ST8402-CS 871111DANNENBAUM ENGINEERING CORPORATION
Novembe[ ]1, ]98?
Mr. Wayne Ginn
Glen. Inc.
Consulting Engineers
17103 Preston Road, Suite
Dallas, Texas /5248
Re: Denton Tap Road at Denton Creek
Oear Wayne:
As you are aware, we are cu've':~y preparing construction plans for :he
widening of the existing Denton 'a; Road bridge across Denton Cree~,. As 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 on information contained in the 1986 report titled Denton
Creek Floodplain Study, prepared for the City of Coppell by Anderson
Engineers, Inc. In this report it was recommended that design discharges De
computed using the HEC-1 computer model assuming a fully developed watershed.
The water surface profiles presented in the report were based on discharges
computed using the HEC-] model. The lO0-year water surface level immedia:ely
upstream of Denton Tap Road, for a discharge of 19.730 cfs. was shown to De
elevation 458.58 (see Figure 2-5, 1986 report), which is approximately :.4
feet below the existing low steel of the bridge.
During our reevaluation ! called Mr. Cart Anderson to be sure that the
information contained in the 1986 report had not been revised. I was informed
Dy Mr. Anderson that he had restudied the discharges on Denton Creek. He had
qetermined that an analysis based on USGS gage records for the gaging station
at SH121 should De used to compute the lO0-year discharges along Denton
Creek. With this type of analysis, the lO0-year discharge increased from the
i9,730 cfs value previously determined, to 27,430 cfs. The ]O0-year ~&ter
surface level (design base flooo~ upstream of the bridge increasec ko
elevation 462.96. With the elevation, the bridge would not have the required
freeboard and woula have to De re: aced to meet the requirements of the new
city ordinance.
] have reviewed the methods used by Mr. Anderson to develop the higher
discharges, The method utilized historic USGS gaged stream flows a~ SH 121 to
predict discharges for specific ~e~uencies. The historic flows in Denton
Creek have been regulated by a ~F,~,s of Engineers flood control reservoir.
Lake Grapevine. since ]957. A~ gaged flows at SI( 12! subsequent to 1952
(r. Wayne Ginn
November 11. 1987
) Page two
reflect the attenuating effect that the lake has on flows in the creek. Gaged
flows prior to 1952 do not reflect the effect that the lake has on creek
flows. In order to determine what flows would have occurred prior to 195Z. if
the lake had existed at that time. a routing through the lake would be
required. This routing would undoubtedly show that the pre-1952 discharges at
the gage would have been significantly less if the lake had been in place.
It 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
lO0-year discharges. ] believe that if the historic pre-1952 flows were
modified to account for the effect of the lake. as they should be, the
lO0-year discharge would be significantly lower, lhis lower discharge would
result in a lower lO0-year water surface profile in Denton Creek. The
existing Denton Tap Road bridge would then have the required freeboard over
this lowered profile.
We hereby request that the lDO-year discharges be recomputed using gage
records modified to reflect the effect of Lake Grapevine, or that discharges
based on the HEC-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.
PD:cr:04055
2201-01
Sincerely,
DAN~ENBAUM ENGINEERING CORPORATION
· / x
Phil Oeaton. P.E.
Project Manager
cc: Mr. Tom Tanner