Windsor Estates-CS 990903i DEPARTMENT OF THE ARMY
FORT WORTH DISTRICT, CORPS OF ENGINEERS
P.O. BOX 17300
FORT WORTH, TEXAS 76102-0300
September 3, 1999
REPLY TO ~
ATT=%TION OF ~,
Hydrology/Hydraulics Section ." .?...
Coppell Department of Public Works
Floodplain/CDC Administrator
Kenneth M. Griffin
P.O. Box 478
Coppell, Texas 75019
Dear Mr. Griffin:
My staff has reviewed the Corridor Development Certification application for Windsor Estates on Denton Creek
for the proposed landfill and housing development. Thc followSng comments are pro~Sdcd for your information:
I. An FJEC-P,.AS backwater nm was performed for improved conditions reflecting a split in flow with water
leaving the study reach and not continuing down stream along its natural course.
2. The existing conditions HEC-RAS backwater nm for the same set of conditions was not performed with
split flow discharges; this made it useless for comparison purposes.
3. Two I-IEC-2 models, EXWE4CDC.DAT (existing condition) and PRWE4CDC.DAT (proposed condition)
were used for comparison of valley storage and water surface elevation changes by Halff Associates, both
including split flow discharges. It was these models that were used in this evaluation.
4. The study reach took place between sections 5 and 14, with split flow weir analysis between sections I. 1
and 13. The HEC-2 analysis showed a 4.38 % decrease in valley storage for the 100-year event flood
between sections 5 and 14. The water surface elevation over the study reach ckopped from 0.01 foot to 0.07
foot for this event.
5. In order to determine ifthere was inducement of water surface elevation increase downstream of the study,
new hydrology would have to be examined as part of the proposed model first, before any such judgement
could be conclusive. There was a fa/lure to keep the 100-year reduction in storage at 0 % and this implied
the existence of a possible inducement.
6. Upstream of section 14 in fids stud)' on Denton Creek, the HEC-2 models showed a drop in storage with a
corresponding decrease in water surface elevation for the 100-year event flood when comparing the
proposed to the existing model; thc reduction upstream of thc stud)' was 1.96 %. The water surface also
was lowered from 0.04 foot to 0.03 ioot.
7. No computer rtms were made for thc SPF.
For further information, please contact Mr. Michael Collier, Hydrology/Hydraulics Section, at (817) 978-2221,
e,,ctension 1692.
Sincerely,
~ Darrell R. Alverson, P.E.
' Chief, Design Branch
Ill 8616 NOFITHWEST PLAZa DRIVE
Halff Associate
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September 17. 1999
AVO 17669
Mr. Kenneth M. Griffin
Floodplain/CDC Administrator
Coppell Department of Public Works
P.O. Box 478
Coppell, Texas 75019
RE: Corridor Development Certification application [or Windsor Estates
Dear Mr. Griffin:
Halff Associates has reviewed the comments made by the Corps of Engineers on the above
rererenced project and would like to make the following response:
1. This project is on Denton Creek ,a tributary of the Elm Fork-Trinity River. It falls into the
CDC Regulatory Zone only because The Elm Fork 100-year event will pond flood waters on
Denton Creek adjacent to this site. The 100-year flood plain in the area is defined by floods
originating on Denton Creek rather than the Elm Fork. All Denton Creek backwater models
(FEMA and City of Coppell) are HEC-2 format. Therel~ore no HEC-RAS modeling was
performed. In addition. HEC-RAS could not be used duc to its lack of a split flow by weir
option. HEC-2 must be used because of its abilit3 to accurately determine split I'low
discharges caused by Denton Creek flood waters overtopping the Dallas Gun Club road.
That portion of Denton Creek 100-year flood waters will combine with Elm Fork flood
waters, rather than rejoin Denton Creek.
2. Split flow discharges were established using the revised existing conditions HEC-2 model.
These discharges were held for proposed conditions. This seems reasonable in light of the
accuracy of the hydrolo$ic estimates and allows for a more direct determination of proposed
project hydraulic impacts.
3. HEC-2 models were used only for water surface elevation comparisons.
4. The valley storage numbers provided by the Corps are accurate based on model results alone
but ignore storage in areas which are not effective. It' the non-effective flow areas are
included in the valley storage calculations, the project reflects an actual gain in valley storage.
: . For example, on the right overbank between cross-sections 6 and 9, there is a pond in the
non-effective flow area. During a 100-year flood c,,ent. 14.1 acre-feet of valley storage
occurs in this area that the hydraulic model does not account for. This is true for the area on
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Mr. Kenneth M. Griffin
Coppell Department of Public Works
September 17, 1999
Page '~
the right overbank between cross-sections 14 and 20980. We calculate that there is
approximately 9.7 acre-ft of valley storage in this non-effective flow area that is not included
in the HEC-2 model. In all. there is approximately '~''
, _.,,.8 acre-ft of valley storage that the
HEC-2 model does not account for which more than offsets the 16.9 acre-ft loss mentioned
by Corps. Our valley., storage calculations are based on standard embankment computation
techniques which provide more accurate calculations of valley storage than those based on
HEC-2 or HEC-RAS models in our opinion.
5. As discussed above, valley storage for the 100-year event is actually increased by on the
proposed project theretbre exceeding CDC requirements for 100-year flood valley storage.
6. As discussed in 4 above, there is no loss in valley storage for the 100-year flood event caused
by the proposed project when non effective flow areas are included.
7. A Standard Project Flood (SPF) discharge tbr this reach of Denton Creek could not be found
in existing documentation. Craig Lofton with the Corps of Engineers stated in a conversation
in May 1999 that the old SPF (1978) for Denton Creek x~'as 36500. The 500 year flood
discharge is 36200 and was substituted for the SPF with thc approval of the City Engineer of
Coppell.
If you have any more questions, or comments, please contact mc at (214) 346-6220.
Sincerely,
HA LFF ASSOCIATES, INC.
Waher Skipwith
Vice-President
cc: Kelly Jordan