ST8601-CS 900821 (2)IKimley. Horn J
Kimley-Horn and Associates, Inc.
ENGINEERS · PLANNERS · SURVEYORS
12660 Coit Road, Suite 200 Dallas, Texas 75251
214386-7007 Facsimile 214 239-3820 ~;~/~,
August 21, 1990
Ms. Shohre Daneshmand
Engineer
City of Coppell
Post Office Box 478
Coppell, Texas 75019
Re: Denton Creek Split Flow Analysis at the McArthur Boulevard Bridge
Dear Ms. Daneshmand:
As we have discussed, this project was undertaken to correct the floodway at the proposed
McArthur Boulevard bridge. In 1985 Dannenbaum submitted a proposed revision to FEMA
that redefined the floodway in this area. Their floodway was along the banks of the Vista
Ridge channel. This left Denton Creek outside the floodway. According to the old city
ordinance (#219) Denton Creek could be filled completely. At present, Dannenbaum (Carter
and Burgess) is completing MacArthur Boulevard under the requirements of the old
ordinance, and Dan Dowdey and Associates has requested permission to develop in the same
area, again under the old ordinance.
The present floodway analysis, which is currently being reviewed by FEMA, has the floodway
limits set at the south bank of Denton Creek on one side and the north bank of the Vista
Ridge channel on the other side. This places a large area of Vista Ridge property in the
floodway, and places the present design for MacArthur Boulevard in question. Because of
this, we were asked to redefine the floodway to eliminate this problem.
After discussions with Phil Deaton of Carter and Burgess and John Karlsruher of Ginn
Engineers, it was decided that a split floodway with the limits set along the top of the bank
of Denton Creek and the Vista Ridge channel would solve the problem. Contact was made
with FEMA to verify that this floodway concept would be acceptable. As a result, we
developed the floodway revision shown in the attached draft letter to FEMA.
It is our belief that this split floodway would not cause problems with the proposed Lake Park
Addition or the proposed MacArthur Boulevard extension. One item of concern to us, which
we have discussed with Phil Deaton of Carter and Burgess is that of modification to the
Denton Creek floodway channel under MacArthur Boulevard. According to FEMA
regulations, work in the floodway cannot cause any increase in flood levels. The MacArthur
Boulevard bridge and channel do not cause any increase; however, the roadway embankment
in the floodplain does cause a significent rise in flood levels. Mr. Deaton believes that FEMA
will look at the bridge and roadway as separate projects. If this is true, there is no problem.
However, if FEMA considers this as one project they may not approve the increases. We may
need to discuss this with Phil Deaton and FEMA.
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8- 90/FEMACopl.rwm/919 I. 15
Ms. Shohre Daneshmand
2 August 21, 1990
We suggest that you send a letter similar to the attached draft letter with attachments to
FEMA. If we can provide further information please call me.
Sincerely,
KIMLEY-HORN AND ASSOCIATES, INC.
Ronald W. Morrison, P.E.
Senior Hydrologist
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Attachment
8-90/FEMACopl.rwm/9191.15
DRAFT LETTER
August 21, 1990
Mr. Matthew Miller, P.E.
Risk Studies Division
Federal Insurance Administration
Federal Emergency Management Agency
500 C Street, SW
Washington, D.C. 20472
Re:
Coppell Revised Flood Insurance Study
Case No. 6 - 89 -236
Dear Mr. Miller:
This letter is in regard to the ongoing review of the City of Coppell Flood Insurance Study.
During conservations with Dannenbaum Engineers Inc. (now Carter and Burgess Engineers,
Inc.) we discovered that there is a particular area of concern along Denton Creek.
Dannenbaum had obtained a revision of an area in Lewisville adjacent to Coppell along
Denton Creek. Our Coppell analysis floodplain contained these changes. However,
Dannenbaum's model and revision showed Denton Creek outside of the floodway. This would
allow filling of Denton Creek, which is not keeping with the desires of the City of Coppell.
To correct this matter we have completed a revised floodway model. This model incorporates
a split flow floodway. One side of the floodway matches the Dannenbaum channel while the
other side follows the banks of Denton Creek, leaving an island between the two areas. We
have completed a separate floodway model through this area. Since it is not possible to model
a continuous floodway model with this split flow, we have completed four models to represent
this split flow area. They are:
1. Floodway model downstream of the floodway split. (Attachment 1)
Floodway model from Cross Section 223+70 (start of split flow) through the
Dannenbaum (Lewisville side) floodway, ending at Cross Section 255+30 (end of
the split flow). (Attachment 2)
Floodway model from Cross Section 223+70 (start of split flow) through Denton
Creek (Coppell side) ending at Cross Section 255+30 (end of the split flow).
(Attachment 3)
Mr. Matthew Miller, P.E. - 2 - August 21, 1990
4. Floodway model from the split flow upstream to the end of the floodway model.
(Attachment 4)
We have included the models, floodway delineations, and other details of the hydraulic models
to support this modification to our origional submittal. Attachment 5 contains the present
floodway delineation and proposed modification.
A summary of the results of the floodway analysis is included in Table 1 on the following
page.
On behalf of the City of Coppell we request that you incorporate this floodway refinement
in the Flood Insurance Study revision you are presently reviewing. Please call if you have
questions.
Sincerely,
K1MLEY-HORN AND ASSOCIATES, INC.
Ronald W. Morrison, P.E. ~
Senior Hydrologist
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Attachment
8-90/FEMACopl.rwm/9191.15