Stormwater retention basins store water accumulated during a rain event and release the water at a controlled rate to prevent or limit downstream flooding and/or limit downstream waterway erosion.
Sedimentation ponds and sedimentation basins (hereinafter referred to as sedimentation basins) are designed to trap stormwater sediment within the boundaries of a property and prevent the sediment from flowing into downstream waterways. However, sedimentation basins are also often also used to meter the flow of stormwater to downstream waterways, thereby reducing the potential for flooding and downstream erosion.
Some basins have a discharge opening or orifice with a fixed cross sectional flow area located near the bottom of the basin. As the water level in the basin increases during a storm event, the rate of water discharged from the basin (which is essentially proportional to the square root of the water depth) increases with water level. Because the basin may fill quickly during a storm event, water is discharged from the basin at a maximum rate when the basin is at its most full condition (which is normally soon after the storm event has occurred).
Stormwater events often cause downstream flooding and also scour sediment from the bottom and sides of waterways. The greater the flow of water, the worse the problem becomes downstream. Site development, which tends to include impermeable surfaces such as parking lots, roofs, and the like, normally acts to increase the rate of site discharge and contributes to downstream flooding and erosion problems.
Typically, all of the sources of stormwater for a given waterway are discharging at their maximum rates during or shortly after a rain event. These sources include underground and aboveground stormwater storage systems, conventional stormwater collection systems, and also overland flows (sheet flows). Given that site development increases the potential for downstream flooding and stormwater volume related problems, it is often advantageous to postpone or delay the onset of the maximum rate of discharge from a basin after the storm event, or to limit the maximum rate of discharge from the basin after a storm event.
Faircloth, Jr. U.S. Pat. No. 5,820,751 discloses a device that maintains a drain inlet orifice at a constant distance below the water surface of a basin despite raising or lowering of the water level in the basin. Because the inlet orifice is maintained at a constant distance below the surface, the water flows into the orifice at essentially a constant flow rate that is also essentially independent of the basin's water level. Even after the basin fills with water during a storm event, water discharges at that constant flow rate. This can be advantageous because it eliminates the early peak flows from the basin and protects downstream waterways from those peaks. This is especially important in that the peak flows from the various sources which contribute to the flow of a given waterway normally are all releasing at their maximum rate at the same time.
Even with a constant flow device controlling one or more sources, downstream waterways will still be subjected to maximal flows from sources that are not volume/rate controlled. These sources may include conventional stormwater collection systems and normal flows across natural surfaces leading to the streams and rivers.
There is a need, therefore, to delay the peak flows from a basin until preferably well after the rain event. The basin would preferably release water at a relatively lower rate early in a storm event near the start of the basin discharge period, and would discharge a relatively higher rate (including the maximum rate) more towards the end of the basin discharge period.