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Mr. Jeff Schwarz December 4, 2001 Page 3 My geomorphic analysis concludes that the construction of the Brantner grade control (1948) and in- channel mining (through 1980) upstream of the "point of failure" resulted in flow being directed at the reconstructed bank, adjacent to the Bull Seep Slough. In-channel mining and construction of the Brantner Diversion Dam influence on the system increased the channel's tendency to meander and resulted in the development of a left bank point bar (west side and opposite from the May 5, 2001 point of failure). This bar developed to be a more significant feature through the 1980s and 1990s. As it grew and became more pronounced, it served to redirect even minor flood flows against the reconstructed channel bank and as such, strongly influenced the bank failure of May 5, 2001. The above analysis suggests that the bank, which isolates the Bull Seep Slough from the river, would continue to be attacked by the river, and in the absence of additional reinforcement, would ultimately fail. A review of several aerial photos prior to Mobile Premix Concrete's realignment 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 a[ [his location would indicate that the levee was being overtopped on a fairly regular basis. The continued attack on the levee by frequently 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 alignment of the Bull Seep Drain irrelevant to such failure. Photographic data from January, 2001 (Figure 3.1) indicate that the "reconstructed" channel bank which isolates ttte Bul! Seep Slough from the river had been overtopped and breached prior to lanuarv 2001. The presence of apre-existing breach may have been the direct cause of failure during the May ~, 2001 event. Parr B Hydrologic/Nydrnulie Analyses As part of our forensic analysis of the bank failure, LA developed flood hydrology data. Annual peak flow data for the Henderson Gage were obtained via the lntemet and a Log Pearson Type [[I Analysis was performed to establish a Flood t~equency curve. The Public Domain software FREQMAN was utilized for the analysis. The analysis indicated that the 2-year, 5-year, and 10-year events were approximately 5,800 cfs, 9.200 cfs, and 12,700 cfs, respectively. Therefore, the event of May 5, 2001 of 6,100 cfs had a return period of approximately 2 years. LA contacted Epp Associates (EA) to survey cross sections at the locations of the north and south breaches as well as to determine the elevation at rite top of the Metro Aeration Weir and the Brantner Diversion Dam. Survey data collected by EA was augmented using topographic mapping obtained from ICON (Urban Drainage's consultant). These data indicated that the top of the levee ranged in elevation from 5038.5 to 5039.9 feet at the northern breach and approximately 5038.5 to 5041 feet at the southern breach. Several cross sections will be discussed in the following paragraphs. The cross section location map is presented as Figure 3.2. Cross section data for [he north and south breach areas were entered into the computer programs XSPRO and Flowmaster Version 6.1. XSPRO is a DOS based program that was developed by the Bureau of Reclamation to evaluate flow in irregular channels. Flowmaster is a Windows based program, which calculates normal depth hydraulics for Flow in irregular channels. The roughness coefficient utilized in the normal depth analyses was taken from the HEC-2 Floodplain modeling performed by Gingery and Associates in 1977 and was set at 0.035. The slope was set at