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<br />e <br /> <br />e <br /> <br />e <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />HYDRAULIC STRUCTURES <br /> <br />6. The main stilling basin should be depressed 1 to 2 feet deep in order to stabilize the jump. A row <br />of boulders should be located at the basin end to create a sill transition to the downstream invert <br />elevation. It is advisable to bury riprap for a distance of 10 feet downstream of the sill to minimize <br />any erosion that may occur due to secondary currents. <br /> <br />7. Do not use slopes steeper than 4:1. Slopes flatter than 4:1 improve appearance while steeper <br />slopes will reduce structure stability. With high public usage, flatter slopes will help mitigate <br />reverse roller formation from higher tailwater depths that can cause submerged hydraulic jump <br />formation. <br /> <br />8. Simplified design criteria are provided in Table HS-4 for grouted sloping boulder drops. These <br />criteria are valid only where the channel flow conditions meet criteria in the MAJOR DRAINAGE <br />chapter. <br /> <br />TABLE HS-4 <br /> <br />Grouted Sloping Boulder Drops: Minimum Design Criteria for Grass-tined Channels <br />Meeting the District's Maximum Depth and Velocity Criteria <br /> <br />Design Parameter Drop Height (Hd) 5 Feet or Drop Height (Hd) Greater Than 5 <br /> Less Feet <br />Maximum drop slope 4Hto 1V 4H to 1V <br />Minimum boulder depth Use V, to size' Use V, to size" <br />Grout thickness-D, %Dr ~Dr <br />Basin depression 2 feet Do sequential depth analysis <br />Grouted rock approach-L, 8 feet 8 feet <br />Basin length-Lb .. <br />Erosive 25 feet 25 feet <br />Non-erosive 20 feet 20 feet <br />Basin width-B Same as crest width <br />Trickle and low-flow zone Install large boulders in center basin zone to break up high flow <br />provisions stream <br />Trickle zone protection width 3bj or b' (whichever is smaller; see Figure HS-7) <br />below drop <br />Other provisions A buried riprap zone shouid be installed for 2Hd (10 feet minimum) <br /> downstream of the basin <br /> <br />. Use critical velocity in low-flow and main channels to size boulders. <br /> <br />.. Use drawdown velocity at Hd to size low-flow and main channel section boulders. <br /> <br />Sizing of boulders for the simplified grouted sloping boulder procedure is based on the following: <br /> <br />1. This procedure can be used only for channels designed using the specified maximum velocities <br />and depths for grass-lined channeis in this Manual (see the MAJOR DRAINAGE chapter). <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-23 <br />