Coppell Trade-CS040725 (2) M4EATEE
ENI INEERIN6
Memorandum
DATE: July 25, 2004
TO: Ken Griffin, P.E.
City of Coppell
FROM: Dayton C. Macatee, P.E.
RE:
40.655-acre Industrial Tract
Sandy Lake Road, 200' West of Freeport Pkwy.
Preliminary Drainage Discharge Analysis
A preliminary drainage analysis was prepared for the watershed containing the referenced
40.655-acre tract. This property lies at the downstream end of a 137-acre watershed. The entire
watershed west and south (upstream) of the subject tract was modeled as fully-developed. The
total runoff for the fully-developed watershed prior to development of the subject tract is 673 cfs
and 840 cfs following development.
The subject tract is traversed by a natural stream having an average grade of 0.53%. The stream
will be realigned and channelized during development, and will have an average grade of 0.40%.
The channel improvements will transition back to the existing stream alignment beginning
approximately 250 feet south (upstream) of the north property line of the subject tract. The
transition will be completed roughly 25-50 feet upstream of the north property line. A rock
blanket will be constructed adjacent to the north property line to control stream velocity and
inhibit erosion.
Runoff calculations for conditions both before and after development of the 40.655-acre tract
were developed. Flow characteristics at the property line were modeled for both conditions.
Halff Associates' Drainage Area Map is attached for informational purposes regarding the
portion of the watershed lying upstream of the subject tract. We annotated this sheet to include
calculations for fully-developed conditions. A Site Plan for the subject tract is attached.
The results of the predevelopment model show a velocity of 3.52 fps and a stream depth of 0.97
feet at the north (downstream) property line. The results of the post-development model show a
velocity of 3.23 fps and a stream depth of 1.24 feet at the same location. The decrease in
velocity results in less erosive conditions downstream.
The results of this summary are based on the City's topographic maps ~t from an on-the-
ground survey.
Attachments: S~te Plan ~-~AYTON C. ~:?J~ ~
6440 N. Central Expressway, Suite 414 ~ Dallas, Texas 75206 ~, Tel (214) 373- ~73-6580
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