Mansions Phase 1-CS 971022I)UNAWAY ASSOCIATES. IN('.
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October 22, 1997
Mr. Ken Gdffin, P.E.
Assistant City Manager / City Engineer
City of Coppell
255 Parkway Blvd
Coppell, Texas 75019
Re: Mansions on the Lake
Floodplain Reclamation Study
Mr. Gdffin:
Thank you for sending the review letter and the bound reports for this project. I am writing to
respond to the recommendations made in the review letter and to suggest how me might
proceed.
Comment 1. FEMA effective model duplication.
We requested and received the effective model for this area from FEMA. I have attached a copy
of the transmittal note and a copy of the label on the diskette we received from FEMA' consultant.
They sent two models of Denton Creek. Both of these models were identical through the vicinity
of our project. The only difference between the models was a very slight difference in the
downstream water surface elevation. The flows, fdction factors, ground data, etc. were all
identical.
Comment 2. Proposed Condition Model
The topic of how much conveyance can be assumed in an overbank pond is a very subjective
one. It will depend on factors such as the length of the pond (parallel to flow direction) and the
depth of flow above the adjacent overbanks. In this case, our pond is about 1000' long, and the
depth of flow will be about 8' above the normal surface of the pond. These two factors will
guarantee that most, if not all, the water on the pond will be moving in the downstream direction
during the 100-year flood.
The review letter recommended we take a conservative approach in our modeling, which we have
done. Our conservative assumptions include using a friction factor for the pond area that is only
slightly reduced by the addition of the ponds. If you disallow the assumption that water in the
bottom of the pond is also moving downstream with the overbank flow, you end up assuming that
moving water is passing over the top of standing water. In that case, you would have to
2351 W. NW HWY · SUITE 3280 · DALLAS, TEXAS 75220 · (214) 654-0123 · FAX (214) 654-0122
Mr. Ken Gdffin
October 22, 1997
page 3
estimate a friction factor for water moving across water. (Which would be quite Iow) Rather than
do that, it is customary to use a higher friction factor but offset it by including most or all of the
flow area of the pond. We assigned a friction factor of 0.045 to the pond areas. This is
conservative when you consider that there is essentially no vegetation in a pond.
Another reason for using 5 - 6' of pond depth is that the original model did the same thing at the
existing pond, which we are enlarging. It used the entire pond depth for conveyance. Aisc, the
final HEC2 model for The Village at Cottonwood Creek, Phase VI, (prepared by Mike Boyd at
Nathan D. Maier Engineers) used the full depth when modeling the pond immediately
downstream of our site. We used the same methodology that was already established for the
Comment 3. FEMA Forms .
/
We will revise the forms to address/these comments. Please note that FEMA requires upstream
tie-in to be 0.5' rather than 0.05'. /
I hope that my explanations will resolve the concerns with the HEC2 models. With regard to
comments 1 and 2, I don't think they merit changing the models. I am requesting that I be
allowed to make the appropriate changes relating to comment number 3 in the review letter and
proceed with my submission to FEMA.
Please call if you have any questions or to tell me how you would like to proceed.
If you have any questions or need additional information, please call.
Cordially yours,
Dunaway Associates, Inc.
Rusty Si)liars, P.E.
Vice Pre~sident
encl.
1996 CIVIL UNIT
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