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Dec-Q4-01 O2:23P • • P-04 Mr. Jeft'Schwarz December 4, 2001 Page 3 My geomorphic analysis concludes that the construction of the Branmer grade control (1948) •rttd in- channel mining (through 1980) upstream of tltc "point of failure" resulted in flow being directed at the reconstructed bank, adjacent to the Aull Sccp Slough. In-channel mining and construction of the Rrantncr Diversion T)am influence on the system increased the channel's tendency to meander and resulted in the development of a Icfl bank point bar (west side and opposite tYom the May 5, 2001 point of failure). This bar developed to be a more significant f"eature thmugh the 19ROs and 1990x. As it grew and becerrte mare pronounced, it served to redirect even minor Flood flows against the reconstructed channel bank and as such, strongly inllucnccd the bank failure ofNlay 5, 2001. The above analysis suggests that the bank, which isolates the Bull Seep Slough fmm the river, would continue to be amcked by the river, and in the absence of additional reinforcement, would ultimately fail. A review of several aerial photos prior to Mohilc Premix COnCTI'll''J reali6mmenl of the Bull Seep Drain Ditch to its present location shows the presence of water near the Bull Seep Slough inlet. The continued presence of water at this location would indicate that the levee was being overtopped on a fairly regular basis. T'he continued attack on the levee by freytrently recurring flows as well as regular overtopping indicate that at some point in time the levee would fail due to natural processes, thus making the existing alignmen! of the Bull Sccp Drain irrelevant to such failure. Photo~aphic data from January, 2001 (Figure 3.1) inclicale that the "reconstructed" channel bank which isolates the Bull Seep Slough from the river had been overtopped and breached prior to Januarv 2001. The presence of apre-existing breach may have been the direct cause of failure during the May 5, 2001 event. Part B HydrolugidAydraalic Analyses As part nt' our forensic analysis of the bank failure, i.A developed flood hydrology data. Annual peak flow data fur the Henderson Gagc were obtained via the lntcmct and a Lug !'earsan 'Type Ili Analysis was pertormed to establish a tood tTequency curve. The Public Domain software FRf•.QMAN was utili~cd for tlrc analysis. The analysis indicated that tl+c 2-year, ,S-ycaz, and 10-year events wero approximately 5,800 cfs, 9,200 cfs, and 12,700 cfs, respectively. Therefore, the event of May 5, 2001 of G,100 cfs had a return period of approximately 2 years. LA contacted Lpp Associates (CA) to survey cross sections et the locations of the north and south breaches as well as to dctcrrninc the elevation at the top of [Ire Metro Aeration Wcir and the Arantncr Diversion llam. Survey data cullecled by !;A was augmented using topographic mapping obtained from ICON (Urban Drainage's consultant). These data indicated that the top of the levee ranged in clevatiort from SU38.5 to SU39.9 feet at the northern breach and approximately SU38.5 to SU41 feet at the southern hn:ach. Several cross sections will be discussed in the following pt+ragraplts. The cross section location map is presented s5 Figure 3.2. Cross section dale for the north and south breach areas were entered into the computer progretns XSPRO and Plowrttaster Versiart 6.1. XSPRO is a DOS Rased pmgram [list was developed by the Bureau of Reclamation to evaluate flow in irregular channels. Flowrnasrer is a Windows based program, which calculates nomtal depth hydraulics t'Dr tlow in irregular channels. The roughness coel3icient utilized in the normal depth analysis was taken from the HF.C:-2 Iloodplain modeling performed by Gingery and Associates in 1977 and Was StT at 0.035. The slope was set at