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<br />normal velocity Vn is 7 feet/sec. for erosion resistant soils and 5 <br />feet/sec. for easily eroded soils. <br />2. The trickle channel shown in this case is a rectangular <br />concrete channel. The concrete channel ends at the upstream end of the <br />upstream riprap apron. A combination cut-off wall and foundation wall, <br />to give the end of the trickle channel additional support, is provided. <br />The water is allowed to "trickle" through the upstream apron and through <br />the crest wall (discussed below). Riprap trickle channels would simply <br />feather into the upstream apron. <br />3. A lO-foot long riprap apron is provided upstream of the cutoff <br />wall to protect against the increasing velocities and turbulence which <br />result as the water approaches the sloping portion of the drop <br />structure. The same riprap and bedding design should be used as is used <br />for the portion of the drop structure downstream of the cut-off wall. <br />4; The crest wall is a very important part of the drop structure. <br />It has several purposes, one of which is to provide a level rigid <br />boundary section which distributes the flow evenly over the entire width <br />of the structure. This is extremely important since the selection of <br />the riprap is based upon the unit discharge, and without the wall, flow <br />concentrations could result which would greatly exceed the design <br />di scha rge. <br />The trickle channel is ended at the upstream end of the upstream <br />apron to prevent the trickle channel from concentrating additional water <br />at a point during high flows, thus exceeding the design unit discharge. <br />The apron and the crest wall combine to disperse the concentrated flow. <br />The trickle flows must be allowed through the crest wall to prevent <br />ponding. A series of notches in the wall will allow the trickle flows <br /> <br />~!t.: <br /> <br />,f; <br />