2018-06-25_REVISION - M2011001 (2)

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April 5,2018 Page 5 • There is no erosion protection along the length of the riverside berm or along the bank of the lake. The lack of erosion protection increases the probability that the riverside berm will be breached during flood events which overtop the berm. • The heavy riprap at the entrance and exit of the spillways does not extend to the limits shown on the As-Constructed Drawings by Tetra Tech dated June 16, 2016. There was evidence of voids and settlement in most of the spillway riprap. This indicates the filter layer may be absent or was not installed correctly and is not functioning correctly to prevent fines from washing out. The riprap is not graded correctly to produce a dense interlocking mat. The lack of a functioning filter and interlocking mat make undermining and washout of the riprap likely during high spillway discharges. • The concrete cut-off walls should be founded on sheet pile cut-offs to prevent undermining and washout of the structure during high spillway discharges. CONCLUSIONS AND RECOMMENDATIONS The probability is high that the riverside berm surrounding the reclamation area will be overtopped and breached during flood events greater than the 2-Year Flood. The high probability is based on the narrow width of the riverside berm, minimal freeboard, lack of erosion protection and inadequate spillways to reduce differential heads. Previous breaches of the riverside berm resulted in significant damages to the site including severe erosion of banks, sediment deposition in the lake and washout of the spillway structures. Similar damages are possible if the berm is breached in the future. The detrimental impacts of mining pit capture by a river are well documented(See USBR Rehabilitation of Floodplain Mining Pits: Interim Report Detailing Initial Plans and Procedures 2005). We recommend the following improvements be made to stabilize the site and prevent breaching of the riverside berm during future flood events. 1. The low area in the riverside berm located at the northwest corner of the lake should be uniformly graded to elevation 4686.0 feet which is approximately equal to the pre-mining elevation and is above the 2-Year Flood elevation of 4685.3 feet. 2. The riverside berm should be protected with riprap on both the riverbank and pitside for the critical section of the berm extending from the inlet spillway approximately 3,200 feet around the river bend at the northwest comer of the lake. The riprap erosion protection should conform to the typical sections and requirements in the UDFCD Guidelines. 3. The inlet and outlet spillways should be increased in width to 100 feet as required by the UDFCD Guidelines. Concrete cut off walls for the spillway should be founded on sheet pile designed in accordance with the UDFCD Drainage Criteria Manual. Riprap erosion protection should meet the requirements of the UDFCD Guidelines and have a granular filter designed in accordance with the UDFCD Drainage Criteria Manual.