Deforest Court-CS010425 HalffA ciates
8616 NORTHWEST PLAZA DRIVE
DALLAS, TEXAS 75225
SSO
FAX (214) 739-0095
City of Coppell Engineering 04 26 0 ..... L t~.~,'~ J I
255 East Parkway Blvd. - - i ~ u v: ~ v R c~ ¥ D '" -
Coppell, TX 75019 1.~ Wr'4/r°
RE: Deforest Court Development Drainage Issues 81(i~~-~ -- .~ ,qr 0 q
As you know, drainage fi.om the proposed Deforest Court development and the property ~
of Mr. Cooper located to the west will discharge into the valley storage pond located on
the Windsor Estates development. We have analyzed the effects of the 5-year and 100- ~,~
year storms with and without the installation of a flap gate on the pond discharge pipe. ItL--r~'~'
is our opinion that use of the flap gate is beneficial from a drainage and
management standpoint for the reasons stated in the following paragraph.
Installation of the above mentioned flap gate will result in an improvement in the
drainage of smaller storms from Mr. Cooper's property. An analysis of the 5-year
rainfall event without the installation of the flap gate results in a 5-year HGL elevation of
449.35 at the inlet structure proposed on Mr. Cooper's property. The installation of a flap
gate will reduce the 5-year HGL elevation to 447.00, therefore reducing the amount of 0t [~ ,t~
flooding on Cooper's property for the 5-year and similar sized flood events. An analysis
of the 10D-year event on the same drainage system results in a 100-year HGL of 452.90
with and without the installation of the flap gate. Therefore, failure of the flap gate to
work properly during the 100-year event will not result in additional flooding to Mr.
Cooper's property. ~
It is also our opinion that the installation of a flap gate will make the most efficient use of
the valley storage provided by the pond. The flap gate prevents the pond from filling as a
result of small floods or the first part of larger floods, preserving the storage for the flood
peaks, which will have a greater effect on reducing the possibility of increased
downstream flooding. In addition, Mr. Kelly Jordan has agreed to place the
responsibility of maintenance for the proposed flap gate solely on the Windsor Estates
Home Owners Association. This will reduce the City of Coppell's maintenance costs.
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Please call if you have any questions.
Sincerely,
ItALFF ASSOCIATES, INC.
Walter E. Skipwi . .
Vice President
cc: Kelly Jordan, Castellan Development