Laserfiche WebLink
1 <br />BMP C202: Channel Lining <br />Purpose <br />To protect channels by providing a channel liner using either blankets or riprap. <br />Conditions of Use <br />When natural soils or vegetated stabilized soils in a channel are not adequate to prevent <br />channel erosion. <br />. When a permanent ditch or pipe system is to be installed and a temporary measure <br />is needed. <br />In almost all cases, synthetic and organic coconut blankets are more effective than <br />riprap for protecting channels from erosion. Blankets can be used with and without ' <br />vegetation. Blanketed channels can be designed to handle any expected flow and <br />longevity requirement. Some synthetic blankets have a predicted life span of 50 <br />years or more, even in sunlight. <br />• Other reasons why blankets are better than rock include the availability of blankets <br />over rock. In many areas of the state, rock is not easily obtainable or is very expens- <br />ive to haul to a site. Blankets can be delivered anywhere. Rock requires the use of <br />dump trucks to haul and heavy equipment to place. Blankets usually only require <br />laborers with hand tools, and sometimes a backhoe. <br />. The Federal Highway Administration recommends not using flexible liners <br />whenever the slope exceeds 10 percent or the shear stress exceeds 8 Ibs/ft2. <br />Design and Installation Specifications <br />See BMP C122: Nets and Blankets (p.288) for information on blankets. <br />Since riprap is used where erosion potential is high, construction must be sequenced so <br />that the riprap is put in place with the minimum possible delay. <br />Disturbance of areas where riprap is to be placed should be undertaken only when <br />final preparation and placement of the riprap can follow immediately behind the ini- <br />tial disturbance. Where riprap is used for outlet protection, the riprap should be <br />placed before or in conjunction with the construction of the pipe or channel so that <br />it is in place when the pipe or channel begins to operate. <br />The designer, after determining the riprap size that will be stable under the flow <br />conditions, shall consider that size to be a minimum size and then, based on riprap <br />gradations actually available in the area, select the size or sizes that equal or <br />exceed the minimum size. The possibility of drainage structure damage by children <br />shall be considered in selecting a riprap size, especially if there is nearby water or <br />a gully in which to toss the stones. <br />2014 Stormwater Management Manual for Western Washington <br />Volume // - Chapter 4 - Page 338 <br />