DR1502-ST160206Stream Stabilization Report
Stream G6
Prepared for: City of Coppell
February 2, 2016
NDM Job No. 15041
Stream G6
Stream Stabilization Report 2
TABLE OF CONTENTS
1.0 INTRODUCTION ............................................................................................................................................ 3
1.1 PURPOSE ......................................................................................................................................................... 3
1.2 BACKGROUND ................................................................................................................................................. 3
1.3 OVERVIEW ....................................................................................................................................................... 4
2.0 DATA COLLECTION AND FIELD RECONNAISSANCE ........................................................................................ 4
3.0 HYDROLOGIC AND HYDRAULIC ANALYSIS ..................................................................................................... 5
3.1 HYDROLOGIC ANALYSIS ................................................................................................................................... 5
3.2 HYDRAULIC ANALYSIS ...................................................................................................................................... 5
4.0 DESIGN ANALYSIS AND CONCEPT PLAN ........................................................................................................ 8
5.0 PERMITTING ............................................................................................................................................... 18
6.0 CONCLUSION AND RECOMMENDATION ..................................................................................................... 18
7.0 REFERENCES ............................................................................................................................................... 19
EXHIBITS
Figure 1: Location Map .................................................................................................................. 3
Figure 2: Alternative 1 Site Overview ............................................................................................. 9
Figure 3: Alternative 1 Site Overview ........................................................................................... 10
Figure 4: Alternative 1 Stream Profile .......................................................................................... 11
Figure 5: Alternative 2 Site Overview ........................................................................................... 13
Figure 6: Alternative 2 Gabion Structure ...................................................................................... 14
Figure 7: Alternative 2 Gabion Structure ...................................................................................... 15
Figure 8: Alternative 2 Typical Cross Section................................................................................ 16
APPENDICES
A Integrated Environmental Solutions Waters of the United States Delineation
B Blackland Prairie Channel Slope Discharge Curve
C Opinion of Probable Cost Calculations
Stream G6
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1.0 INTRODUCTION
1.1 PURPOSE
The purpose of this report is to summarize results from the field analysis, hydraulic evaluation and
developed concept plan for the Stream G6 stabilization project in the City of Coppell. This report
provides technical data to support the proposed stream stabilization concept plan.
1.2 BACKGROUND
The project site is located between East Belt Line Road and East Sandy Lake Road in Coppell, TX, see
Figure 1. The project scope begins at South Moore Road and continues downstream approximately
1600-feet to Lot 12 of Northlake Woodlands East, Phase 3. Existing conditions on Stream G6 include
bank erosion, exposed utility lines and channel degradation. Slopes of the existing channel range
from mild to adverse (3.59% to -0.10%) and the existing channel alignment is mostly straight with
minor bends. The existing channel is also positioned between multiple neighborhood developments
with homes and pools close to either side of its banks.
Figure 1: Location Map
Stream G6
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1.3 OVERVIEW
This study is based on the concept plan described in the City-Wide Storm Water Management Study
for the City of Coppell, Texas, by Albert H. Halff Associates, Inc. (Halff Associates 1991). Data
collection, Field Survey and Reconnaissance was the first task performed on Stream G6. During this
task Nathan D. Maier (NDM) collected existing topographic maps, property and easement
information, subdivision plates, utility plans, storm drain plans, sanitary sewer plans and noted
several items of concern in the project site. Hydrologic and hydraulic analyses were then performed
to determine the baseline flood elevations and channel velocities within Stream G6. Based upon
these results a concept plan was developed, coupled with a hydraulic analysis, to assess the
impacts of the proposed improvements on water surface elevations and channel velocities.
