BMP Monitoring Sites - Urban Drainage and Flood Control District

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UDFCD BMP MONITORING SITES Orchard Pond EDB For a perforated outlet, calculate the required area per row based on WQCV and the depth of perforations at the outlet. The lowest perforations should be set at the water surface elevation of the outlet micro-pool. The total outlet area is calculated by multiplying the area per row by the number of rows. Minimize the number of columns and maximize the perforation hole diameter when designing outlets to reduce chances of clogging by accepting the orifice site that will empty the WQCV in 36 to 44 hours. Trash Rack Provide a trash rack of sufficient size to prevent clogging of the primary water quality outlet. Size the rack so as not to interfere with the hydraulic capacity of the outlet. Using the total outlet area and the selected perforation diameter (or height), determine the minimum open area required for the trash rack. Use one-half of the perforated plate’s total outlet area to calculate the trash rack’s size. This accounts for the variable inundation of the outlet orifices. Basin Shape Two-Stage Design Shape the pond whenever possible with a gradual expansion from the inlet and a gradual contraction toward the outlet in order to minimize short circuiting. It is best to have a basin length to width ratio between 2:1 to 3:1. To achieve this, it may be necessary to modify the inlet and outlet points through the use of pipes, swales or channels to accomplish this. Always maximize the distance between the inlet and the outlet. A two-stage design with a pool that fills often with frequently occurring runoff minimizes standing water and sediment depostition in the remainder of the basin. The two stages are as follows:
UDFCD BMP MONITORING SITES Orchard Pond EDB A. Top Stage: The top stage should be one or more feet deep with its bottom sloped at 1 to 2 percent toward the trickle flow channel. B. Bottom Stage: The dry weather water surface of the active surcharge volume of the bottom stage should be 0.5 feet or more below the bottom of the top stage, but no less than 4-inches below the invert of the upstream trickle channel, and store no less than 0.5 percent of the WQCV. Provide a permanent micro-pool below the active storage volume of the lower stage in front of the outlet. The pool should be ½ the depth of the top stage depth described above, or 2.5 feet, whichever results in the larger depth. Line bottom of the micro-pool with concrete paving at least 6-inches thick or with grouted boulders grouted to the top of the boulders. Low-Flow Channel Basin Side Slopes Dam Embankment Conveys low flows from the forebay to the bottom stage. A concrete-bottomed channel is recommended to provide maintainability. Otherwise line its sides with buried Type VL soil riprap and bottom with concrete. Make it at least 4-inches deep if concrete lined sides and 8-inches if buried riprap sides are used. At a minimum provide capacity equal to twice the release capacity at the upstream forebay outlet. Basin side slopes should be stable and gentle to facilitate maintenance and access. Side slopes should be no steeper than 4:1 and the use of flatter slopes is recommended: the flatter, the better and safer. Design the embankment not to fail during a 100-year and larger storms. Embankment slopes should be no steeper than 3:1, preferably 4:1 or flatter, and planted with turf forming grasses. Poorly compacted native soils should be excavated and replaced. Embankment soils should be compacted to at least 95 percent of their maximum density according to ASTM D 698-70 (Modified Proctor). Spillway structures and
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BMP Monitoring Sites - Urban Drainage and Flood Control District

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