MINIMIZING POND SIZE USING AN OFF-SITE POND IN A CLOSED BASIN: A STORM FLOW MITIGATION DESIGN AND EVALUATION

Construction of large on-site detention pond to manage the stormwater runoff in the project area is not only expensive, but also a waste of developable land. To minimize the pond size, a systematic design of storm flow routing followed by model verification is necessary. This study presents a challenging stormwater management design for a site located in a complex urban setting at Tallahassee, Florida, USA. The site, a 4.6-hectare (11.4 acre) wooded area, was developed into a swimming pool complex resulting in increased post-development runoff. This increased runoff was managed by designing an on-site pond, minimized by placing it in series with an existing downstream off-site pond of a closed basin. The available storage of the downstream pond was efficiently used to reduce the upstream pond size. To minimize the on-site pond, the design considered re-arrangement and re-sizing of pre-development basins that allowed releasing some portion of post-development runoff below its pre-development level in the directions where it was allowed to drain. The excess runoff generated from the area was routed through the on-site pond and discharged into the existing off-site pond, where all runoff was retained to meet the guidelines of a closed basin. The short duration simulation results (8-hr and 24-hr design storms) confirmed significant off-site runoff reduction for the postdevelopment condition. Besides short duration simulations, the extended simulation results (for the entire 1-yr period) also revealed that the on-site and off-site ponds can jointly manage all extreme runoff including the runoff of a historical extreme wet year.