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<br />to do this. The size and number of notches will depend on the design <br />discharge of the trickle channel. Note that they are offset from the <br />trickle channel to permit flow of water through the upstream apron. The <br />voids in the riprap below the notch inverts are expected to silt in <br />rapidly or they can be filled at the time of construction. The crest <br />wall can also be used to reduce or eliminate seepage and piping and the <br />failures which can result from these problems. <br />The two most common types of wails used will probably be reinforced <br />concrete or sheet pile. The design of the wall is a structural problem <br />which will not be addressed here. The depth of the wail should be at <br />least to the bottom of the bedding material and could be deeper if <br />necessary for the control of piping. <br />It will usually be necessary to place the top of the crest wall a <br />distance P above the upstream channel bottom. This is done to create a <br /> <br />higher water surface elevation upstream, thus reducing the drawdown <br />effects normally caused by a drop structure. P can be determined from <br />Table 2. P is not considered in the total allowable vertical drop. <br />5. The riprap chute portion of the drop structure and the <br />downstream apron can be sized using Table 1. The way to use the table <br />is to compute q = VnYn, enter the table at the next highest value of 9, <br />~ <br />in the left-hand column, determine the allowable slopes for the three <br />riprap classifications in the row for that q and select the best <br />combination of riprap classification and slope using site and cost <br />considerations. The length of the downstream apron La and the depth of <br />the riprap DR can also be obtained from Table 1. The riprap must be <br />placed on bedding and filter fabric as shown in Figure 2. The two foot <br />long filter fabric cutoffs help prevent piping failures. The riprap <br /> <br />~ <br />