5.2.-5.4. PCSR 09-13-2011 - Laserfiche WebLink

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Runoff from DS-1 will be routed under the maintenance road through a 24" RCP before discharging into P-1. An 18" CPEP outlet pipe from P-1 will control stormwater run-off discharge. Details of the outlet structure are shown on the drawings. P-2 is a proposed sedimentation basin on the southeast corner of the SDA. P-2 will accept the runoff from the eastern half of the expansion including downslope structure (DS-2). Runoff from DS-2 will be routed under the maintenance road through a 24" RCP before discharging into P-2. A 12" RCP will discharge P-2 to P-1. The design parameters for the sedimentation basins are listed below: Bottom Top Inlet Pond Elevation Elevation Elevation Outlet Total Storage (ft) (ft (ft) Elevation (ft) Capacity (ac-ft) P-1 986 1000 994 993 10.4 P-2 1016 1028 1026 1021 11.5 Routing the 25-year, 24-hour storm event through the sedimentation basins provides the following information. The sedimentation basins design incorporates permanent dead storage, which is the volume below the normal outlet, that is equal to or exceeds the entire runoff volume to the pond which would result from the 25-year, 24-hr rainfall event. The calculations below assume a dry pond at the start of the storm and assume that the dead storage has infiltrated, which may sometimes occur between storm events. Peak Peak Peak Top Permanent Peak Storage Pond Inflow Outflow Elevation Elevation Dead Storage From Storm cfs) (cfs) (ft (ft ac-ft (ac-ft) P-1 77.0 1.6 993.6 1000 3.1 0.2 P-2 58.3 0.7 1021.4 1028 2.7 0.4 The table below takes a more realistic look at the sedimentation basin designs far the 25-year storm event by running the HydroCad model with the assumption that the pond dead storage is full. This is a more conservative approach that takes into account the potential for back to back T:\0742\1406Uune11 CityApp\Comment response\Response_08181 ]\Attachment t \Surface Water Management Memo 08101 l.doc 4