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Chapter II <br /> slopes. Downstructures are subject to snow damage as the pressure <br /> of the snow on top of the structure tends to pull it downward, <br /> causing separation of the band connecting the downstructure to the <br /> culvert. Downstructures are not advisable in avalanche areas , and <br /> may not be advisable in some areas of heavy snow. The ground <br /> below the downstructures should be stabilized with riprap and/or <br /> other flow dissipators. Downstructures should extend no further <br /> than the point at which the outfall can be accessed for riprap <br /> placement and culvert repair. <br /> Riprap <br /> The ground below the culvert outfalls will be riprapped <br /> with rock material for a distance extending 8-10' from <br /> the outlet. The rock material will be of variable size <br /> and shape. <br /> Geo-Hydro, a consulting firm retained by Mid-Continent to <br /> carry out an extensive geotechnical review of the road <br /> system, noted that erosion problems are "common" in Coal <br /> Basin, and suggested using riprap with "different <br /> thickness of riprap for different slopes. " <br /> The design prepared by Morrison Knudsen (Vol. 3-A, II-B- <br /> App. 7 Att. 2) calls for riprap sizing of gravel to <br /> cobble size (6" to 1' thick) material below culvert <br /> outlets. Below downdrain structures, on steeper slopes, <br /> the design calls for gravel to cobble sized rock bedding <br /> beneath larger size riprap including 2 to 3 foot <br /> boulders. <br /> Mid-Continent's experience suggests that riprap sizing <br /> and shape should be variable, and should be mixed rather <br /> than segregated into layers. Large size rock material <br /> should be included for steeper slopes and areas of <br /> considerable flow velocity. <br /> Maintenance of Culverts <br /> Culverts will be inspected to determine whether repair or <br /> replacement is necessary. Replacement of culverts will <br /> be judged necessary when the culvert has been damaged so <br /> ` as to fail to pass flows, or when the culvert has been <br /> worn and rusted to the point that leakage endangers the <br /> integrity of the road. On light use roads, rolling dips <br /> may be installed in place of rusted culverts. <br /> 19(a) Revised 12/91 <br />