CRDC 1402-CS150721
engineers
surveyors
landscape architects
www.tnpinc.com
July 21, 2015
Mr. Ken Griffin and Mr. Michael Garza
City of Coppell – Department of Engineering
265 Parkway Boulevard
Coppell, TX 75019
RE: Tailwater Impacts of Denton Creek on Woodridge Channel
Dear Mr. Griffin and Mr. Garza,
The purpose of this letter is to provide a discussion of the results of a sensitivity analysis performed
for the proposed Woodridge Channel project, based on increased tailwater constraints posed by
Denton Creek. The impetus for this analysis was borne by the series of flood events in late May and
early June 2015 in which several properties along the Woodridge Channel (near the crossing of
Sandy Lake Road) were inundated as a result of high water surface elevations in Denton Creek.
These high elevations were the result of weeks of rain preceding the flood event and the subsequent
discharges from the Grapevine and Lewisville reservoirs, both upstream of the project vicinity. TNP
was asked to verify what, if any, impacts the higher tailwater constraints could have on the design
of the proposed Woodridge improvements. The following paragraphs outline our understanding of
the field conditions and their impacts on the proposed Woodridge project.
It should be noted that the reach of Woodridge contracted for design improvements by TNP is
located approximately 2,270 feet upstream of the stream confluence with Denton Creek
(specifically, the TNP project reach begins approximately 750 feet upstream of MacArthur and
proceeds downstream to the downstream face of MacArthur) The subject vicinity is located on FEMA
FIRM Panel 48113C0155K. Woodridge Channel itself does not appear on the map and therefore
has no mapped Special Flood Hazard Area. However, the map does depict the floodplain extents
for Denton Creek and the Elm Fork Trinity River. This mapping indicates that several properties near
Woodridge and Sandy Lake Road are prone to 500-year flooding from Denton Creek. The 100-
year BFE in Denton Creek at the point of confluence with Woodridge is approximately 445 feet.
Based on topography along Woodridge, this elevation would result in a backwater condition
upstream in Woodridge. As conveyed to TNP during a meeting on June 15, 2015, gauge data and
field observations by City of Coppell staff indicated that flood elevations as high as 447 feet were
experienced during the June floods.
In order to understand the true impacts of the various backwater scenarios, a hydraulic model was
prepared for Woodridge Channel from the upstream end of the TNP project limits to a point
downstream of Sandy Lake Road. This model represents the existing channel geometry along the
entire reach, including within the limits of TNP’s current project. The limits of the hydraulic model can
be seen on Exhibit 1. Flows used in the model are based on the “Woodridge Channel
Improvements Concept Study” (TNP; April 2014). Four tailwater boundary conditions were
evaluated in this model, the results of which are shown on Exhibit 2:
1. Normal Depth (blue line);
2. Fixed backwater elevation based on frequency mixing (443.3 feet; 50-year tailwater in
Denton Creek; green line);
3. Fixed backwater elevation using FEMA 100-year BFE in Denton Creek (445 feet; pink line);
4. Fixed backwater elevation of 447 based on City observations (red line);
Fort Worth1100 Macon StreetFort Worth, Texas 76102817.336.5773 Dallas17304 Preston Road, Suite 1340Dallas, Texas 75252214.461.9867
Denton1517 Centre Place Drive, Suite 320Denton, Texas 76205940.383.4177 Sherman200 North Travis Street, Suite 500Sherman, Texas 75090903.870.1089
Registered by the Texas Board of Professional Engineers, Firm No. F-230 Registered by the Texas Board of Professional Surveyors, Firm No. 100116-00
Based on this exhibit, it can be seen that increased tailwater elevations result in extended
backwater impacts upstream into Woodridge Channel. However, the impacts of the tailwater are
negligible beginning at the downstream side of MacArthur Boulevard. Specifically, the difference in
tailwater at the downstream side of MacArthur is 0.08 feet between Condition 3 and Condition 4
discussed above. It is our assertion that tailwater elevations up to the 447 elevation observed by
the City will have minimal impact on the functionality of the improvements currently being designed
by TNP. Furthermore, backwater impacts downstream of MacArthur cannot be mitigated without
improvements to Denton Creek. It should be noted that this analysis did not consider any tailwater
conditions other than those described herein.
Please do not hesitate to contact me if you have any questions.
Sincerely,
tnp
teague nall & perkins
Niraj A. Acharya, P.E., CFM
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Source: Esri, DigitalGlobe, GeoEye, i-cubed, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP,swisstopo, and the GIS User Community
Woodridge Channel
Denton Creek
Exhibit 1
N
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1 inch = 200 feet
Legend
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Exhibit 2