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DI~AFT - JUNE 21, 1989 <br /> <br />Have an outlet control structure that effectively prevents floatin£ <br />debris and oils from entering the downstream conveyance system. <br /> <br />Control of Streambank Erosion and Streambed Degradation <br /> <br />Streambank erosion and streambed degradation control measures must: <br /> <br />Be employed whenever the net sediment transport for a reach of <br />stream is greater than zero or whenever the stream's natural <br />tendency to form meanders directly threatens damage to structures, <br />utilities or natural amenities in public areas. <br /> <br />Include effective energy dissipation devices or stilling basins to <br />prevent streambank or channel erosion at all stormwater outfalls. <br />Specifically: <br /> <br />Co <br /> <br />Outfalls with outlet velocities of less than 4 fps that <br />project flows downstream into the channel in a direction at <br />least 30' from the normal flow direction generally shall not <br />require energy dissipators or stilling basins, but they may <br />need some riprap protection. <br /> <br />Energy dissipators shall be sized to provide an average outlet <br />velocity of no more than 6 fps. If riprap is also used, the <br />average outlet velocity may be increased to 8 fps. <br /> <br />Riprap stilling basins shall not be used where outlet <br />velocities exceed 8 fps. <br /> <br />Specify riprap consisting of natural angular stone suitably graded <br />by weight for the anticipated velocities. <br /> <br />2302047/LRR~IO.WP/SAM <br /> <br /> <br />