Parks Coppell ABQ-CS 880916 GINN, INC. CONSI LTING ENGINEERS
September 16, 1988
Mr. Russell Doyle, P.E. S6[~2~ ~
City Engineer
City of Coppell
P.O. Box 478
Coppell, TX 75019
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 time.
If we can be of further assistance, please call.
Sincerg~
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 118 · Dallas, Texas 75248 · Phone 214/248-4900
t '
DANNENBAUM ENGINEERING CORPORATION L ~ ~ ~ /
79~ ELMBRO~ DRIVE. SUITE 2~ DALLAS, TEXAS 75247 (214) ~8-0145
~ay 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:
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
Phil Deaton, P.E.
Project Manager
PD:cr:O850PD
2201-01
DANNENBAU/VI, ENGINEERING CORPORATION
November '1, ]987
Mr. Wayne Ginn
Ginn. Inc.
Consulting Engineers
17103 Preston Road. Suite 100
Dallas, Texas 75248
Re: Denton lap Road at Denton Creek
Dear Wayne:
As you are aware, we are cu~v preparing construction plans for the
widening of the existing Denton a; Road bridge across Denton Creex. As a
result of the recently adopted Soppell Flood Plain Ordinance, we have Peen
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 immediacely
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
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 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 station
at SH121 should be used to compute the lO0-year discharges along Denton
Creek. With this type of analysis, the lO0-year discharge increased from ~he
i9.730 cfs value previously dete~qned, to 27,430 cfs. The l$O-yeaF water
surface level (design base ilooo', upstream of the brioge increasec to
elevation ~62.96. With the elevat:c~,, tme bridge would not have the required
freeboard and would nave to be re: aced to meet the requirements o~ tr~e
city ordinance.
1 have reviewed the methods used by Mr. Anderson to develop the higher
discharges. The method utilized h~$teric USGS gaged stream flows at SH !21 to
predict discharges for specific ~--ecuencies. lhe h~storic flows in Denton
Creek have been regulated by a ~:?;,s of Engineers flood control reservoir.
Late Grapevine. since ]952. a~ ~;aged flows at SH ]2] subseq~ent to 1952
Wayne Ginn
ar ]], 1987
Page two
reflect the attenuating effect that the lake has on flows in the creek. Gaged
flows prior to ]952 do not reflect the effect that the lake has on creek
flows. In order to determine what flows would have occurred prior to 1952, 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. 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-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.
DAN,~;£NBAUM ENGINEERING CORPORATION
Phil L~eaton, P.[.
Pr'o~ ect Manager
PD:cr:04055
2201-01
cc: Mr. Tom Tanner