2.0 DATA COLLECTION AND FIELD RECONNAISSANCE
NDM, with assistance from the City of Coppell, performed data collection and field reconnaissance
for Stream G6. Data collected for this study site included existing topographic maps, property and
easement information, subdivision plates, utility plans, storm drain plans, sanitary sewer plans and
aerial imagery data. Two sets of topographic maps were received from the City of Coppell collected
in 2001 and 2007. Both of these were originally obtained from North Central Texas Council of
Governments (NCTCOG). The 2001 topographic data set was collected using an Airborne LIDAR
providing a vertical accuracy of 5.9-7.8 inches and was provided as 2-foot contours. The 2007
topographic data set was produced using stereo images to automatically generate a digital
elevation model which was used to produce 2-foot contours. Property and easement boundaries,
subdivision plates, utility plans, storm drain plans and sanitary sewer plans were all obtained from
the development plans for Northlake Woodlands East No. 1, 3, 4. Aerial Imagery data from 2013
was provided from the City of Coppell.
NDM performed a limited field survey from South Moore Road to Lot 12 of Northlake Woodlands
East, Phase 3. Five cross-sections were surveyed along with locations of any utilities and trees six
inches and larger from South Moore Road to Lot 7. Stream flowline data was also taken from Lot 7
to the end of the project area.
Lastly a field visit was made from several NDM employees. During this visit the stream was
examined for adverse conditions such as large bank erosion zones, existing stream protection and
exposed utilities.
Stream G6
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3.0 HYDROLOGIC AND HYDRAULIC ANALYSIS
3.1 HYDROLOGIC ANALYSIS
There currently are no available hydrologic models to provide flow data for this study area.
However, the 1-percent-annual-chance flood event discharge was provided at select locations in
the City-Wide Storm Water Management Study for the City of Coppell, Texas, by Albert H. Halff
Associates, Inc. (Halff Associates 1991). The discharge points from the 1991 Halff study were
located at the Moore Road culvert and Bethel School Road. Both discharge locations were outside
the scoped region so a gage transfer method was implemented. The equation below was used to
transfer peak discharge data from Bethel School Road upstream to the end of Stream G6s scope.
𝑄1 =𝑄2 √𝐴1
𝐴2
(1)
Where,
Q1 = Estimated discharge at ungaged watershed 1
Q2 = Known discharge at gaged watershed 2
A1 = Area of watershed 1
A2 = Area of watershed 2
Using this approach a 1-percent-annual-chance flood event discharge of 1090 cfs was calculated
and used in the hydraulic analysis.
Lastly a bank full flow condition was developed for the study area. This was completed by running
multiple flows through the baseline condition model until water surface elevations (WSEL) were at
approximately bank full levels. A discharge of 350 cfs was determined to be representative of bank
full conditions.
3.2 HYDRAULIC ANALYSIS
During the hydraulic analysis of Stream G6 four separate HEC-RAS models were developed. These
included a baseline condition and three other proposed alternative models. A fourth alternative is
provided in this section but no modeling was conducted as it did not evaluate modifying the
channel slope or cross section geometry.
Baseline Condition:
The baseline condition model covers the study area from downstream of Moore Rd to Lot 12 of
Northlake Woodlands East, Phase 3. In order to create a stream centerline alignment for this model
a combination of topography and imagery maps were utilized to trace the existing Stream G6
flowline. Next 11 cross sections were placed at locations where NDM survey was completed along
this alignment. From Moore Rd to Lot 7 of Northlake Woodlands East, Phase 3, five cross sections
Stream G6
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were placed aligning with the detailed channel survey. The survey elevation data were then
combined with 2001 topographic elevation data to produce station elevation data for these five
cross sections. Further downstream from Lot 7 to Lot 12 of Northlake Woodlands East, Phase 3, six
cross sections were placed aligning with six channel flowline points recorded by the NDM survey.
Since detailed channel survey was not completed at these locations and the 2001 topographic
elevation data did not provide accurate channel delineation, an average cross section from surveyed
cross sections was developed and placed into each cross section station elevation data set.
Alternatives:
Once a baseline model was established for this study region it was then used to create three
additional alternative models. These models were modified as necessary to capture the proposed
channel modifications while preserving cross section locations to allow for comparison between the
baseline and alternatives. Tables 1 to 4 below display results from the baseline and all three
alternative models under bank full and 100 year flow conditions. It should be noted that all data
used for this conceptual analysis was best available and results presented herein do not represent
final design values.
