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Dec-04-O1 02:24P ~ r Mr. Jiff Scbwar~ December 4, 2001 Page S rela[cd Lo the existing location of the Bull Seep Drain is irrelevant fur the following reasrnts: It is clear that a Swale existed on lhu inboard side of the levee prior to MYM relocation of the Bull Sccp Urain, which would have conveyed breech flows into the slough T'Ite southern breach location was such that breach flows would have been directed into the Bull Seep Drain even if it was located permitted location (reference b;xhibit G-1 from Permit M-78- U52); and, Based rnt t}te remnant field evidence, which existed after the May event, it was clear that the ittajority of (low was conveyed through the northern breach. Additional Modeling Data it is believed that the north breach was first created during a hood event which occurred during July 17 and 18, 2000. LA obtained hourly data for this evcrtt (derived by establishing an average flow role for each hour from the I S-ntinttte data) frnrn the State Engineer's Office. The Itnurly data indicate a peak flow of 8390 ct's at 5:00 AM on July 17, 2000. 't'he actual peak was probably higher than 8,390 cfs given the fact that this discharge is an average hourly peak. it should be noted that there was another evert which occurred on August 17 and 18, 2000 whero the peak discharge exceeded 7,000 cfs. P-06 1b improve LA's confidence in the normal depth analyses and to examine the potential for backwater in the Bull Seep Slough, a step-backwater analysis was pertbrnted using tlrc Army Corps of Engineers IfEC- RAS model (see Attachment A for cross sections and the output files). LA utilized the IfEC-2 data deck from the South F'lalte River floodplain study performed by Gingery and Associates, 1977 es a bate fur their HEC-kAS mortal. 1'he model utilized information frnrn cross sections 30, 31, and 33. Roughness coefficients utilized fur the main channel and overbook areas were the same as the original 1977 fluvtl study. The following ntoditications were rrtade to the Gingery and Associates, 1977 HEC-2 model. Cross section 31 was modified to include the bank, which was reconstructed along the west end of the Trei pit attd borders the Bull Seep Slough. This feature did not exist in 1977. 2. 'Che 1977 flood shdy utilized air invert clcvatfon al the Branter Diversion structure of 5021.5 feet (reference Cross Section 31). The survey perfomted by Epp and Associaks indicated that the top of the Brantner Diversion structure: was SU31.ti feet. ]t appears that the original cross section was cut along the base of the Branfier Diversion structure. During the Gingery modeled 100-year event, the effect of the diversion structure on the water surface profile way ntinintal. When modeling events such as the May 5 flood event (2-year), the impact of the Brantner Diversion structure on the water surface profile is significant. To adeyuately model upstream conditions Cur art event similar to the May S event, LA created a cross section of the lop oC the Brantner Diversion Srnneture from the EA survey data and ICON mapping. 3. LA added a cross section (see cross section 32) at floc north brcacln location utilizing survey data tiom EA supplemented by 2 fool wnlvur mapping supplied by ICON Engineering. 4. LA added a moss section at the confluence of floe Soutlt Platte River and the outlet of the Bull Seep Slough (see cross section 30.5) to establish a reasnnnable approximation of the backwater