2012-11-09_REVISION - M2010049 (3)

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Western Sugar Reclamation Land Development Project Flood Analysis 1*14 Broil] N►•iM11110IdV.11 :i•� This report discusses a presentation pertaining to flood analysis for the Western Sugar Land Development Project given by Mr. Douglas J. Trieste, P.H. of Flow Technologies, LLC at a meeting between the Department of Reclamation, Mining, and Safety and Varra Companies, Inc. on September 21, 2012. Mr. Trieste is a consultant to Varra Companies, Inc. The presentation was a follow -up from a July 21, 2012 meeting between DRMS and Varra Companies, LLC where Mr. Trieste suggested a method for using a dam breach erosion model to estimate headcutting and erosion from flood flows into a gravel pit. 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. 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. • The largest peak discharge on record (107 years of record) at the Cache LaPoudre River at Greeley stream gage (Colorado Department of Natural Resources ) is 6,090 cfs as compared to the FEMA Base Flood (100 -yr flood) of 10,600 cfs. Thus, the FEMA Base Flood appears to be high. In any case, the above headcuting and erosion analysis was performed with the larger FEMA 100 -yr discharge which adds conservatism to the analysis. Note that if the FEMA Base Flood was lower, the Site may not even flood during a 100 -yr event. • When the ultimate pit is achieved, it will be allowed to fill — and remain full - creating a reservoir and thereby eliminate headcutting. In addition, a flood mitigation plan was discussed and solutions agreed upon to minimize risk of headcutting in the gravel pits that may capture the Cache LaPoudre River (CLPR) should a large flood occur. That flood mitigation plan will be discussed in a separate document provided by Varra Companies, Inc. pg. 3