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DRAINAGE CRITERIA MANUAL MAJOR DRAINAGE • fabric. Seepage parallel with the fabric might be reduced by folding the edge of the fabric vertically downward about 2 feet (similar to a cutoff wall) at 12-foot intervals along the installation, particularly at the entrance and exit of the channel reach. Filter fabric has to be lapped a minimum of 12 inches at roll edges with upstream fabric being pi aced on top of downstream fabric at the lap. Fine silt and clay has been found to clog the openings in filter fabric. This prevents free drainage which increases failure potential due to uplift. For this reason, a double granular filter is often a more appropriate bedding for fine silt and clay channel beds. See Figures 5-3 a through 5-3 c for details on acceptable use of filter fabric as bedding. 5.4 Channel Linings Channel linings constructed from ordinary riprap, grouted riprap, or wire encased rock to control channel erosion have been found to be • cost effective where channel reaches are relatively short (less than 1/4 mile). Situations for which riprap linings might be appropriate ~~ are: 1) where major flows, such as the 100-year flood are found to ` produce channel velocities in excess of allowable non-eroding values (5 feet per second for sandy soil conditions and 7 feet per second in erosion resistant soils); 2) where channel side slopes must be steeper than 3:1; 3) for low flow channels, and 4) where rapid changes in channel geometry occur such as channel bends and transitions. Design criteria applicable to these situations are presented in this section. Section 5.4 emphasizes design requirements associated with ordinary riprap, while Section 5.5 contains additional design considerations specifically related to wire enclosed rock. Both Sections 5.4 and 5.5 are valid only for subcritical flow conditions where *.he Froude number is 0.8 or less. 5.4.1 Roughness Coefficient • The Manning's roughness coefficient (n) for hydraulic computa- tions has been found to be about 0.035 for wire enclosed rock and may ~4' 11 1G A7