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<br />"Hydraulic Design Criteria for Riprap Chutes and Vertical Drop <br /> <br />Structures" . <br /> <br />While Dr. Stevens was conducting his investigations the UD&FCD <br />staff was independently analyzing rock sizing needs in drop structures <br />using techniques other than standard design charts (1) (2). Techniques <br />used included boundary shear analysis, momentum/lift analysis, lift <br />coefficient analysis, and scaling up of lift and drag forces measured on <br />a small particle. UD&FCD analysis indicated that rock size requirements <br />can be significantly reduced; however, this lacked reliable laboratory <br />or field verification. Thus, when Dr. Stevens submitted his report, <br />UD&FCD was in the position to better evaluate its content and to develop <br />concise charts and detailed design guidelines. The information and <br />guidelines that follow are state-of-the-art for use of riprap in drop <br />structures. Since the aim of UD&FCD for use of these structures was <br /> <br />primarily for adequate function in ephemeral major drainageways during <br />the lOO-year type of event, the guidelines contain the minimum <br />requirements and each component part cannot be minimized in importance. <br /> <br />GENERAL DISCUSSION OF SLOPING DROP STRUCTURE DESIGN <br /> <br />--!.. <br /> <br />The design chart for the sloping drop structure is based upon the <br />unit discharge (q) of the approach channel, the riprap classification <br />and the slope of the drop structure, and is valid only for sub-critical <br />flow in the approach channel. The unit discharge is found by taking the <br />average or normal channel velocity (Vn) for the lOO-year discharge times <br />the normal depth of the channel (Yn). Since the Urban Storm Drainage <br />Criteria Manual (USDCM) maximums for a grass lined channel in erosion <br /> <br />,3 <br />