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ec-04-O1 O2:24P Mr. Jtff Schwxri gecembcr 4, 2nn 1 Page 4 O.OOlS fUfl, which was slightly greater than the measured had slope (S-.001 ft/ft: established using the Epp and Associates survey data), LA's divergence t'rartt measured beet slope is justified in that the water surface energy slope rather than thu channel slope is an essential clement in this calculation. Untter lower flood flow conditions, the effect of the B+•antner will also act to steepen the energy slope. '1'!te decision to use a slope of 0.0015 is further validated by field evidence that supports channel's conlaittment of events tower tltart 6000 cfs. Al a slope of 0.001, tltc river would overtop the bank al 5000 cfs, rather than 6500 cfs. Historic evidence doesn't support this. Assuming there are no pr•e-existing "breaches" in the cltanrtcl bank, stage discltargc curves were developed at the norlhem and southern "breach cross section location" (Table5 1 and 2). The normal depth analysis fur the northern breach indicates that the levee begins to overtop ++l 6,500 cfs (Reference Table 1 where a stage Iteigltt of 10 feet is equal to 5038.8 fee[). Nottnal depth analysis of the southern breach indicates that the Ievice will nut be overtopped wail the discharge is in excess of R,000 cfs (please reference Table 2 where a stage height of 9 feet equates Io 5038 feet), l+t January 2001, :+ photo was taken by Urban Drainage (Figure 3.1) which indicates the presence of a+t erosion col (i.e., the area of the May 5, 2001 breach) at the north location. LA has estimated that this "cut" was approximately four (4) feet deep attd 10 feet wide (see attached photo). Assuming the top of bank is elevation of 5039, the invert of the cut is 5035. This would suggest that flow woteld begin to enter the "col" and discharge tin+n the South Platte River to the Bull Seep Slough, when the main channel is flowing around 2800 cfs. P_O5 Tables 3 aitd 3A present hydraulic analyses of the assumed dimensions initial breach using the weir equation (:I,N`~. Depending on the assumed siic of the erosion cut (4 versus 6 feel steep), sorttewltere from 266 ufs to 490 cfs stay ltavc passcll through the "erosion col" al the northern breach location assuming that the geometry of the "erosion cut" remained unchanged during the May 5, 2001 event. This is aft unlikely scenario and LA concludes that as more and tttore Clow passed through the May 5, 2001 breach, the erosion cut progressively enlarged u»til failure was achieved. Because the Icvee was constructed of non-engineered fill material, once water started flowing through the breach, failure occurred quite rapidly. []Itimalcly the breach enlarged to an opening approximately six (6) to seven (7) feet deep by 100 feet Inrtg based on Geld observations of the breach repair which occurred at tltc Cnti of May, 2001, Table 4 presents a rating table for tltc hrcach utilizing these enlarged dimensions. Normal depth analyses were performed for the Bull Seep llrain et cross sections approximately 200 fact upstream and 800 feet downstream of the north breach (Tattle 5 and 6 respectively). Tn enrnple[e tltc 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 capacity (400 cfs) of the Bull Secp drain. Yermit M-78-052 (Ml'M's reclamation permit) required a capacity of 350 ct's. TablC 6 reflects the conveyaitee capacity of the pre-flood Bull Seep Slough below the Nortlt Breach. Neither the existing nor the permitted Bull Seep I)rsin ("fable 5), upst+•emn of the Rull Sccp Slough could convey the flood count of May 5, 2001 (>I500 cfs: Ta61e 6). Witlt this said, the location of the Bul] Seep Drain would not make any difference with respect to downstreartt material damage within the Bull Seep Slough. Il is I.A's ttltderstattdirtg tl+at a hrc:ach did exist at the sout}terrt location at the time oY'tlte May 5, 2001 event. The dimensions of this breach at the lime of the May flood evcrtt are unknown but field evidence indicates that; (I) the invert of [his breach was higher than the nnrthcm breach; (2) the conveyance capacity of the river cross section at [his lucaliun w:ts greater titan ita downstream counterpart; and (3) due to the geometrical configuration of thin hrcuh (ot~ channel from the South Ylatte) that development of the opening would 6e limited. The potential impact of the southern hreach on the Bu(I Seep Slough as