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<br /> <br />Mr. Jeff Schwarz <br />December 4, 3001 <br />Page 5 <br />related to the existing location of [he Bul! Seep Drain is irrelevant for [he following reasons: <br />• It is clear that a Swale existed on the inboard side of the levee prior to MPM relocation of the Bull <br />Seep Drain, which would have conveyed breach flows into [he slough; <br />• The southern breach location was such that breach Flows would have been directed into the Bull <br />Seep Drain even if it was located permitted location (reference Exhibit G-1 from Permit M-78- <br />052); and, <br />• Based on the remnant field evidence, which existed after the May event, it was clear that the <br />majority of flow was conveyed through the northern breach. <br />Additional Modeling Data <br />It is believed that the north breach was first created during a Flood even[ which occurred during July 17 <br />and 18, 2000. LA obtained hourly data for this event (derived by establishing an average flow rate for <br />each hour from the l~-minute data) from the State Engineer's Office. The hourly data indicate a peak <br />flow of 8390 cfs at x:00 AM on July 17. 2000. The actual peak was probably higher than 8.390 cfs given <br />the fact that this discharge is an average hourly peak. It should be noted that there was another event <br />which occurred on August 17 and 18. 2000 where the peak discharge exceeded 7,000 cfs. <br />To improve LA's confidence in the normal depth analyses and to examine the potential for backwater in <br />the Bull Seep Slough, a step-backwater analysis was performed using the Army Corps of Engineers HEC- <br />RAS model (see Attachment A for crass sections and the output files). LA utilized the HEC-Z data deck <br />from the South Platte River floodplain study performed by Gingery and Associates, 1977 as a base for <br />their HEC-RAS model. The model utilized information from cross sections 30, 31, and 33. Roughness <br />coefficients utilized for the main channel and overbank areas were the same as the original 1977 Flood <br />study. The following modifications were made to the Gingery and Associates, 1977 HEC-2 model. <br />Cross section 31 was modified to include the bank, which was reconstructed along the west end <br />of the Frei pit and borders the Bull Seep Slough. This feature did not exist in 1977. <br />2. The 1977 flood study utilized an invert elevation at the Branter Diversion structure of 5021.5 feet <br />(reference Cross Section 31). The survey performed by Epp and Associates indicated that the top <br />of the Brantner Diversion structure was X031.6 feet. It appears that the original cross section was <br />cut along rite base of the Brantner Diversion structure. During the Gingen• modeled 100-year <br />event, the effect of the diversion structure on the water surface profile was minimal. When <br />modeling events such as the May 5 Flood event (2-year), the impact of the Brantner Diversion <br />structure on the water surface profile is significant. To adequately model upstream conditions for <br />an event similar to the May 5 event. LA created a cross section at the top of the Brantner <br />Diversion Structure from the EA survey data and ICON mapping. <br />3. LA added a cross section (see cross section 32) at the north breach location utilizing survey data <br />from EA supplemented by 2 foot contour mapping supplied by ICON Engineering. <br />~. LA added a cross section ar the confluence of the South Platte River and the outlet of the Bull <br />Seep Slough (see cross section 30.x) to establish a reasonable approximation of the backwater <br />