Table 1: Water Surface Elevations for Base flow Conditions
Bank Full Flow Conditions
(Q = 350 CFS)
Station
Water Surface Elevations (ft) Difference
(Alternative - Baseline)
Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3
1686.85 488.15 487.62 487.62 487.27 -0.53 -0.53 -0.88
1534.38 485.88 485.54 485.49 485.55 -0.34 -0.39 -0.33
1426.89 485.31 484.73 485.32 485.32 -0.58 0.01 0.01
1264.80 482.35 483.79 484.09 484.09 1.44 1.74 1.74
983.53 477.48 478.01 477.97 477.97 0.53 0.49 0.49
802.70 476.78 476.54 476.54 476.54 -0.24 -0.24 -0.24
705.24 476.22 475.65 475.65 475.65 -0.57 -0.57 -0.57
563.96 475.1 474.48 474.48 474.48 -0.62 -0.62 -0.62
432.04 474.15 474.03 474.03 474.03 -0.12 -0.12 -0.12
179.28 471.55 471.34 471.34 471.34 -0.21 -0.21 -0.21
0.00 469.54 469.84 469.84 469.84 0.3 0.3 0.3
Stream G6
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Table 2: Average Channel Velocities for Base flow Conditions
Bank Full Flow Conditions
(Q = 350 CFS)
Station
Average Channel Velocity (ft/s) Difference
(Alternative - Baseline)
Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3
1686.85 7.95 7.03 7.03 8.25 -0.92 -0.92 0.30
1534.38 5.15 6.51 6.64 4.95 1.36 1.49 -0.20
1426.89 3.46 6.68 4.51 4.00 3.22 1.05 0.54
1264.80 8.66 5.22 6.31 6.31 -3.44 -2.35 -2.35
983.53 5.42 2.97 7.31 7.31 -2.45 1.89 1.89
802.70 4.84 4.96 4.96 4.96 0.12 0.12 0.12
705.24 5.94 6.75 6.75 6.75 0.81 0.81 0.81
563.96 7.58 6.68 6.68 6.68 -0.90 -0.90 -0.90
432.04 4.72 4.83 4.83 4.83 0.11 0.11 0.11
179.28 8.85 8.92 8.92 8.92 0.07 0.07 0.07
0.00 6.68 5.64 5.64 5.64 -1.04 -1.04 -1.04
Table 3: Water Surface Elevations for 100 year Flow Conditions
100 yr Flow Conditions
( Q = 1090 CFS)
Station
Water Surface Elevations (ft) Difference
(Alternative - Baseline)
Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3
1686.85 490.47 489.94 489.94 489.94 -0.53 -0.53 -0.53
1534.38 487.41 487.5 489.01 489.04 0.09 1.6 1.63
1426.89 487.35 487.24 489.18 489.18 -0.11 1.83 1.83
1264.80 485.17 485.36 485.86 485.86 0.19 0.69 0.69
983.53 480.09 481.08 481.8 481.8 0.99 1.71 1.71
802.70 478.58 478.61 478.61 478.61 0.03 0.03 0.03
705.24 477.82 477.61 477.61 477.61 -0.21 -0.21 -0.21
563.96 476.62 476.56 476.56 476.56 -0.06 -0.06 -0.06
432.04 476.15 476.05 476.05 476.05 -0.1 -0.1 -0.1
179.28 473.74 473.87 473.87 473.87 0.13 0.13 0.13
0.00 471.49 472.03 472.03 472.03 0.54 0.54 0.54
Stream G6
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Table 4: Average Channel Velocities for 100 year Flow Conditions
100 yr Flow Conditions
(Q = 1090 CFS)
Station
Average Channel Velocity (ft/s) Difference
(Alternative - Baseline)
Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3
1686.85 8.85 9.46 9.43 9.43 0.61 0.58 0.58
1534.38 8.73 9.68 6.01 5.34 0.95 -2.72 -3.39
1426.89 3.53 5.38 3.03 2.97 1.85 -0.5 -0.56
1264.80 9.84 9.38 12.76 12.76 -0.46 2.92 2.92
983.53 9.71 4.71 9.2 9.2 -5 -0.51 -0.51
802.70 9.26 8.48 8.48 8.48 -0.78 -0.78 -0.78
705.24 9.97 9.9 9.9 9.9 -0.07 -0.07 -0.07
563.96 10.33 8.85 8.85 8.85 -1.48 -1.48 -1.48
432.04 5.03 5.18 5.17 5.17 0.15 0.14 0.14
179.28 10.98 10.5 10.5 10.5 -0.48 -0.48 -0.48
0.00 9.44 7.99 7.99 7.99 -1.45 -1.45 -1.45
4.0 DESIGN ANALYSIS AND CONCEPT PLAN
When comparing each topographic map to existing ground information, it was found that the 2001
data matched the best. However, the 2007 map provided a channel alignment that agreed with
existing conditions the best. Overall, the final channel alignment used during the design analysis
was based upon a combination of each topographic map along with aerial imagery.
