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ST9401-CS 871111~)ANNENBAUM ENGINEERING CORPORATION Novem~,e~ ~1. ]987 Mr. Wayne Ginn Glen, Inc. Consulting Engineers 17103 Preston Road, Suite 100 Dallas, Texas 752a8 Re: Denton Tap Road at Denton Creek Dear Wayne: As you are aware, we are cu~e~Y preparing construction plans for che widening of the existing Denton -a2 Road bridge across Denton Cree~. 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 NEC-1 model. The lO0-year water surface level immediately upstream of Denton Tap Road, for a ~ischarge of 19,730 cfs, was shown to be elevation 458.58 (see Figure 2-5, 1986 report), which is approximately feet below che existing low steel of cne 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 t~at he had resCud~ed %he discharges on Denton Creek. He ~ad de%ermined that an analysis based or, USGS gage records fo~ the gaging stacion at SH121 should De used %o compute the lO0-year discharges along Denton Creek. With this type of analysis, the lO0-year discharge increased from tr~e i9,730 cfs value previously determ!nee, to 27,430 cf$. The ]O0-sear water surface level (~esign oase flooo', upstream of the bridge increase¢ elevation ~62.9~, With the elevaZ:c~, the bridge would not nave the required freePoard and wouie nave to De re~ aced to meet %~e requirements ol the new city ordinance. I have reviewe~ the methods ~sed by Mr. Anderson to develop the higher discharges. The method utilized hqstoric USGS gaged stream flows at SH !21 to predict discharges for specific ~?~uencies. The historic flows in Denton Creek have been regulated by a ~r~s of Engineers flood control reservoir, Lake Grapevine. since ]952. ~' ~aged flows at SH 12~ subsequent to 1952 Mr. 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 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 !948 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-J952 flows were modified to account for the effect of the lake, as they should be, the lO0-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 requlrec 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 H£C-1 model be adopted. We also request that new water surface profiles be computed using the revised discharges. We recuest 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:O~055 2201-01 Sincerely, DA~N~NBAUM ENGINEERING CORPORATION Phil Oeaton, P.E. P~ojecz Manager cc: Mr. Tom Tanner DENTON TAP ROAD