PERMFILE72058 - Laserfiche WebLink

Home Browse Search

6.5 Vegetative Linings 6.5.1 General Vegetative linings can be a practical, economical method of channel pro- faction in regions where the vegetation can be grown. Minor erosion damage to a vegetative lining often repairs itself rhere a rigid-type lining would progressively deteriorate unless repaired; however, it is well known that vegetative linings do not withstand large shear forces, nor do they easily survive long periods of submergence. Therefore, under these conditions, vegr tative linings may be impractical and other Linings such as rock riprap should be utilized. Often campoeite linings consisting of rock riprap in areas of high shear or long term submergence and vegetation in the remainder of the crass section can be utilized to reduce coats. Intermittently spaced vegeta- tive diversions are commonly used on surface mine operations for long elopes of backfill areas and waste sites to collect drainage without gully erosion. 6.5.2 Design Procedure - Maximum Permissible Velocity Since about 1935, many flow tests over common American and Australian grasses have been performed and summarized by Cox and Palmer (1948), Flee and • Palmer (1949), and Fastgate (1966). In each test depth scour end general appearance of the channel was noted. Whenever conditions were such that unac- ceptable rates of scour and destruction of the channel lining occurred, the mean velocity of flow was noted. Then she maximum mean velocity the channel withstood without significant damage was suggested as the maximum permissible velocity. Velocities tabulated in Table 3 of the "Handbook of Channel Design for Soil and Water Conservation" are reproduced in Table 6.2. It should be noted that maximum permissible velocity Ls generally less for Steeper slopes. 711 so, velocities stated were often exceeded without damaging the axperiaental channels from which the data were derived. Of course, these channels were usually prepared with great care and under ideal conditions, resulting in vegetative linings of greater density and uniformity than those found in the field. Therefore, the designer should typically use slightly lower velocities to provide for a margin of error. Design of vegetated channels is complicated by the fact that the relative roughness is a function of depth or hydraulic radius. The Soil Conservation Service hoe identified the degree of retazdance by vegetation height according • to data given in Table 6.3. Design charts given in Figures 6.Sa to 6.Se can