Alternative 1:
Alternative 1 was developed as a single solution to prevent additional stream degradation. This
consists of placing grade control structures at key locations along the channel to create a stabilized
slope condition, as shown in Figures 2 and 3. In order to determine an appropriate stable slope for
Stream G6 the Blackland Prairie (Appendix B) and GED No. 54 (Watson 1999) slope methods were
applied to this stream. Once calculated, the two were then averaged producing a stable slope of
0.45%. Each drop structure will provide a hard control point in the stream keeping the channel
bottom from incising or eroding while providing a stabilized channel slope. Due to dynamic stream
conditions at this site each grade control structure must be designed in a way that does not create
adverse impacts to the upstream, downstream or adjacent property owners. Figure 4 shows a
profile view of Stream G6 with a conceptual design of the size and location of all proposed drop
structures. The opinion of probable cost for this alternative is $172,190.
Figure 2:
Alternativ
e 1 Site
Overview
Figure 3:
Alternati
ve 1 Site
Overview
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Figure 4:
Alternati
ve 1
Stream
Profile
Stream Stabilization Report 12
Alternative 2:
Alternative 2 includes all components from in Alternative 1 along with the addition of gabion walls
in two reaches. These four gabion walls have been proposed at key locations where the existing
banks have eroded to steep and near vertical slopes. The first structure will begin at lot 5 and end
at lot 6 of the Northlake Woodlands East, Phase 3 neighborhood, Station 11+04 to 13+17. As shown
in Figures 5 and 6, this proposed structure consists of two gabion gravity walls separated by a
gabion mattress to provide channel protection. The second proposed gabion structure will run from
Lot 6 to Lot 7 of Northlake Woodlands East, Phase 3, Station 09+03 to 10+02. This proposed
alternative will included a gravity wall on the southwest side of the stream along with a gabion wall
on the northeastern side, as shown in Figures 5 and 7. Due to the height and weight of this second
proposed wall, tie-backs will be required at two tiers along the back of the northeastern wall. Each
tie-back will be placed at approximately 10-foot intervals along the wall requiring 20 individual tie-
backs for the completed wall. Figure 8 shows typical cross sections for both gabion structures and
Figure 9 shows the stream profile. The opinion of probable cost for this alternative is $434,360.
Alternative 3:
Alternative 3 includes all components from in Alternative 2 along with a slope lay back from the
channel centerline through the right over bank facing downstream. This slope was cut at 4:1 (H:V)
to provide a stable bank slope and aims to provide flooding relief and reduced velocities at the
upstream end of the site. The slope lay back begins at station 16+00 and continues downstream to
station 14+13. Refer to Figure 2 for stationing reference. The opinion of probable cost for this
alternative is $455,360.
Alternative 4:
Alternative 4 consist of lining approximately 2,050-feet of channel with 18-inch think rock riprap
from the existing gabions upstream of Bethel Rd. to the existing rock riprap downstream of the S.
Moore Rd. culvert at station 16+34. No other channel improvements or bank stabilization
improvements are included in this alternative. The opinion of probable cost for this alternative is
$447,890.
Summary:
Table 5 compares the opinion of probable cost for the 4 alternatives evaluated. Opinion of probable
cost calculations do not include engineering and permitting costs and are provided in Appendix C.
