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HYDRAULIC STRUCTURES DRAINAGE CRITERIA MANUAL (V. 2) 2.0 CHANNEL GRADE CONTROL STRUCTURES (CHECK AND DROP STRUCTURES) e 2.1 Planninq for the Future Channel grade control structures (typically check structures and drop structures) should be designed for future fully developed basin conditions. In the use of a natural channel. the effects of future hydrology and potential downcutting must be included so that the natural channel is property stabilized. Urbanization will create a base flow that, over time, will cause downcutting if not managed with grade control structures. "Drop structures" are broadly defined. They establish a stable stream grade and hydraulic condition. Included are structures built to restore damaged channels. those that prevent accelerated erosion caused by increased runoff, and grade control drops in new channels. Drop structures provide special hydraulic conditions that allow a drop in water surface and/or channel grade. The supercritical flow may go through a hydraulic jump and then return to subcritical flow. The focus of these criteria is on channel drops with primary emphasis on grass-lined channels. Check structures may be used to stabilize the natural low-flow channel in an unmodified floodplain. Thus, check structures also require additional consideration of the wider major flood path extending around the structure abutments. e Specific design guidance is presented for the following basic categories of drop structures: baffle chute drops (BCD), grouted sloping boulder drops (GSB). and vertical hard basin drops (VHB). All drop structures should be evaluated after construction. Bank and bottom protection and adjustments may be needed when secondary erosional tendencies are revealed. It is advisable to establish construction contracts and budgets with this in mind. Use of standardized design methods for the types of drops suggested herein will reduce the need for secondary design refinements. The design of the drop structure crest and provisions for the trickle or low-flow channel directly affect the ultimate configuration of the upstream channel. A shallow and/or dispersed trickle configuration will tend to result in some aggradation and a wetter channel bottom than might be associated with a wetland channel bottom. However, the wetland channel design would not contain a trickle channel because the low flows would be spread out uniformly across the entire channel bottom. A higher unit flow will pass through the trickle or low-flow area than will pass through other portions of the channel cross section. This situation must be considered in design to avoid destabilization of the drop and the channel. e HS-6 06/2001 Urban Drainage & Flood Control District