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e e e DRAINAGE CRITERIA MANUAL (V. 2) HYDRAULIC STRUCTURES channel. Baffle chute drops are best suited for grass-lined channels and should not be used for boatable streams. The basic design criteria and details are given In Figure HS-11 (adapted from Peterka 1984). Remaining structural design parameters must be determined for specific site conditions. Recommended design procedures are as follows: 1. Determine the maximum inflow rate and the design unit discharge, q = g . W 2. An upstream channel transition section with vertical wingwalls constructed 45 degrees to the flow direction causes flow approaching the rectangular chute section to contract. It is also feasible to use walls constructed at 90 degrees to the flow direction. In either configuration, it is important to analyze the approach hydraulics and water surface profile. Often, the effective flow width at the criticai cross section is narrower than the width of the chute opening due to flow separation at the corners of the abutment (see Section 5.0). 3. The entrance transition should be followed by a rectangular flow alignment apron, typically 5 feet in length. The upstream approach channel velocity, V, should be as low as practical and less than critical velocity at the control section of the crest. Figure HS-11 b gives the USBR- recommended chute entrance velocity. In a typical grass-lined channel, the entrance transition to the rectangular chute section will produce the desired upstream channel velocity reduction. The chute elevation (shown in Figure HS-11b) should only be above the channel elevation when approach velocities cannot be controlled by the transition. Extra measures to prevent upstream aggradation are required with the raised crest configuration. 4. Normally, the baffles should be sized at height, H, equal to 0.8 times critical depth at peak flow. The chute face slope should be 2:1 for most cases but may be reduced for low drops or where a flatter slope is desirable. For unit discharge applications greater than 60 cfs/ft, the baffle height may be based on two-thirds of the peak flow; however, the chute sidewalls should be designed for peak flow (see Step 8 below). Baffle block widths and spaces should equal approximately l.5H but not less than H. Other baffle block dimensions are not critical hydraulically. The spacing between the rows of baffle block should be H times the slope ratio. For example, a 2:1 slope makes the row spacing equal to 2H parallel to the chute floor. The baffle blocks should be constructed with the upstream face normal to the chute floor. 5. Four rows of baffle blocks are required to establish full control of the flow. At least 1 Y. rows of baffles should be buried in riprap where the chute extends below the downstream channel grade. 06/2001 Urban Drainage & Flood Control District HS-31