2008-02-04_GENERAL DOCUMENTS - M1989056

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Mr. Rex Burns Page Four May 1, 1998 We computed the sediment transport rate using the Meyer-Peter bedload formula. This formula was developed for coarse grained sediments in situations where no energy is lost to bedforms. In the steep erosion zone, it is unlikely that bedforms would occur. The formula is shown below: gsv3 = 39.25gv3 S - 9.95d50 where: gs = sediment transport rate, in Ib/(sec-ft) q = unit discharge, in cfs/ft S = channel slope (we used energy slope since that really governs sediment transport) d50 = median sediment size Since most of the erosion will take place in the overburden material, we used the d50 of that material rather than the pit material. We assumed that the overburden d50 is 0.0625 mm (corresponding to very fine sand). The sediment transport rate was computed for the right overbank for each step of the discretized hydrograph. Using the transport rate and the width of the proposed gravel pit (1,100 feet along the north edge), the weight of sediment transported was computed for each step in the discretized hydrograph and summed to quantify the sediment transported during the entire flood. Using an in-place soil density of 110 Ib/ft3, volume of erosion equals almost two million cubic feet. Assuming an erosion depth of 6 feet across the full with of the gravel pit, the erosion length (or the distance the erosion face would advance upstream) would be approximately 300 feet. if the north-to-south length of undisturbed ground along the north edge of the gravel pit exceeds this quantity, the flood hydrograph should not have enough time to remove a sufficient amount of sediment to form a channel cutoff. However, we feel that increasing this length by 50 percent is reasonable given the uncertainties involved in sediment transport analyses. Thus~he safe undisturbed leng±h would be approximately +440 feet. Table 2 displays pertinent information concerning the sediment transport rates, volumes, and length of channel erosion. Impact on Mining Plan The applicant intends to begin mining the Timnath Gravel Pit at the southeast end, proceeding first west, then north. The mined areas are to be restored behind the mining operations with the overburden material from the site as mining advances. The final grades in the restored areas will be approximately 15 feet lower than existing grades, and will accommodate ponds and wetlands. The significant aspect of the restoration plan (relative to potential channel cut-off) is that the area will not be returned to existing grades, so the area downstream of mining activities can not be counted on to resist channel cut-off. In other words, the cut-off channel will essentially already exist downstream of the mining operations. Thus, only the undisturbed terrain upstream of mining operations can be assumed to be effective in preventing a channel cut-off. BURNS-4A.LTR 34-0648.00