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REPLY
DATE
SIGNED
TOPS ®FORM 3601 urHa iN us.a.
SENDER ... RETAIN THIS COPY
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GI'~T~, I~'C.
September 16, 1988
Mr. Russell Doyle, P.E.
City Engineer
City of Coppell
P.O. Box 478
Coppell, TX 75019
CONSI'L'I'I~(~ E1GI'~EI~:RS
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Re: MacArthur Blvd. Extension across Denton Creek
Dear Russell:
This letter is in response to your Rapid Letter, dated September
9, 1988, copy attached. Our recent conversations with Mr.
Deaton, of Dannenbaum Engineering Corporation, indicate that they
will now accept the higher discharges and design base flood
elevations for Denton Creek at Denton Tap Road. Their intent is
to completely replace the existing bridge over Denton Creek at
Denton Tap Road and design it to the current requirements of the
City of Coppell Floodplain Management Ordinance.
Additionally, since there has been no Conditional Letter of Map
Revision issued for the proposed bridge improvements prior to
enactment of the Floodplain Management Ordinance, there will be
no grandfathering of the proposed Denton Tap Bridge structure at
Denton Creek.
The ABQ property along Cottonwood Branch, west of and adjacent to
Denton Tap Road, has been "grandfathered", with respect to the
current Floodplain Management Ordinance.
We have not received any plans of the drainge facilities for the
MacArthur Boulevard extension as described in the attached and
must assume that you are not seeking our comments pertaining
thereto at this tire.
If we can be of further assistance, please call.
Sincere
~/---
H. Wayne Ginn, P.E.
JCK/HWG/dsp
cc: Frank Trando~/
Shohre Daneshmand
John C. Karlsruher
Kevin Peiffer
File 241-044
17103 Preston Road • Suite 100 • LB ] 18 • Dallas. Texas 75248 • Phone 214/248-4900
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DANNENBAUM ENGINEERING CORPORATION f ~ ~ ~ ~ ~ ~
" 7950 EIMBROOK DRIVE, SUITE 750 DALLAS, TEXAS 75717 (111) 6380115
May 2, 1988
Ms. Shohre Daneshmand
City of Coppell
P.O. Box 478
Coppell, Texas 75019
Re: MacArthur Blvd. Extension Across Denton Creek
Dear Ms. Daneshmand:
~~ 11
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As you requested in our recent meeting I am providing you with a drainage
area map for the proposed drainage facilities along the MacArthur Blvd.
extension across Denton Creek north of DeForest Lane. I have also inclosed
preliminary profiles of the proposed drainage structures.
At our meeting we also discussed the hydraulic studies prepared for the
City of Coppell by Anderson Engineers, Inc. Attached herewith for your review
is a copy of a letter I sent to Mr. Wayne Ginn concerning a problem I have
with the discharges used in the study.
If you have questions concerning this letter, please call.
Sincerely,
DANNENBAUM ENGINEERING CORPORATION
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Phil Denton, P.E.
Project Manager
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2201-01
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DANNENBAUM ENGINEERING CORPORATION
Io50 ELMPROOK DRrv[_SU'*! 7`~. D/~1 lAS TEXAS 'S )~' f71e~6380u5
November :1, 1987
Mr. Wayne Ginn
Ginn. Inc.
Consulting Engineers
17103 Preston Road, Suite 100
Dallas, Texas 75248
Re: Denton Tap Road at Denton Creel.
Dear Wayne:
As you are aware. we are cu--=-_'y preparing construction plans for .he
widening of the existing Denton _a~ Road bridge across Denton Creep:. t.s a
result of the recently adopted Coppell Flood Plain Ordinance, we have been
reevaluating the hydraulic adequacy of the existing bridge. Our initial
evaluation was based en information contained in the 1986 report titled Denton
Creek Floodplain Stud_y. prepared for the City of Coppell by Anderson
Engineers, loc. In this report it was recommended that design discharges be
computed using the NEC-1 computer model assuming a fully developed watershed.
The water surface profiles presented in the report were based on discharges
computed using the HEC-1 model. The 100-year water surface level immediately
upstream of Denton Tap Road, for a discharge of 19.730 cfs, was shown to be
elevation 458.58 (see Figure 2-5, 1986 report), which is approximately C.4
feet below Lhe existing low steel of the bridge.
During our reevaluation I called Mr. Carl Anderson to be sure that the
information contained in the 1986 report had not been revised. I was informed
by Mr. Anderson that he had restudied the discharges on Denton Creek. He had
determined that an analysis based on USGS gage records for the gaging stat~~on
at SH121 should be used to compute the 100-year discharges along Denton
Creek. With this type of analysis, tree 100-year discharge increased from the
19,730 cfs value previously deter:ir,ed, to 27,430 cfs. Tree 1;.C-year water
surface level (design base flooa'I upstream of the bridge increasec to
elevation 467_.96. With the elevat~c.-,. the bridge would not have the required
freeboard and would have to be r~;.'aced to meet the requirements of tree r,~w
city ordinance.
1 have reviewed the methods used by Mr. Anderson to develop the higher
discharges. The method utilized historic USGS gaged stream flows at SH 121 to
predict discharges for specific ~-eeuencies. The historic flows in Denton
Creek have been regulated by a .."~~,~s of Engineers flood control reservoir,
lake Grapevine, since 195. a' ::aged flows at SH l2] subsequent tc :952
. „c$
~,~~^ r. Wayne Ginn
November 1l. 1987
Page two
reflect the attenuating effect that the lake has on
flows prior to 1952 do not reflect the effect
flows. In order to determine what flows would have
the lake had existed at that time, a routing
required. This routing would undoubtedly show that
the gage would have been significantly less i` the 1
flows in the creek. Gaged
that the lake has on creek
occurred prior to 1952. if
through the lake would be
the pre-1952 discharges at
ake had been in place.
[t so happens that for the gage period 1948 through 1984 which was used by
Mr. Anderson in his analysis. the highest, third highest, and fourth highest
historical flows occurred prior to 1952. in his analysis, these flows were
not modified to reflect the effect that Lake Grapevine would have on them.
Use of the unmodified flows has resulted in an artificially high prediction of
100-year discharges. I believe that if the historic pre-1952 flows were
modified to account for the effect of the lake, as they should be, the
100-year discharge would be significantly lower. This lower discharge would
result in a lower 100-year water surface profile in Denton Creek. The
existing Centon lap Road bridge would then have the required freeboard over
this lowered profile.
We hereby request that the 100-year discharges be recomputed using gage
records modified to reflect the effect of Lake Grapevine, or that discharges
.~ based on the NEC-1 model be adopted. We also request that new water surface
profiles be computed using the revised discharges.
We request your earliest assistance in resolving this matter. We cannot
continue with the design of the Denton Tap Road bridge until we are confident
of the discharge and water surface levels which the City will use in reviewing
our design. Any revised information should also be provided to FEMA. Please
call me if you have any questions or wish to discuss this matter further.
Sincerely,
DA~:P:E~;BAUM ENGINEERING CORPOP,ATION
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Fh-;~.1 Denton, P.c.
Pro~pc? Manaoer
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2201-01
cc: Mr. Tom Tanner