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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />Rock protection, assumed here as Type M riprap, should continue from the chute outlet to a <br />minimum distance of approximately 4H at a riprap layer depth of 2.0 feet to prevent eddy currents <br />from undennining the walls. Additional rows of baffles may need to be buried below grade to <br />allow for downstream channel degradation. Determine if the downstream channel grade has <br />been stabilized to determine how many rows of baffles may need to be buried. <br /> <br />e <br /> <br />6. The baffle chute wall height (measured normai to the floor slope) should be 2.4 times the critical <br /> <br />depth based on peak discharge. The wall height will contain the main flow and most of the <br /> <br />splash. The designer of the area behind the wall should consider that some splash may occur, <br /> <br />but extensive protection measures are not required. <br /> <br />7. Determine upstream transition and apron sidewall height as required by backwater analysis. <br />Lower basin wingwalls generally should be constructed normal to the chute sidewalls at the chute <br />outlet to prevent eddy current erosion at the drop toe. These transition walls should be of a <br />height equal to the channel normal depth in the downstream channel plus 1 foot and length <br />sufficient to inhibit eddy current erosion. <br /> <br />8. The trickle flow channel should be maintained through the entrance transition apron, approach, <br />and crest sections. It may be routed between the first row of baffle piers. The trickle channel <br />should start again at the basin rock zone that should be slightly depressed and then graded up to <br />transition into the downstream channel to focus the low flows into the trickle channel. Figure HS- <br />12c illustrates one method of designing the trickle channel through the crest. <br /> <br />e <br /> <br />9. The conventional design shown in Figure HS-11a results in the top elevation of the baffles being <br />higher than the crest, which causes a backwater effect upstream. Figure HS-12a may be used to <br />estimate the extent of the effect and to determine corrective measures such as increasing the <br />upstream freeboard or widening the chute. Note that blocks projecting above the crest will tend <br />to produce upstream sediment aggradation. Channel aggradation can be minimized by the trickle <br />channel treatment suggested in Step 8. <br /> <br />Another means of alleviating these problems is by using the Fujimoto entrance developed by the <br />USBR and illustrated in Figure HS-12b. The upper rows of baffles are moved one row increment <br />downstream. The important advantage of this entrance is that there is not a backwater effect of <br />the baffles. The serrated treatment of the modified crest begins disrupting the flow entering the <br />chute without increasing the headwater. More importantly, this configuration provides a level <br />crest control. The designer may either bring the invert of the upstream trickle channel into this <br />crest elevation, widening the trickle channei as it approaches the crest, or he or she may have a <br />lower trickle channel and bring it through the serrated crest similar to Step 8. <br /> <br />e <br /> <br />HS-32 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />