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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />1.7 Transitions and Constrictions <br /> <br />e <br /> <br />Channel transitions are typically used to alter the cross-sectional geometry. to allow the waterway to fit <br />within a more confined right-of-way, or to purposely accelerate the flow to be carried by a specialized high <br />veloc~y conveyance. Constrictions can appreciably restrict and reduce the conveyance in a manner that <br />is either detrimental or beneficial. For example, a bridge, box culvert. or constriction may increase the <br />upstream flooding by encroaching too far into the floodplain conveyance. whereas in another situation a <br />hydraulic control structure can be employed to purposely induce an upstream spill into an off-stream <br />storage facility. <br /> <br />1.8 Bends and Confluences <br /> <br />General considerations for lined channels and conduits are discussed in the MAJOR DRAINAGE chapter <br />of this Manual. Additional emphasis is added herein for certain situations. Channels and conduits that <br />produce supercritical flow may require special structural or design considerations. This discussion is <br />limited since these types of structures are generally associated with hydraulic performance exceeding the <br />recommended criteria for grass-lined channels. Extensive study, specialized modeling andlor analysis <br />may be required for these situations. <br /> <br />On the other hand, confluences are commonly encountered in design. Relative flow rates can vary <br /> <br />disproportionately with time so that high flows from either upstream channel can discharge into the <br /> <br />downstream channel when it is at high or low level. Depending on the geometry of the confluence. either <br /> <br />condition can have important consequences. such as supercritical flow and hydraulic jump conditions, <br /> <br />and result in the need for structures. <br /> <br />e <br /> <br />1.9 Rundowns <br /> <br />A rundown is used to convey storm runoff from high on the bank of an open channel to the low-flow <br /> <br />channel of the drainageway or into a detention facility. The purpose is to control erosion and head cutting <br /> <br />from concentrated flow. Without such rundowns, the concentrated flow will create erosion. <br /> <br />1.10 Enerav DissiDation <br /> <br />The energy of moving water is known as kinetic energy, while the stored energy due to elevation is <br />potential energy. A properly sloped open channel will use up the potential energy in a uniform manner <br />through channel roughness without the flow being accelerated. A grade control structure (i.e., drop and <br />check) converts potential energy to kinetic energy under controlled conditions. Selection of the optimum <br />spacing and vertical drop is the work of the hydraulic engineer. Many hydraulic structures deal with <br />managing kinetic energy-to dissipate it in a reasonable manner, to conserve it at structures such as <br />transitions and bridges, or occasionally to convert kinetic to potential energy using a hydraulic jump. <br /> <br />e <br /> <br />HS-4 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />