The retention of stormwater on-site with the goal of mimicking pre-development hydrology is increasingly being required or encouraged for new and redevelopment projects.
The recently adopted Ventura and Orange County Municipal Separate Storm Sewer System (MS4) NPDES permits require retention on site of runoff from storms up to the water quality design storm via infiltration, evapotranspiration and/or harvest and use. To date, the retention of stormwater on site has primarily been accomplished via infiltration and, to a much more limited extent, evapotranspiration. In only a few cases has rainwater harvesting and non-potable use been employed on a site scale; typically when it has been included it has been part of meeting LEED requirements. The feasibility and desirability of retaining stormwater on site up to a specific design storm has not been vetted technically on a national or regional scale. For example, there has been almost no consideration of the natural water balance in technical guidance. Often infiltrated volumes must be increased over natural conditions in order to match pre-development surface runoff volumes, yet there has been little consideration for whether increasing infiltration over natural conditions may be an issue. There has also been almost no assessment of the circumstances necessary for rainwater harvesting systems to work well for stormwater management. This paper presents some of the considerations for retaining on site and proposes conceptual criteria for determining whether it is feasible and/or desirable to do so. The paper reviews and discusses the general precipitation and runoff patterns for California and the west coast; the natural water balance and changes to that balance under developed and low-impact development conditions; under what conditions infiltration is feasible and desirable with examples of evaluations; what levels of evapotranspiration can be achieved; and finally presents some examples of the use of rainwater harvesting as a means to retain stormwater on site. Example modeling scenarios of rainwater harvesting for irrigation and toilet flushing are presented from Southern California and Portland, Oregon to highlight both the stormwater management results as well as impacts on potable water demand. Implications related to reclaimed water use will also be presented.
- Geosyntec Authors: Aaron Poresky, Eric Strecker
- Title: The Feasibility and Desirability of Stormwater Retention On Site in California and on the West Coast
- Event or Publication: Proceedings of the 2010 International Low Impact Development Conference, American Society of Civil Engineers (ASCE)
- Practice Areas: Water and Natural Resources Assessment, Management, and Restoration
- Date: 2010
- Location: San Francisco, California