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Dec,-Q4-01 O2:24P • • P-06 Mr. Jeff Schwarc December 4, 2001 Page 5 related to the existing location of the Bull Seep Drain is irrelevant for the following reasons: It is clear that a swap existed on the inboard side of the levee prior to M1'M relocation of the Bull Seep Drain, which would have conveyed breach flows into the slough; The snutltern breacft location was such that breach flows would have been directed into the Btrll Seep Drain even if it was located permitted location (refcrcrtcc Exhibit Ci-1 t}ottt Permit M-78- 052); and, Based ort t}te remnant field evidence, which existed after the May event, it was clear that the majority of Iluw was conveyed through the ttortltertt breach. Additional Modeling Data It is helieved that the north breach was first created during a flood event which occurred during luly 17 end 18, 2000. LA obtained hourly data for this event (derived by establishing an average Iluw tale fur cash hour from the 15-minute dato) frnrtt the State Engineer's Uffice. The hourly data indicate a peak flow of 8390 cf's at 5:00 AM on July 17, 2000. 't'he actual peak was probably higher than 8,390 cfs given the fact that t}tis discharge is an average hourly peak. it s}tould be noted that there was attotlter event which occurred oft August 17 and 18, 2000 when: the peak discharge exceeded 7,000 cfs. 1b improve LA's confidence in the nonnel depth analyses and to examine the potential for backwater in the Bull Seep Slough, a step-backwtter analysis was pertbrnted using tltc Army Corps of Engineers i1EC- RAS tnodcl (sue Atlat:hment A for cross sections attd the output files). LA utilized the IfEC-2 data deck from the South Platte River floodplain study performed by Gingery :tnd Associates, 1977 as a hasc fur their HI:C-RAS mutlel. 1'lte model utilized information from Cross sections 30, 31, and 33. Roughness coefficients utilized fur the maim channel attd overhattk arCas were the same as the original 1977 fluuti study. 't'he following tnoditicatiotts were ntadc to the Gingery and Associates, 1977 HE:C-2 model. Cross section 31 was tnoditied to include the bank, which was reconstructed along the west end of the Prei pit attd borders the Bull Seep Sluttgh. This feature did not exist in 1977. 2. 'fhe 1977 flood study utilized aft invert elcvatlon al the Branter Diversint structure of 5021.5 feet (reference Cross Section 31). 't'he survey perfomted by Epp and Associates indicated that the top of the Brantner Diversion structure; was 5031.6 feet. It appears that the original Cross section was cut along the base uC the Brattfier Diversion structure, During the Gingery modeled 100-year event, the effect of the diversion structure oft the wafer surface profile was ntittintal. When modeling events such as the May 5 !loud event (2-year), the impact of the Brantner Diversion structure on the water surface profile is significant. To adeyttately model upstream Conditions fur aft event similar to the May S event, LA created a cross section at the lop of the Branhter Diversion Stnteture from the EA survey daut and 1CON mapping. 3. LA added a en~ss section (see cross section 32) at the north breach location tttilizittg survey data from EA supplemented by 2 tout contour mapping supplied by IC~C1N k.ngitteering. 4. LA added a cross section at the confluence of the Sotttlt Platte River and the outlet of the Bull Seep Slough (see cross section 30.5) to establish a reasnttablc approximation of the backwater