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<br />usually be necessary to place the top of the wall a distance P above the <br />upstream channel bottom. This is done to create a higher water surface <br />elevation upstream, thus reducing the drawdown effects normally caused <br />by a sudden drop. P can be determined from Table 2. <br />5. The riprap stilling basin is designed to force the hydraulic <br />jump to occur within the basin. It is designed for essentially zero <br />scour. The floor of the basin is depressed an amount B below the <br />downstream channel bottom, excluding the trickle channel. This is done <br />to create a deeper downstream sequent depth which helps keep the <br />hydraulic jump in the basin. This arrangement will cause ponding in the <br />basin. The trickle channel can, depending on its depth, relieve all or <br />some of the ponding. The riprap can also be buried and vegetated to <br />reduce the ponded area to a smaller size. <br />The riprap basin can be sized using Table 3. The way to use the <br />table is to determine the required height of the drop C, the normal <br />velocity of the approach channel Vn, and the upstream and downstream <br />normal depths Yn and Y2. Both channels must have the same geometry and <br />Y2 must be equal to Yn in order to use the table. Enter the row which <br />contains the correct C, Vn and Yn and Y2 and select the riprap <br />classification and all necessary dimensions from that row. <br />f:: ~ <br />The riprap must be placed on bedding and filter fabric as shown in <br />Figure 3. The riprap should extend up the channel side slopes a <br />distance of Y2 + l' as projected from the downstream channel. The basin <br />side slopes should be the same as those in the downstream channel (4:1 <br />or flatter) up to the Y2 + l' location, above which riprap slopes as <br />steep as 2:1 are allowable. <br /> <br />It <br />