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75202
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FREESE - NICHOLS
MEMORANDUM
TO: Mike Martin, P.E.
FROM: Scott Maughn, E.I.T.
SUBJECT: Coppell Road Construction - Rock Filter Dam
DATE: May 24, 2001
On May 21, 2001, I received a call from Larry Davis about the reasoning and design impacts of a
permanent rock filter dam that was installed in the channel east of Coppell Road near
Westminster Way. Mr. Davis expressed to me residents' complaints and concerns about
standing water just upstream of the rock filter dam. He explained that the City was receiving
complaints about the standing water causing mosquito infestation and stagnant water problems in
the channel that is immediately behind homes. Mr. Davis inquired about the function and
possible removal of the filter dam.
After speaking with Alan Greer, P,E,, Project Manager, it was determined that the filter dam was
put in place to help control the amount of silt that had been previously accumulating in the
downstream box culverts, The amount of silt that was present in the downstream box culvert
prior to construction was becoming a problem, and the filter dam was designed to help control
the silt on a permanent basis. The rock filter dam was not designed as a hydraulic structure to
control the velocity of flow in the channel for high frequency storms. The concrete energy
dissipaters tlpstream of the rock filter dam are in place for hydraulic reasons to reduce the erosive
velocities on the higher frequency stoEms. The rock filter dam was installed to improve water
quality and provide sediment control for the "first flush" rainfall events (approximately the first
inch of storm water runoff). The filter dam is controlling the slit and trash deposits from causing
maintenance issues downstream and providing some velocity protection on very Iow frequency
storms.
The removal of the rock filter dam by the City for eliminating stagnant water and to please the
surrounding residents will not significantly affect the hydraulic integrity of the channel. It will
remove permanent water quality and sediment control which was a concern and was designed
into the project. The removal of the rock filter dam may eliminate ponding water but cause other
future maintenance concerns downstream of the culvert. A possible alternative would be to leave
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FR£EiSE AND NICHOLS · 1701 MARKET ST., #500, LB51 · DALLAS, TEXAS 75202.2001
TELF~PHONE: 214-920-2500 · FAX: 214-9'20-2565
~0'~ SU~qUO IN~ S£:TT TOO~-P~-AUN
Coppell Road Construction - Rock Fi:ter Dam
May 24, 2001
Page 2 of 2
the rock filter dam in place and install PVC drain pipes through the rock filter to drain the
ponding water. Please call Alan Greet or myself to discuss this alternative or any other questions
you may have,
Thank You
Scott Maughn~