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,.,v f 13 ~ Se lumber 17 ?+ c ne plan for the flood control berm includes installation of appropriately sized riprap attnoring at locatic where the Floodplain Study indicates that Flow velocities in excess of ~ feet per second may occur. However, placement details for the riprap have not been provided, In order for the Division to approve t berm armoring plan, the Operator must provide details on riprap layer thickness, bedding material gradation specification and bedding Layer thickness, and a detail illustrating how and to what depth ripra; will be keyed into existing ground. The Division of Minerals and Geology and the Urban Drainage and Flood Control District have both pubtislted guidance that includes riprap design examples for this type n installation. The Operator must also provide a discussion and(or illustration of the burro cross-section showing how and where the riprap will be placed. In particular, will to riverside, pit side, and berm Gres all be armored? if the pit side of the berm wit! be armored, how far will the riprap be carried dawn into excavated pit in order tv prevent erosion and breaching of the berm if flows from the 100-year flood whc to spit! Duct into the pit. The Division recommends that the riprap on the pit side of the berm be carried down at least to the contact of overburden at the top of the dense sand and gravel deposit. Under certain circumstances, pit side and berm crest armoring requirements can be minimized by providing dips in the berm crest tv concentrate flood flows into the pit through an armored side channel spillway and riprap ru down into the pit. However, this cos[ saving measure does not appear to be feasible for the h9amm Cree operation due tv the need to exclude the 50-year tlovd white allowing the 100-year flood into the pit and the relatively small floodwater surface elevation difference between these two Flood events. fn other words, there is not enough freeboard vn the 50-year flood berm to incorporate side channel spillways. The Floodplain Study states that "a"control sill will be installed at the downstream end of the active pit provide a cr>ntrolled overflow location to pass flood Flows nun-erosivcfy through the pit. Is only noe control sill to be provided? Won't several sills be necessary as the four proposed phases of the mining operation proceed? The Operator must describe the general location for each of the sills to be required. series of sills will be required at each location if there is an elevation drop of more than three feet betve~ each subsequent silt or between a sill and the discharge point where flood flows will reenter the river. T requirement must be considered in the Operator's response to this memo. Finally, the Operator must address the pre-existence or, alternatively, the need to construct channels to concentrate t OU-year flood flows and force the Plow u~ pass veer the control sill(s) as the floodwaters pass through the pits. In summary, the Operator has proposed a plan that will prevent the main stem of the Colorado River fro being captured and redirected through the pits following a 100-year flood. The proposed berms are designed to be protective of native fish habitat, with only minimal resultant increase in flood surface elevations. C?nee the Operator provides the information requested above, which is considered tv be clarification rather than changes to the plans under review, the Division can approve the plan, Given thl pending decision date for the Mamm Creek application, the Division may issue an approval with the stipulation that the issues raised in this memo be addressed within some reasonable timeframe. The onl item that must be clarified prior to the decision date, and can not be addressed through a conditional approval, is whether or not the Operator will commit to installation of flood control structures (berm. riprap, control sill(s)) at the beginning of gravel mining, since this issue has a direct bearing on the bone amount that will be specified at the time ofapplication approval. cc: Carl Mount, DMG C lbfq Dvcumam;ibtamm Crcck FIVV4pi'aio Issue; doe