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Dec-04-O1 02:23P <br />Mr.1eft' Schwarz <br />December 4, 2001 <br />Page J <br />My gemnorphic analysis concludes that the construction of the AranmCr grade control (1948) and in- <br />channel mining (through 1480) upstream nt'tlte "point of failure" resulted in flow being directed at tltc <br />reconstructed bank, adjacent to the Aull Sccp Slough. ln-channel mining and construction ofthc Hrantncr <br />Diversion T)am influcnw on the system increased the channel's tendency to meander and resulteJ in the <br />development of a Icll bank point bar (west side and opposite torn the May 5, 2001 point of failure). This <br />bar developed to be a more significant tbature through the 1980s and 1990x. As it grew and became more <br />pronounced, it served to redirect even minor flood flows against the reconstructed channel bank and as <br />such, strongly influenced the bank failure of May 5, 2001. <br />The above analysis suggests that the bank, which isolates the Bull Seep Slough t'rom the river, would <br />continue to be attacked by the river, and in the absence of additional reirtforecmcnt, would ultimately fail. <br />A review of several aerial photos prior to Mohilc Premix Concrete's realibmmenl of the Bull Seep Drain <br />Ditch to its present location shows the prescnce of water scar tltc Bull Seep Slough inlet. The continued <br />prescnce of water at This location would indicate that the levee was being overtopped o» a fairly regular <br />basis. The continued attack on the levee by frequently recurring flows as well as regular overtopping <br />indicate that at some point in time the levee would fail due to natural processes, thus making the existing <br />alignment of the Bull Sccp Drain irrelevant to such failure. <br />Photographic dale from January, 2001 (Figure 3.1 }indicate that the "reconslruoled" channel bank which <br />isolates the Bull Seep Slough from the river had been overtopped and breached urior to Jartuarv 20(11. <br />The presence of apre-existing breach may have been the direct cause of failure during tt+c May 5, 2001 <br />event. <br />Purl B HydrolugidHydrnulic Analyses <br />As part nt' our forensic analysis of the hank failure, I.A developed flood hydrology data. Annual peak <br />flow data for the Hcndctson Gagc were obtained via the Internet and a Lug Pearson 't'ype 111 Analysis was <br />performed to establish a flood ti•equency curve. The Public Domain software FRI;QMAN was utili~cd <br />for tl+c analysis. The analysis indicakd that the 2-year, 5-year, and 10-year events were approximately <br />5,800 cCs, 9,200 cfs, and 12,700 cfs, respectively. 7'hercfore, the event of May 5, 2001 of G,100 cfs had a <br />return period of approximately 2 years. <br />LA contacted Cpp Associates (CA) to survey cross sections et the locations of the north and sotttlt <br />breaches as well as to dctcrrninc the elevation at the top of tl+c Metro Acraliort Weir and the Arantncr <br />Diversion Dam. Survey data collected by JrA was augmented using topographic mapping obtained from <br />ICON (Urban Drainage's consultant). These data indicated that the top of the levee ranged in elevation <br />from SU38.5 to 5039.4 feet :tt the northern breach and approximately SU38.5 to 5041 feet at the southern <br />breach. <br />Several cross sections will be discussed in the following paragraphs. The cross section location map is <br />presented as Figure 3.2. Crass section data fur the north ;md south breach areas were entered into the <br />computer programs XSPRO and Plowmaster Version 6.1. XSPRO is a DOS based program that was <br />ceveloped by the Bureau of Reclamation to evaluate flow in irregular cltertttels. Flowmaster is a <br />Windows based program, which calculates normal depth hydraulics f'or flow in irregular channels. <br />P_04 <br />The roughness coefficient utilized in the normal depth analyses was taken from the HF,C:-2 Iloodplain <br />modeling performed by Gingery and Associates in 1477 and was set at 0.035. The slope was set at <br />