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<br />design, bedding requirements etc. Another useful publication is "Design <br />Guidelines & Criteria, Channels & Hydraulic Structures on Sandy Soil". <br />This publication has as its subject ordinary riprap drop <br />structures. Other types of drop structures, when properly designed, are <br />acceptable to the District. The designer should select the type of drop <br /> <br />structure based on cost, location, aesthetics, right-of-way, etc. <br /> <br />These criteria are for the 100-year discharge with no factor of <br />safety. Use of these criteria for more frequent discharges will result <br />in more frequent failures and should be avoided. <br /> <br />BACKGROUND <br /> <br />In early 1980 the UD&FCD started investigating the hydraulics of <br /> <br />riprap drop structures. It was discovered that the readily available <br /> <br />and commonly used standard riprap selection charts resulted in <br /> <br />unrealistically large riprap for drop structures. This resulted from <br /> <br />the fact that the flow depth to rock size ratios were very small which <br /> <br />indicated that typical drop structure conditions were working outside <br /> <br />the design range of these commonly available charts. As a result, it <br /> <br />was determined that the UD&FCD needed specialized expertise to assist, <br />~. <br />" <br /> <br />with the development of design guidelines. In late 1980 the District <br />contracted with M.A. Stevens, Ph.D. to research the subject and to <br /> <br />provide the District with guidance and recommendations. Dr. Stevens <br /> <br />drew on the published findings by such specialists as C.D. Smith in <br /> <br />Canada, H. Oliver in England, A.G. Anderson, et.al. of St. Anthony Falls <br /> <br />Hydraulic Laboratory and on his own expertise and publications. This <br /> <br />effort resulted in a letter report by Dr. Stevens to UD&FCD entitled <br /> <br />z <br />