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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />2.2 DroD Selection <br /> <br />e <br /> <br />The primary concerns in selection of the type of drop structure should be functional hydraulic <br />performance and public safety. O1her considerations include land uses, cost, ecology, aesthetics, and <br />maintenance, and environmental permitting. <br /> <br />Table HS-1 presents information to assist in the selection of appropriate drop structures applicable for <br />various situations. Generally, the drops in any group are shown in order of preference. Comparative <br />costs are often close, and on-site conditions, safety, and aesthetics may weight the selection toward a <br />drop structure other than the first alternative indicated. <br /> <br />TABLE HS-1 <br /> <br />Non-Boatable Drop Structure Selection for 3- to 5-Foot Vertical Drops <br />and 0 to 15,000 cfs <br /> <br />1. High and low public usage with likely downstream degradation. <br />a) Baffle chute drop <br />b Grouted slo in bouider dro <br />2. Low public usage and no likely downstream degradation. <br />a) Grouted sloping boulder drop <br />b) Vertical hard basin drop <br />c Baffle chute dro <br />3. High public usage and no likely downstream degradation. <br />a) Grouted sloping boulder drop <br />b Baffle chute dro <br /> <br />e <br /> <br />From an engineering design standpoint, there are two fundamental systems of a drop structure: the <br />hydraulic surface drop system and the foundation and seepage control system. The material components <br />that can be used for the foundation and seepage control system are a function of on-site soils and <br />groundwater conditions. The selection of the best components for design of the surface drop system is <br />essentially independent of seepage considerations and is based on project objectives, channel stability, <br />approach hydraulics, downstream tailwater conditions, height of drop, public safety, aesthetics, and <br />maintenance considerations. Thus, foundation and seepage control system considerations are discussed <br />separately. One factor that influences both systems is the extent of future downstream channel <br />degradation that is anticipated. Such degradation can destroy a drop structure if adequate precautions <br />are not provided. <br /> <br />2.3 Detailed Hydraulic Analysis <br /> <br />2.3.1 Introduction. Analysis guideiines are discussed in this section to assist the engineer in addressing <br />critical hydraulic and seepage design factors. For a giyen discharge, there is a balance between the crest <br />base width, upstream and downstream flow velocities, the Froude number in the drop basin, and the <br />location of the jump. These parameters must be optimized for each specific application. <br /> <br />e <br /> <br />HS-10 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />