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e e e DRAINAGE CRITERIA MANUAL (V. 2) HYDRAULIC STRUCTURES TABLE HS-6 Vertical Drops With Grouted Boulder Basin: Minimum Design Criteria for Grass-Lined Channels Meeting District Criteria Oesion Parameter Criterion Boulder slze-D: 18 inch minimum dimension Grout thickness-D. 10 inches" Basin deoression-B (see Fioure HS-101 1.5 ft Basin lenoth-Lb (see Fioure HS-10) 25ft ApDroachlenoth--La 10ft buried riprap Trickle flow zone provisions Install large boulder or baffles in center zone to break up high flow stream, or apply separate water surface analysis Other provisions A buried riprap zone should be installed for 10ft minimum downstream of the droo basin . Boulder size refers to the minimum dimension of all boulders measured in any direction. .. Bury all grouted boulders on side slopes by filling to top of boulder with lightly compacted in-situ soil and capping with a minimum of 4 inches of top soil. Vegetate buried surfaces with native grasses. 2.5 Baffle Chute DroDs The USBR has developed design standards for a reinforced concrete chute with baffle blocks on the sloping face of the drop, commonly referred to as baffled apron or baffie chute drops. There are references such as Hydraulic Design of Stilling Basins and Energy Dissipators (Peterka 1984) and Design of Small Canal Structures (Aisenbrey, et al. 1978) that should be used for the design of these structures. A baffle chute drop was constructed on Harvard Gulch that can be inspected for long-term performance (Wright 1967). The hydraulic concept involves flow repeatedly encountering obstructions (baffle blocks) that are of a nominal height equivalent to critical depth. The excess energy is dissipated through the drop by the momentum loss associated with reorientation of the flow. A minimum of four rows of baffle blocks is recommended to achieve control of the flow and maximum dissipation of energy. Guidelines are given for sizing and spacing the blocks. Designing for proper approach velocities is critical to structure performance. One advantage of this type of drop is that it does not require tailwater control. However, the designer does need to consider local flow and scour patterns in the transition back to the channel. Optimal performance occurs for a unit discharge of 35 to 60 cfslft of chute width, which happens to be a well-matched design for the District's grass-lined channel criteria. Refer to Rhone (1977) for guidance on higher unit discharge and entrance modifications to address backwater effects. 06/2001 Urban Drainage & Flood Control District HS-29