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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />There is a balance between the crest shape chosen, upstream channel stability, and the configuration <br />downstream of the drop which will result in optimal or reasonable energy dissipation, Further, there <br />is usually a single configuration of drop crest, upstream channel slope and base width which will result <br />in channel performance at the maximum flow conditions allowable by the UDFCD grass lined channel <br />criteria, <br /> <br />Note that the simplified methods in the revised criteria do not specifically address hydraulic <br />performance in the trickle zone, where higher unit discharge and energy will occur, The flow <br />characteristics in the trickle channel should be reviewed, Figures in the revised criteria provide basic <br />drop structure layout guidelines, while the remaining design criteria are provided in the specific <br />sections for each type of drop structure, <br /> <br />These relationships provided a good starting point for design, The designer should recognize the high <br />likelihood of erosion and stability problems with channels that perform at the maximum allowable <br />channel conditions. <br /> <br />RESEARCH NEEDS.. SECTION XV <br />Items 51,52,53 and 55 in the original 1986 remain as valid research and monitoring concerns, <br />Basically, they relate either the trickle channel an its incorporation in the drop and resulting effects <br />and control measures upstream and downstream, or with seepage and other surface related hydraulic <br />pressures under the structure, In any suspected or real problem situation, these factors should be <br />monitored and analyzed. <br /> <br />1-8 <br />