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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 />It is reasonable to retain a whitewater boating specialist to assist in the design criteria for a hydraulic <br />structure on a boatable stream. In particular, reverse rollers are to be avoided (USACE 1985). <br /> <br />1.3 Channel Grade Control Structures <br /> <br />Grade control structures, such as check structures and drop structures, provide for energy dissipation and <br /> <br />thereby result in a mild slope in the upstream channel reaches. The geometry at the crest of these <br /> <br />structures can effectively control the upstream channel stability and. to an extent. Its ultimate <br /> <br />configuration. <br /> <br />A drop structure traverses the entire waterway, including the portion that carries the major flood. A check <br />structure is similar, but is constructed to stabilize the low-flow channel (i.e., one carrying the minor or <br />lesser flood) in artificial or natural drainageways. It crosses only the low-flow portion of the waterway or <br />floodplain. During a major flood. portions of the flow will circumvent the check. Overall channel stability is <br />maintained because degradation of the low-flow channel is prevented. Typically. the 2-year flows are <br />contained in the protected zone so that the low-flow channel does not degrade downward. potentially <br />undermining the entire waterway. <br /> <br />1.4 Wetland Channel Grade Control <br /> <br />Wetland channels, whether low-flow channels or from bank to bank. require modest slopes not exceeding <br />about 0.3%. Grade control structures are often required for stability. Due to the environmental nature of <br />the wetlands. the grade control structures are planned and designed to be compatible with a wetland <br />environment. Wetland channels do not need a trickle channel, but where used, the trickle channel should <br />not lower the wetland water table more than 12 inches. <br /> <br />1.5 Conduit Outlet Structures <br /> <br />Design criteria given in this chapter are for structures specifically designed to dissipate flow energy at <br />conduit outlets to the open waterway. These types of structures are typically located at storm sewer <br />outlets. Design criteria for culverts and storm sewers that discharge in-line with the receiving channel are <br />described in the MAJOR DRAINAGE chapter of this Manual. <br /> <br />1.6 BridCles <br /> <br />Bridges have the advantage of being able to cross the waterway without disturbing the flow. However, for <br /> <br />practical, economic, and structural reasons, abutment encroachments and piers are often located within <br /> <br />the waterway. Consequently, the bridge structure can cause adverse hydraulic effects and scour <br /> <br />potential that must be evaluated and addressed as part of each design project. <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-3 <br />