Vista Ridge 1st P-CS 871111DANNENBAUM ENGINEERING CORPORATION
November ]1, ]98?
Mr. Wayne Ginn
Gi~n. Inc.
Consulting Engineers
17103 Preston Road, Suite 100
Dallas, Texas /5248
Re: Denton Tap Road at Denton Creek
Dear Wayne:
As you are aware, we are currently preparing construction plans for the
widening of the existing Denton -a; Road bridge across Denton CreeK. 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 be
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-1 model. The lO0-year water surface level immedia=ely
upstream of Denton Tap Road, for a discharge of ]9.730 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 I called Mr. Carl Anderson to be sure that the
information contained in the 1986 reDort 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 fo~ the gaging station
at SH121 should be used to compute the lO0-year discharges along Denton
Creek. With this type of analysis, the ]O0-year discharge increased from the
i9,730 cfs value previously determined, to 27,430 cfs. The fOe-year water
surface level (design base flooo~ upstream of the bridge increasec zo
elevation 462.96. With the eleva~:cr,, [ne bridge would not have the required
freeboard and woulo nave to be reziaced to meet the requirements of lrme new
city ordinance.
I have reviewed the methods used by Mr. Anderson to develop the higher
discharges. The method utilized historic USGS gaged stream flows at SH 12l to
predict discharges for specific ~e~uencies. The historic flows in Denton
Creek have been regulated by a ;r~[,s of Engineers flood control reservoir,
La~:e Grapevine. since ]952. ~ ~aged flows at SIJ 12] subsequent to 195Z
Nayne Ginn
November 1], ]987
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 ]952. 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 ]984 which was used by
Mr. Anderson in his analysis, the highest, third highest, and fourth highest
historical flows occurred prior to ]952. 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
]O0-year discharges. I believe that if the historic pre-J952 flows were
modified to account for the effect of the lake, as they should be, the
]O0-year discharge would be significantly lower. This 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 lO0-year discharges be recomputed using gage
records modified to reflect the effect of Lake Grapevine, or that discharges
based on the HEC-I 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 Ta · · '
of the discharge and water s-~-~ , .... P.~o~d ~rldge until we are con '
our desinn ^ ......... ~ .... ~ ,~ve~s which the City w~ ...... [ld~nt
call me i~ ]-- ~"y rev~seo lntormation should also be ~r~,.~]]] Y~ ,n reviewing
~uu nave any questions or wis~ *~ ~- [. -,u~u ~o FEMA. Please
,, ~v u,scuss thlS matter further
PD:cr:O~055
2201-01
Sincerely.
DA~ENBAUM ENGINEERING CORPORATION
/ i~' //~__j ~-
~, ,. ~, ~'~/
Phf] Deaton. P.E.
Pro~ect ~anager
cc: Mr. Tom Tanner