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Mr. Jeff Schwarz December d, 2001 Page 4 0.0015 ft/ft, which was slightly greater than the measured bed slope (5=.001 ft/ft: established using the Epp and Associates survey data). LA's divergence from measured bed slope is justified in that the water surface energy slope rather than the channel slope is an essential element in [his calculation. Under lower Flood Flow conditions, the effect of [he Brantner will also ac[ [o steepen the energy slope. The decision to use a slope of 0.0015 is fitrther validated by field evidence that supports channel's containment of events lower than 6000 cfs. A[ a slope of 0.001, [he river would overtop the bank a[ 5000 cfs, rather than 6500 cfs. Historic evidence doesn't support [his. Assuming there aze no pre-existing "breaches" in the channel bank, stage discharge curves were developed at the northern and southern "breach cross section location" (Tables 1 and 2). The normal depth analysis for the northern breach indicates that the levee begins to overtop at 6,500 cfs (Reference Table I where a stage height of 10 feet is equal to 5038.8 feet). Normal depth analysis of the southern breach indicates that the levee will not be overtopped until the discharge is in excess of 8,000 cfs (please reference Table 2 where a stage height of 9 fee[ equates to 5038 feet). In January 2001, a photo was taken by Urban Drainage (Figure 3. I) which indicates the presence of an erasion cut (i.e., the area of the May 5, 2001 breach) at the north location. LA has estimated that this "cut" was approximately four (a) feet deep and 10 feet wide (see attached photo). Assumin: the top of bank is elevation of 5039, the invert of the cut is 5035. This would suggest that flow would begin to enter the "cut" and discharge from the South Platte River to [he Bull Seep Slough, when the main channel is flowing around 2300 cfs, Tables 3 and 3A present hydraulic analyses of the assumed dimensions initial breach using the weir equation CLH"'. Depending on the assumed size of the erosion cut (4 versus 6 feet deep), somewhere from 266 cfs to X190 cfs may have passed through the "erosion cut" at the northern breach location assuming that the geometry of the "erosion cut" remained unchanged during the May 5, 2001 event. This is an unlikely scenario and LA concludes that as more and more flow passed through the May 5, 2001 breach, the erosion cut progressively enlarged until failure was achieved. Because the levee was constructed of non-engineered fill material, once water started flowing through the breach, failure occurred quite rapidly. Ultimately the breach enlarged to an opening approximately six (6) to seven (7) feet deep by 100 feet long based on field observations of [he breach repair which occurred at the end of May, 2001, Table d presents a rating table for the breach utilizing these enlarged dimensions. Normal depth analyses were performed for the Bull Seep Drain at cross sections approximately 200 feet upstream and 800 feet downstream of the north breach (Table 5 and 6 respectively). To complete the normal depth analysis, a roughness coefficient of 0.035 and an average slope of 0.003 fUft was assumed. Table 5 identifies the actual pre-Flood capaciq~ (=t00 cfs) of the Bull Seep drain. Permit M-78-052 (MPM's reclamation permit) required a capacity of 350 cfs. Table 6 reflects the conveyance capacity of the pre-flood Bull Seep Slough below the North Breach. Neither the existing nor the permitted Bull Seep Drain (Table 5), upstream of the Bull Seep Slough could convey the flood event of May 5, 2001 (> 1500 cfs: Table 6). With this said, the location of the Bull Seep Drain would trot make any difference with respect to downstream material damage within the Bull Seep Slough. It is LA's understanding [ha[ a breach did exist at the southern location at the time of the May 5, 2001 event. The dimensions of this breach at the time of the Mav flood event are unknown but field evidence indicates that; (1) the invert of this breach was higher than the northern breach; (2) the conveyance capacity of the river cross section at this location was greater than its downstream counterpart; and (3) due to the geometrica: configuration of this breach (off-channel from the South Platte) that development of the opening would be limited. The potential impact of the southern breach on the Bull Seep Slough as