenich (2001), the permissible shear stress for the proposed riffle material (cobble) and rock
steps (boulders) are 3 lbs/sq ft and 10 lbs/sq ft, respectively. Permissible velocity for cobble and
boulders is 4 ft/s and 14 ft/s, respectively. Overbank flows were also analyzed for shear stress and
velocity. Permissible shear stress and velocity for native grasses are 1.2 lbs/sq ft and 4 ft/s,
respectively. For erosion control blankets (coir fiber), the permissible shear stress and velocity are
2.25 lbs/sq ft and 3 ft/s. So, the analysis of the concept plan provides some assurity to the stability
of the proposed design and materials.
Table 5. Shear Stress estimates and related statistics for the 2-year event from the hydraulic
model of the Rich Branch Site
Existing
Existing
Existing
Proposed
Proposed
Proposed
Shear
Statistic
Shear LOB
Shear ROB
Shear Channel
Shear LOB
Shear ROB
Channel
(lbs/sq if)
(lbs/sq ft)
(lbs/sq if)
(lbs/sq if)
(lbs/sq if)
(lbs/sq_if)
MAX
4.37
N/A
N/A
4.15
0.14
0.23
AVERAGE
1.54
N/A
N/A
1.32
0.11
Statistic
Existing E.G.
Slope (ft/if)
Existing Flow
Depth (if)
Proposed E.G.
Slope (ft/if)
Proposed Flow
Depth (if)
MAX
AVERAGE
0.0858
0.03
2.25
1.57
0.1721
0.04
2.07
1.02
0.13
Table 6. Velocity estimates and related statistics for the 2-year event from the hydraulic model of
the Rich Branch Site
Existing
Velocity
Existing
Existing
Proposed
Proposed
Proposed
Statistic
Velocity
Velocity
Velocity
Velocity LOB
Velocity ROB
Channel
LOB (ft/s)
ROB (ft/s)
Channel (if/s)
(ft/s)
(ft/s)
(if/s)
MAX
5.8
N/A
N/A
9.34
1.17
1.2
AVERAGE
3.36
N/A
N/A
4.99
0.83
0.85
Statistic
Existing
Froude 4$
Existing Flow
Area (sq if)
Existing Top
Width (if)
Proposed
Froude 4$
Proposed Flow
Area (sq if)
Proposed Top
Width (if)
MAX
AVERAGE
1.02
0.65
34.15
14.21
28.71
15.22
2.63
1.14
25.53
10.25
47.95
16.31
Table 7. Shear Stress estimates and related statistics for the 10-year event from the hydraulic
model of the Rich Branch Site
Existing
Existing
Existing
Proposed
Proposed
Proposed
Shear
Statistic
Shear LOB
Shear ROB
Shear Channel
Shear LOB
Shear ROB
Channel
(lbs/sq ft)
(lbs/sq ft)
(lbs/sq ft)
(lbs/sq ft)
(lbs/sq It)
(lbs/sq_It)
MAX
5.08
N/A
0.92
4.82
0.7
0.9
AVERAGE
2.25
N/A
0.92
1.98
0.45
Statistic
Existing E.G.
Slope (ft/It)
Existing Flow
Depth (It)
Proposed E.G.
Slope (ft/It)
Proposed Flow
Depth (It)
MAX
AVERAGE
0.076248
0.03
3.52
2.45
0.100849
0.03
3.1
1.62
0.44
Table 8. Velocity estimates and related statistics for the 10-year event from the hydraulic model of
the Rich Branch Site
Existing
Velocity
Existing
Existing
Proposed
Proposed
Proposed
Statistic
Velocity
Velocity
Velocity
Velocity LOB
Velocity ROB
Channel
LOB (ft/s)
ROB (ft/s)
Channel (ft/s)
(ft/s)
(ft/s)
(ft/s)
MAX
AVERAGE
6.49
4.48
N/A
N/A
1.97
1.97
11.28
6.84
3.09
1.86
3.12
1.75
Statistic
Existing
Froude #
Existing Flow
Area (sq It)
Existing Top
Width (It)
Proposed
Froude ft
Proposed Flow
Area (sq It)
Proposed Top
Width (if)
MAX
AVERAGE
1.02
0.67
69.01
29.26
44.44
20.65
2.24
1.14
49.8
22.52
58.69
26.58
Significant increases were observed for the 100-yr floodplain elevation, as shown in Table 9. But the
floodplain is predicted to remain within the HOA property, and no private individual properties are
projected to be impacted by the rise. The floodplain map in Appendix E illustrates the affect of the
proposed design on the 100-yearfloodplain at the Rich Branch Tributary project site.
Table 9: Water Surface elevation estimates for the 100-year event from the hydraulic model of the
Rich Branch Tributary Site
Plan
Existing
Proposed
RS Station (ft)
W.S.Elev
W.S.Elev
Change
(ft)
28.66
323.2
324.26
1.06
116.58
199.33
240.86
323.69
324.93
326.9
325.88
326.94
328.21
2.19
2.01
1.31
283.16
317.84
357.72
417.02
328.07
331.06
332.79
334.03
329.5
330.37
332.72
335.07
1.43
-0.69
-0.07
1.04
472.37
518.19
335.44
336.76
335.72
336.78
0.28
0.02
538.41
610.22
337.18
339.04
338.15
339.64
0.97
0.6
650.88
699.12
755.86
339.37
340.11
341.54
340.12
340.78
342.52
0.75
0.67
0.98
841.76
878.95
342.31
343.5
344.87
346.15
2.56
2.65
940.06
989.62
1016.79
344.44
346.58
347.84
347.1
347.47
349.7
2.66
0.89
1.86
1041.53
1070.34
1148.85
350.73
351.67
352.62
350.47
352.95
353.83
-0.26
1.28
1.21
1193.94
1252.87
1315.63
353.1
353.84
355.19
354.87
356.08
356.9
1.77
2.24
1.71