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<br />These criteria result in channels with significantly flatter slopes than the historic <br />floodplains. Grade control and bank stabilization measures are often investigated <br />and constructed for grass channels, wetlands waterways, structural channels, and <br />"preserved" floodplains. <br /> <br />mSTORY OF DROP CRITERIA IN THE DENVER METROPOLITAN REGION <br /> <br />In the original USDCM (UDFCD, 1969), grade control structures were not well <br />addressed. Over the years a variety of drops were constructed including vertical <br />configurations (sheet pile with riprap basins, gabions, shotcrete covered gabions, <br />concrete, boulders or grouted boulder crests with riprap); and sloping <br />configurations (riprap, gabions, grouted riprap, grouted boulders, concrete aprons <br />to soft basins, baffle chutes, concrete aprons to hard dissipator basins often using <br />Saint Anthony Falls, SCS or USBR guidelines). In 1982 UDFCD (UDFCD, 1982) <br />published an interim standard for sloping riprap drops at the request of the <br />development community. The rock sizing was based on the work of Smith, 1965. <br /> <br />Smith had determined an equation predicting when riprap motion would occur and <br />recommended a second equation with a safety factor. UDFCD essentially adopted <br />the first, presumably feeling that some movement during a 100 year flood was <br />reasonable. Many of the riprap drops subsequently constructed experienced <br />significant movement, especially because of poor construction quality and difficulty <br />verifying gradation. Because of the riprap drop problems, the wide range of <br />performance of the numerous other drop types, and related channel problems, the <br />UDFCD initiated an investigation effort in 1986 under the direction of DeGroot. <br />Taggart led the investigation effort for McLaughlin Water Engineers (Taggart, <br />1986). The work established new guidelines in 1990 (UDFCD, 1990) describing <br />four successful drop types. These guidelines were primarily for grass and wetlands <br />waterways, but were also applicable to floodplains and other types of channels. <br /> <br />UDFCD DROP GUIDELINES <br /> <br />Baffle Chute Drops (BCD), Grouted Sloping Boulder Drops (GSB), Vertical Hard <br />Basin Drops (VHB), and Vertical Riprap Drops (VRR) are the primary types of <br />drops recommended for grass lined channels. <br /> <br />Baffle Chute Drqps (BCD) <br />The United States Bureau <br />of Reclamation (USBR) <br />has developed design <br />standards for a reinforced <br />concrete chute with baffle <br />blocks on the sloping face <br />of the drop. The <br />excellent references, "Hydraulic Design of Stilling Basins and Energy Dissipators"; <br />Peterka, 1958, and "Design of Small Canal Structures"; Aisenbrey, et aI, 1978, <br />should be used for the design of these structures. Optimal performance occurs for <br />a unit discharge of 3.25 to 5.6 cms/m (35 to 60 cfs/ft) of chute width, which <br />happens to be a well-matched design for the UDFCD grass lined channel criteria. <br /> <br /> <br />Several channels which utilized baffle drops experienced backwater problems, <br />aggregation and an unintentional formation of a trickle channel upstream of the <br /> <br />2 <br /> <br />Taggart, et al <br />