Table 5: Opinion of Probable Cost for each Proposed Alternative
Proposed Design Total Price
Alternative 1 $172,190
Alternative 2 $434,360
Alternative 3 $455,360
Alternative 4 $447,890
Stream Stabilization Report 13
Figure 5:
Alternati
ve 2 Site
Overview
Stream Stabilization Report 14
Figure 6:
Alternativ
e 2
Gabion
Structure
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Figure 7:
Alternativ
e 2
Gabion
Structure
Stream Stabilization Report 16
Figure 8:
Alternati
ve 2
Typical
Cross
Section
Stream Stabilization Report 17
Figure 9:
Alternati
ve 2
Stream
Profile
Stream Stabilization Report 18
5.0 PERMITTING
In order to determine the status of Stream G6 as a part of the waters of the United States or an
area prone to wetland development, Integrated Environmental Solutions, LLC (IES) was contracted
by NDM to perform an assessment of the study site. This included evaluating U.S. Geological Survey
(USGS) topographic maps, Soil Survey of Dallas County and the Natural Resources Conservation
Service (NRCS) digital soil databases, the Federal Emergency Management Agency (FEMA) Flood
Insurance Rate Map (FIRM) and aerial photographs of the project site. In addition IES performed a
field visit to the site and documented local information including vegetation and stream properties.
Once all information was carefully evaluated, IES determined that Stream G6 can in fact be
considered as a waters of the United States according to their professional opinion and guidelines
provided in 33 Code of Federal Regulations (CFR) 328.3. This was determined based upon the fact
that Stream G6 is a tributary to a Traditional Navigable Water (TNW). The tributary for Stream G6 is
the main drainage for the project site that empties into Grapevine Creek, which ultimately
discharges into Elm Fork Trinity River, a TNW. The complete report from IES is provided as
Appendix A.
6.0 CONCLUSION AND RECOMMENDATION
As requested by the City of Coppell, NDM has completed a conceptual design and analysis to
stabilize conditions on Stream G6. During this process NDM has collected survey information for
the study site, developed four alternative design options, produced conceptual HEC-RAS models to
assess alternatives and developed an opinion of probable cost for each alternative.
All alternatives presented in this report provide different levels of protection for Stream G6 by
applying several common stream restoration and stabilization techniques. Alternative 1 provides
protection to channel degradation using drop structures as hard control points in the stream. This
will keep the channel bottom from incising further and help to maintain a stable slope of 0.45%.
However it will not stabilize the existing steep bank slopes.
Alternative 2 will provide channel bottom protection through drop structures while also providing
channel bank slope protection with the four proposed gabion walls. Alternative 3 will provide the
benefits from Alternatives 1 and 2 with the addition of a slope lay back to provide bank stabilization
and flooding relief at the upstream end of the study site. This helps to reduce velocities at the
upstream side of the stream and provides additional conveyance.
Alternative 4 will cover the entire stream from the existing gabions upstream of Bethel Rd. to the
existing rock riprap downstream of the S. Moore Rd. culvert at station 16+34’ with 18-inches rock
riprap. This will armor the channel bottom but not stabilize the bed slope or channel banks.
Since best available data was used for this study and only a limited survey was conducted, results
from the HEC-RAS models do not represent final design values, but are intended to identify
Stream Stabilization Report 19
potential impacts from the construction of each alternative. Detailed survey and hydraulic
modeling should be used to refine the design elements such that no adverse impacts result from
the final design.
NDM recommends the City of Coppell to consider moving forward with Alternative 3. This design
will provide the greatest protection and help reduce future damages to individual properties and
City utilities.
7.0 References
Halff Associates, Inc., Albert H. City-Wide Storm Water Management Study. Rep. 1991. Print.
Watson, CHESTER C., and DAVID S. Biedenharn. "Design and effectiveness of grade control
structures in incised river channels of north Mississippi, USA." Incised River Channels: Processes,
Forms, Engineering, and Management, SE Darby and A. Simons (Editors). John Wiley and Sons,
West Sussex, United Kingdom (1999): 432.