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Two Rivers Gravel Pit Mine <br /> Riverside Berm Failure Analysis <br /> and Flood Control Mitigation Plan <br /> 7ech"olooicess January 22,2020 <br /> CENTRAL PIT <br /> FILL TIME ELEV VS HEAOCOT ELEV <br /> a390 0 200 R <br /> Bern Crest <br /> -R3=� mmmmmm uNbwes men s ss or <br /> &sdwpa exceeded 100%of affa%wWe at 0.5 houfs.) <br /> 4HO 0 <br /> =5-=0 <br /> �^+ W700 <br /> � Best <br /> 4U'0 .Fill Time <br /> aeao 0 <br /> Length 1M 4.from pit bottom. <br /> Pit Toe tlerm kn vs Hat brsa <br /> 41655 0 <br /> 46500 <br /> 0.5 1 1.5 2 > >._ 4 4.5 <br /> Time{hn <br /> Figure 6 Central Pit <br /> Pit fill time and water surface elevation vs. head cut elevation for 200 ft riverside berm. <br /> 4.2. Northwest and Northeast Pits (Big Thompson River Side) <br /> Northwest Pit results are summarized in Figure 7. Per Figure 7, no soil parameter case results in <br /> head cutting/erosion at an elevation or length that is threatening to river capture. Regardless, hard <br /> armoring will be applied during reclamation to areas susceptible to head cutting/erosion as an <br /> extra measure of protection as explained in Section V, Hard Armoring. <br /> The main difference as to why Northwest Pit head cutting/erosion is less severe than Central Pit <br /> is due to the flood fill time. Figure 7 depicts a steep pit water surface elevation vs time curve as <br /> compared to Central Pit. That is due to a smaller pit volume with respect to flood inflow. Thus, <br /> head cutting/erosion ceases much sooner. <br /> Northeast Pit head cutting was not evaluated. It has a relatively small ultimate pit volume of 659 <br /> acre-ft and compared to Northwest and Central Pits, would fill in a relatively short time period <br /> which would prevent significant head cutting/erosion. <br /> pg. 19 <br />