Northlake WE P1-6-CS 900214Kimley. Horn and Associates, Inc. 12660 Colt Road, Suits 200, Dallas, Texas 75251 · (214) 386-7007
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Mr. Russell Doyle, P.E.
City Engineer
City of Coppell
Post Office Box 478
Coppell, Texas 75019
Re: }"reliminary Engineering Drainage Study of Northlake Woodlands East
Cl~annel North of Bethel School Road Between Hawk Lane and Moore Road.
Dear Mr. Doyle:
The Northlake Woodlands Subdivision is experiencing a number of drainage
and erosion problems. These are evidenced by the photographs included in
appendix 1. The purpose of this report is to identify the probable cause
of the problems and recommend possible solutions. The study is presented
in three parts: Hydrology, Hydraulics, and Summary as follows:
Hydrolo[ly
The watershed area for this tributary is 0.293 square miles in size. A
review of the watershed (see aerial photo, appendix 2) as compared to the
city Future Land Use Map reveals that the watershed is fully developed.
The storm rainfall produces peak discharges at Bethel School Road for this
watershed as follows:
2 yr 10 yr 50 yr 100 yr 500 yr
199cfs 467cfs 708cfs 831cfs 1205cfs
* cfs = Cubic Feet Per Second
These values are the peak for the entire watershed. Values are less at
upstream points, so we have reduced values at upstream cross sections to
more accurately model the stream hydraulics.
Building client relationships since 1967
Drainage Study
City of Coppell
Feburary 14, 1990
Page 2
Hydrauli cs
Existing Condition
In order to determine the cause of the erosion and drainage problems it is
necessary 'to review the hydraulics of the stormwater runoff as it exists.
It is not necessary to model each individual lot along the drainage path to
determine the cause of the drainage and erosion problems, rather we have
determined the hydraulic conditions by using a limited number of surveyed
cross sections to model the flow. The results of the existing condition
model are sufficient to make preliminary determinations of the cause of the
erosion and drainage problems that exist at the site. Erosion results from
excessive velocities and sedimentation occurs when the velocity slows and
deposits the eroded soil being carried along with the flow. Velocities in
excess of 8 feet per second would probably result in erosive conditions on
earthen channels. The model indicates velocities through the channels range
up to 10.4 feet per second, clearly erosive.
The floodplain levels predicted by the model do not appear to be a
significant problem. Although we do not have the finished floor slab
elevations, it appears from the information available that the floor slabs
are well above flood levels.- The 100 year .flood levels in the subdivision
range from a high of 487.3 at Moore Road to a low of 469.3 at Bethel School
Road. These flood elevations are based on the preliminary results of this
study, and need to be refined for the final design.
The one case where the model indicated flooding as a potential problem was
at the Bethel School Road culvert. With the existing ground elevations of
the channel the roadway above the culvert is overtopped in the 100 year
flood by about one half foot. This flooding condition would continue to
worsen as sediment fills the culvert.
Drainage Study
City of Coppell
Feburary 14, 1990
Page 3
Since 'Lhe erosion and sedimentation problems are caused by high velocities,
correcting these problems involves reducing the velocity by either
designing a wider channel, reducing the channel slope, or lining the
channel. (See appendix 3 for the improved condition 100 year floodplain)
.Improvements
It is beyond the scope of this study to analyze individual retaining walls,
and channel treatments. Throughout the reach of this channel individuals
have attempted to correct erosion problems by building retaining walls of
concrete, and railroad ties. In one case we even discovered a "homemade"
concrete lining in the channel bottom (see picture number 16 in appendix
1). Those individual methods of correcting problems without engineering
guidelines often causes additional problems elsewhere.
This report investigated two general solutions to this channel area.
The first is to size a grass-lined channel that will convey the flows at a
non-erosive velocity. Such a channel could be grass-lined with a shallow
3:1 side slope. The second is to use a drop structure to reduce the slope,
thereby reducing the velocities.
Alternative 1: Grass-Lined Channel
A grass-lined channel could be constructed along the channel
generally following the alignment shown in the subdivision construction
plans on file with the city. This channel would have 3:1 side slopes and a
topwidth as shown in appendix 3. This channel would convey flood waters at
a velocity of less than 8 feet per second. The topwidth of the channel
varies from 30 feet near Moore Road to about 60 feet near Bethel School
Road.
Drainage Study
City of Coppell
Feburary 14, 1990
Page 4
As an alternative to the channel above an enclosed concrete drainage box
could be constructed that would accommodate the stormwater flow for the
first four lots upstream from Bethel School Road. This would reduce the 60
foot channel through this area to approximately 20 to 30 feet.
Alternative 2: Drop Structure
A three foot drop structure could be constructed near Moore Road that would
lower the flow line and reduce the velocity. This drop structure would
allow a slightly smaller channel (a few feet). Drop stuctures also allow
more latitude in working the channel around trees and structures.
Summa
This report has addressed the erosion, sedimentation and flooding
considerations. However, it is possible that much of the soil slippage and
other problems are a result of improper compaction, or exceeding the soil
slope stability. Before any construction improvements are made additional
information and design is required. It is possible to construct retaining
walls aad still maintain an adequate channel. However, this cannot be
done without careful consideration of the effects on upstream and
downstream channel conditions. ..This must be done as part of the final
engineering before any improvements could be constructed.
As mentioned, Bethel School Road is flooded by about 1/2 foot at the 100
year event. An additional box equal to the existing box at this culvert
would eliminate this flooding. We are available to discuss this report
further.
Drainage Study
City of Coppell
Feburary 14, 1990
Page 5
If you need additional information, please call me.
Sincerely,
KIMLEY-HORN AND ASSOCIATES, INC.
Ronald W. Morrison, P.E.
Senior Hydrologist
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Kimley. Horn
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12660 Coif Road, Suite 200
Dallas, Texas 75251