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HYDRAULIC STRUCTURES DRAINAGE CRITERIA MANUAL (V. 2) 2. For drops of 5 feet or less in height, use the UD-Channels Spreadsheet to determine the 100- year critical velocities in the low-flow channel and the main channel to size boulders for each section. e 3. For drops greater than 5 feet in height, determine the critical velocities using drawdown calculations to establish the 100-year flow depth at the toe of the drop. 4. For a composite channel, find critical velocity, V" for the channel cross-section segment outside the low-flow section. 5. For a composite channel, find critical velocity, Vm" for the low-flow channel cross-section segment. 6. For a simple trapezoidal or wetland bottom channel, find critical velocity, V" for the channel cross section. 7. Calculate rock-sizing parameter, Rp, for the channel cross-section segment outside the low-flow section or for a simple trapezoidal channel section using the critical velocity estimated for this segment of the cross section: Rp V SO.17 , (S, _1)"66 e in which: S = iongitudinal slope along direction of flow in fl/ft S, = Specific gravity of the rock. Assume 2.55 unless the quarry certifies higher specific gravity. 8. Calculate rock-sizing parameter, RpL' for the channel cross-section segment within the low-flow section using the critical velocity estimated for this segment of the cross section: V SO.17 R - me pL (S, _1)066 (HS-9) g. Select minimum boulder sizes for the segments within and outside the low-flow channel segment of the channel cross section from Table HS-5. If the boulder sizes that result for the iow-f1ow channel and the overbank segments of the cross section differ, decide to use only the larger sized boulders throughout the entire structure, or to specify two sizes, namely, one for the low- flow channel and the other for the overbank segments of the cross section. Consider the e HS-24 06/2001 Urban Drainage & Flood Control District