2012-11-09_REVISION - M2010049 (3)

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Western Sugar Reclamation Land Development Project Flood Analysis X. DISSCUSSION AND CONCLUDING REMARKS Study results indicate that for a major rainfall /runoff event, it is possible to have ample flood warning time for dewatering pumps to be stopped allowing the ultimate pit (for conservatism) to fill before headcutting can reach the river. In addition, in the unlikely event that there is no flood warning and flooding occurs on the Site, and if rising water surface level - due to inflow from both the river and groundwater (pumps stopped when flooding is noticed) - in the pit is considered, there will be less time for headcutting to occur and likely not have sufficient time to erode to the river. In a situation like Great Western Sugar Reclamation Site (and all others), we can't predict the future but we can make some reasonable evaluations and assumptions as to flood consequences and mitigating measures. The direction of extraction for Tract C will be from southwest to north east. Thus, the berm width at the western edge of the Site (breakout area just upstream from Oglivy Ditch Diversion Dam; refer to Appendix, Plate 1, Site Conditions) will be hundreds of feet for many years (approximately 2 to 10) thereby decreasing the probability of a flood occurring when the project reaches the ultimate pit. That is, during the majority of the life of the project, the berm width will be hundreds of feet, with the narrowest berm width being 200 ft. WinDAM B erosion modeling is not an exact science, but the results appear reasonable and within the realm of possibility. Analyses and discussion such as above can not conclusively predict that the pit will erode back to the river. — it is impossible to truly predict natural events. However, information presented in this study adds a high degree of confidence that headcutting due to a 100 -yr flood event will not capture the river; key points are summarized as follows: • Erosion modeling indicates that a 100 -foot berm will not likely erode back to the river. And being that the Tract C pit rim will be at least 200 ft from the river at the breakout area (west side of Tract C just upstream from the Oglivy Ditch diversion dam), analysis considering a 100 -ft berm width adds a margin of safety to the results. Note that although a berm width of 100 ft is referred to with respect to erosion modeling analyses, but in actuality the berm width for Tract C at the river breakout location will be 200 ft. Refer to the Appendix, Plate 1, Site Conditions for a depiction of setbacks and berm widths for the Site. • Direction of extraction will be from southwest to northeast on Tract C which means that extraction begins hundreds of feet from the river. And, it will be approximately 2 to 10 years before the rim of the pit reaches a minimum distance of 200 ft from the river on the west side of Tract. Thus, due to the longer operation time with a wide berm (greater than the 200 -ft minimum) exposed to the 100 -yr flood, that that operation time greatly reduces the probability of erosion and headcuting back to the river. P9. 12