20190304132823244DENTON CREEK DRAINAGE STUDY
CITY OF COPPELL, TX
STUDY AREA
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Drainage Area:
At Denton Tap = 12.3 sq. mi.
Cottonwood Creek = 6.1 sq. mi.
D/S Limit of DCLID = 22.3 sq. mi.
Total = 25.4 sq. mi.
HALFF ASSOCIATES, INC.
Oty 0 Lewisville.
Tinein of
Flower Mound
Upstream Study Umit Oenton County Lrvee
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• Analyze the current conon of the of the channel through visual survey
• Conduct a stability analysis
• Assess stable channel configurations
• Assess sediment contribution and source of sediment
• Physically measure erodibility of banks
• Assess incipient motion of bed material
■■■ HALFF ASSOCIATES, INC.
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■■■ HALFF ASSOCIATES, INC.
12/6/2017
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COMPARISON WIDTHS UPSTREAM & DOWNSTREAM
OF DENTON TAP
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Mean Movement=
1.56 fUday.
sid oev. = i. i s
Minimum=0.47 fl/day
Maximum= 3.375 Way
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��� HALFF ASSOCIATES, INC.
• Sand is suspended less than 6 -
feet with larger flows
• Approximately 7.5% of the sand
is deposited within the DCLID-1
• Minimum 35 -years of deposition
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tGR Plans —t Creek Survey t Flood Control Channel Survey
• Preliminary Erosion Hazard Zone (EHZ)
• Estimated 41 homes in EHZ
• 21.5 protected
• 15 toe erosion/slope problems
• Four (4) no erosion currently
• Further geotechnical investigation needed
to itemize
■■■ HALFF AssoclnTEs, INc.
Summary: Erosion
Corridor Evaluation
-EHZ -
12/6/2017
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• Stable Channel Design Recommendations:
• Bottom Width = 47-53 feet
• Depth = 5-6 feet
• Side Slope = 3H:1V
• Channel Slope = 0.0004 ft/ft (1 ft drop in elevation over 2200 feet)
• Continued downcutting and widening of channel upstream of Denton
Tap expected
• Preliminary EHZ
HALFF ASSOCIATES, INC.
• USACE HEC -HMS model updated to
reflect current lag times, curve
numbers, and drainage areas due to
storm sewer
• Results in 300-1,000 cfs lower
discharges for the 100 -year through
DCLID compared to FEMA and 800-
1,700 cfs lower than USACE
• Preliminary USACE 500 -year flow
increased from 36,200 cfs to 55,500
cfs
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HALFF ASSOCIATES, INC.
17892
A roximate Beginning
of Levee
Approximately 3,300 feet
downstream of MacArthur
18826
20,600
22,000
20,300
Boulevard
MacArthur Boulevard
24519
Approximate End of Levee
Approximately 3,769 feet
downstream of Denton Tap
25499
21,300
21,000
20,200
Road
Approximately 2,500 feet
downstream of Denton Tap
26688
21,300
21,000
13,100
Road
Approximately 1,000 feet
downstream of Denton Tap
28217
21,300
13,500
13,000
Road
Denton Tap Road
12/6/2017
•1r11�V1�VI1[•
• USACE HEC -RAS model updated/modified with survey and updated
topography
• Three hydraulic models developed for study:
• Steady-state to compare to FEMA and USACE
• Unsteady state to incorporate Elm Fork Trinity River backwater effects
• 2 -Dimensional to evaluate low flows and velocities
■■■ HALFF ASSOCIATES. INC.
• Releases from Lake Grapevine and Lake
Lewisville for the May 2015 were used
to calibrate hydraulic models
• 2 USGS Gages
• SH 121 at Denton Creek
• Carrollton Dam on the Elm of the Trinity
River (below Denton Creek Confluence)
• Modeled releases from Grapevine and
Lewisville Lake based on USGS gage
data with 10 -year frequency
• Models reproduced flooding in
Wellington Place Apartments
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HALFF ASSOCIATES, INC.
•2 -year discharge
"10 -year plus releases from Lewisville
• Gage datum at SH 121 is approximately
4 -ft lower than surveyed flowline
(439.11 vs. 443.21)
• May 2015 storm event was reported as a
10 -year event
12/6/2017
HYDRAULIC MODEL CALIBRATION
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EXISTING 2 YEAR VELOCITIES
Denton Creek
Sandy Lake Road
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EXISTING 2 YEAR VELOCITIES
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EXISTING 2 YEAR VELOCITIES
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EXISTING 2 YEAR VELOCITIES
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OCITIES
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EXISTING 2 YEAR VEL
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EXISTING 2 YEAR VELOCITIES
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ALTERNATIVE ANALYSIS
■■■ HALFF ASSOCIATES, INC.
• Located just downstream of Denton Tap Road
• Concrete structure recommended to prevent further
downcutting upstream of Denton Tap Road
• Would result in less sediment transported to DCLID-1
• Estimated Cost = $287,000
■■■ HALFF ASSOCIATES, INC.
• Berm constructed at upstream
junction
• Length and meander mimic existing
channel
® Utilize stable channel design
• Four (4) hard points along channel
• Banks must be protected
• Estimated Cost = $7.4 million
12/6/2017
15
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'''' STRUCTURE PROFILE
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• Located just downstream of Denton Tap Road
• Concrete structure recommended to prevent further
downcutting upstream of Denton Tap Road
• Would result in less sediment transported to DCLID-1
• Estimated Cost = $287,000
■■■ HALFF ASSOCIATES, INC.
• Berm constructed at upstream
junction
• Length and meander mimic existing
channel
® Utilize stable channel design
• Four (4) hard points along channel
• Banks must be protected
• Estimated Cost = $7.4 million
12/6/2017
15
Advantages
• Eliminates erosive velocities
• Excess excavation may be
provided to homeowners
• Increased flood storage
... HALFF ASSOCIATES, INC.
• Divert erosive flows away from bank
• Alignment and placement are key
• Fully submerged at bankfull elevation
• Total of seven (7) barbs
• Estimated Cost = $836,000
... HALFF ASSOCIATES, INC.
Disadvantages
• Expensive
• Environmenta
permitting
negotiations
and ROW
• High 0&M cost
• Stagnant water in existing channel
12/6/2017
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• 12/6/2017
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12/6/2017
12/6/2017
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