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• ~2 <br />ctarage capacity can contain the rwoff volume from the contributing area for <br />the post-mining condition. Thus, placement of the eulwrt is not necessary <br />prior to reclasation of the upstream area. However, • eulwrt to alloy drain- <br />age wder the rosin road downstream should be provided prior to plaeament o! <br />the Nlwrt under the G.E.C. haul road. <br />6.2.E Culvert Outlet Crosson Control <br />Protection at eulwrt outlets is required to prevent scour of the down- <br />stream channel. 7(dequate protection can be provided by placing riprap at the <br />outlet of the eulwrt. The required riprap is based on the outlet velocity of <br />the culvert and channel slope. The outlet wloeitias for existing Nlwrts and <br />proposed culverts is given Sn Table 11. The velocities were estimated by Manning's <br />' egwtion assuming normal depth !os the design flw. i'hese wlws are the maximum <br />that can occur. Generally, the w loeities will be less because of storage build <br />,' <br />up upstream of,the Nlwrts which attenwtes the peak discharge. For Nlverts <br />• that the exit velocity is less than five feet per second, no protection •t the <br />• Nlvert outlet is required. Foz velocity ranges of 5 to 9 feet per second, 9 <br />to 17 feet per sewnd, and 17 to 16 feet per second, riprap sizes of 0.5 feet, <br />1.0 feet, and 1.5 feet, zespeetiwly are required. These sizes were deterniined <br />to haw a factor of safety of 1.5 using the method 'described on pages 118-~25 <br />of Simons and Sentnrk (1977). The factor of salety is for the channel Md and <br />not the side slopes. Depending on channel geometry and alignment, the high ve- <br />locity jet from the culverts will impinge on the channel bottom and may impinge <br />on the side slopes. Therefore, the •ide elopes should also be proteeud. For <br />wloeities in excess of 16 feet per second, tw-foot diameter material should <br />W wed, ha+aver, with such high velocities, the certainty of riprap adequacy <br />is gaestionable. The sizes mentioned refer to the median diameter (DSO) of <br />the material. To ensure pertotmenn , proper gradation is also required, The <br />oaxirua size of the riprap should be approximately tro times the median size <br />and on the order of 20 percent of eha material should ba less than half the see- <br />dian size. To prevent the sand below the riprap iron washing out, a filter of <br />gravel or filter cloth should be placed between the riprap and channel bed. If <br />• gravel filter is used, <br />DSO (filter) <br />• '11 DSO (base) ~ i0 <br />