Windsor Estates-CS010425 Halff A ate
mmm S S O C 1 S
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ENGINEERS · ARCHITECTS · SCIENTISTS { ~ ~ ~
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April 25, 2001 C' L t 1~- , .,i' --
Mr. Mike Mmin, P.E. XI' ¢ ~ ¢ / ~[~),,
City of Coppell Engineering ' ~-:; - C 1 -' '; · ~ ..... ~e
255 East Parkway Blvd. - .... -. - - - -- . ,.'z ~ ~ ~.~-
Coppell, TX 75019 ~
~: Deforest Cou~ Development Drainage Issues ~, ;'~ ' I.~ ~[,r.~
As you ~ow, drainage ~om the proposed Deforest Cou~ development and the prope~y 4 ' ..~,-. :-'
of Mr Coo er located to the west will dischar e into the valley storage pond located on
· p g ~ .-
the Windsor Estates ~evelo ment We have analyzed the effects of the 5-year and 100- '~ ~ ,,
year sto~s w~th ~d w~thout the ~nstallat~on of a flap gate on the pond d~scharge p~pe. It
is our opinion that use of the flap gate is beneficial from a ~ainage ~d floodplain
management standpoint for the reasons stated ~n the follow~n, para~aph.
~stallation of the above mentioned flap gate will result in ~ improvement in the . [-- t
drainage of smaller sto~s from Mr. Coopcr's propc~. An ~alvsis of the 5-ye~
rainfall event without the ]nsta]lauon of thc flap gate results in a 5-year HGL elcvatmn of · ·
449.35 at the inlet structure proposed on Mr. Cooper's prope~'. The installation of a flap
gate will reduce the 5-year HGL elevation to 447.00, therefore reducing the amount of
. ~ . . . . . .~'I~
flooding on Cooper s prope~y for the )-ye~ and szmdar s~zed flood events. ~ ~alys]s
of the 100-ye~ event on the same drainage system results in a 100-year HGL of 452.90 ~'~'
with and without thc installation of thc flap gate. Therefore, failure of the flap gate to ,.~
work properly during the ]00-ye~ event will not result in additional flooding to Mr.
' /
Cooper's prope~'. ~
It ~s also our opinion that the installation of a flap gate will make the most efficient usc of
the valley storage provided by thc pond. The flap gate prevents thc pond ~om filling as a
result of small floods or thc first pa~ of larger floods, presc~ing thc storage for thc flood
pe~s, which will have a ~cater effect on reducing the possibility of increased
downstrcm flooding, h addition, Mr. Kelly Jord~ has a~ecd to place thc
responsibility of maintcn~cc for the proposed flap gate solely on the Windsor Estates
Home Owners Association. This will reduce thc City of Coppc]l's mainten~ce costs.
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Please call if~,ou have any questions.
Sincerely,
ItALFF ASSOCIATES, INC.
Walter E. Skipwith,/P.E.
Vice President
cc: Kelly Jordan, Castellan Development