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• 64 determination of these forces depends on the velocity, which is difficult to determine since the riprap required has a diameter on the same order of mag- nitude as the depth of flow. Bathurst (1979) describes a procedure for calcu- lating velocities in mountain streams with large roughness elements. Using his procedure and knowing the riprap diameter, a velocity was determined. This velocity was then used to check the stability of the riprap. If the rip- rap was calculated to be unstable, a larger size vas tried and the process repeated. This procedure calculates the required riprap size for the side slopes. The riprap size will therefore be more than adequate for the channel . bed. Since both of these channels are steep with large floes, riprap lining over the entire length is required. L Gradation requirements of the riprap were described in Section 6.2.4 for culvert design; the same criteria should be followed for the channels. in r addition, both a gravel filter and filter fabric should be used. The gravel ' layer should be 0.5 feet thick and it is necessary to prevent puncturing the filter fabrick. The filter fabric will provide maximum protection against removal of fine material form the bed and side slopes on which the riprap is placed. Figure 7 shows a typical channel cross section that is totally lined. The riprap is extended outside the banks to prevent against failure if the eater runoff should get out of the banks. ' Placement of riprap is also important. Riprap placement is usually by dumping directly from trucks. If riprap is placed during construction of the embankment, rocks can be dumped directly form trucks from the top of the em- bankaient. Rock should never be placed by dropping down the slope in a chute or pushed downhill with a bulldozer. These methods result in segregation of sizas. c:ith damned ricrap there is a minima o` expensive hand work. Poorly graded riprap with slab-like rocks requires more work to form a compact pro- tective blanket without large holes or pockets. Draglines with orange peel buckets, backhoes, and other power equipment can also be used advantageously to place the riprap. The first two alternatives for PM-CH-15 consider that the channel will be graded on a uniform slope starting at the existing invert elevation of the Magpie Creek diversion channel. Alternative 15-A uses side slopes of 2:1 and alter- . native 15-H uses side slopes of 3:1. The uniform channel bed slopes were deter- mined by setting the largest D50 size of the riprap equal to two feet. Riorap sizes larger than this were considered unavailable. These tvo alternatives keep