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Memo to Wallace Erickson 10 May 3 3001 Flood storage in the pre-mining alluvium is equal to the pore space available in the alluvium above the water table. Atypical sand and gravel deposit will have approximately 35 percent voids (Davis, 1969), so the total pre-mining flood storage available over the approximately 1 1 acres that will become pond surface may be calculated as: 1 lacres x 43,560 ft Z / acre x 5 fee[ x 0.35 = 838,530cu. ft. = 19.25acre -fee[ This is a conservatively high estimate of flood storage in the pre-mine setting, since the upper two feet of the deposit is topsoil and overburden with a lower void ratio than sand and gravel. Flood storage provided by freeboard in the north pond is: 2.3 lacres x 3 feet = 6.93acre -feel Flood storage provided by freeboard in the north pond is: 3.37acres x 0.5 feel = 1.685acre -feel Flood storage provided by freeboard in the south pond is: 5.38acres x 2 feet =10.7bacre -feel Total flood storage provided by freeboard in the post mining configuration is estimated at 19.38 acre- feet, demonstrating that mining, reclamation, and pond filling will not create a reduction in flood storage in the pit area. Another flood related concern with the excavation of the Line Camp Pit would be the potential for scouring of the land berm between the pit and the river during a flood and potentially creating a new flow path for the river through the excavated pits. If this were to happen, down cutting of the main stream channel may result in property damage upstream and downstream of the pit location. The common nomenclature for the land berm between a gravel pit and the river is to call it a "riverbank berm." Concerns with the potential for failure of riverbank berms have prompted Urban Drainage and Flood Control District in Denver and the DMG to issue guidance documents addressing this issue (UDFCD, 1987 and DMG. 1998). One of the protective measures described in both of the guidance documents is to leave a specified minimum crest width from the bank of the river to the edge of the pit. The crest width requirements are specified to protect the riverbank berms from rapid failure during floods. The crest width requirements consider long term stability and safety along streams, but are not necessarily preventative of localized damages to the berms during large floods. Such localized damages may be maintained and/or repaired during low flow. This is a particularly straight forward process on rivers with hydrographs like the Dolores, as will be discussed below. Along the South Platte River through Adams County Colorado, gravel operators are allowed to mine with a 400 foot crest width on the riverbank berm with no other bank armoring or spillway installation required. The 400 foot crest width is the extreme case in terms of setback requirements imposed on gravel mining operators in the State of Colorado. Operators are allowed to mine much closer to the South Platte River in Adams County if they install mitigation measures. By comparison, the Applicant for the Line Camp Pit has proposed mining to within 500 feet of the Dolores River at the north or upstream end of the permit area, and to within 200 feet along a short section of river at the south end of