Design of forebay and micropool for highway stormwater detention basins.
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2016-12-25
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Abstract:A new procedure was developed to design a sediment forebay at the entrance of a stormwater detention basin. It was found that the geometry of the forebay depends on the size of the target particle for settlement in the forebay. The fall velocity of the target particle and the horizontal inflow velocity provide the basis to size the length and depth of the forebay pool. From the gradation curve derived from the soil laboratory, the target particle is recommended to have a diameter of 0.1 to 0.2 mm. It is expected that a forebay will intercept 60 to 70% of sediment load in stormwater. A micropool is designed to provide a continuous suction flow to drain the remaining water in the detention basin after the outlet screen become clogged. In this study, dry samples of float deposits were collected from the screens in detention basins, and they were analyzed in the soil laboratory to construct the gradation curves. It was found that D90=0.3 mm and the saturated specific gravity is 0.8 to 0.9. The new concept of float velocity was derived in this study. Using the float velocity as the basis, the micropool surface area and flow depth can be determined. As recommended in this study, the flow depth in micropool shall include evaporation loss through the period of inter event time, dead storage for solids, and suction head to sustain the flow. The screens covering WQCV outlet plates have long been a maintenance issues due to clogging and UDFCD now recommends only three or four orifices to maximize the orifice size and minimize clogging of the orifice plate. Implementation 1. New design procedures for forebay sizing and micropool design will be incorporated into CDOT‘s Hydraulic Design Manual. Technical papers will be prepared to translate the findings into engineering design charts. A condensed version of this report will be presented in conferences. 2. Sediment gradation samples collected at locations upstream and downstream of the EDB 502L will be submitted to the CDOT’s maintenance. EDB 502L is conservatively built and does satisfy the design criteria for both water quality and flood mitigation purposes.
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