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Stormwater as a Resource: Rainwater Harvesting in the City of Los Angeles

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Abstract:

The City of Los Angeles has developed a robust stormwater program that entails not only water quality improvements, but also includes the enormous potential of rainwater harvesting. Rainwater harvesting includes at - or near-surface storage in rain barrels and cisterns, as well as groundwater recharge. This paper covers 1) the City 's programs to date, 2) potential end uses and treatment requirements, 3) potential water supply, water quality, water conservation, and energy conservation benefits, and 4) new techniques and technologies to improve efficiencies.

For decades Los Angeles has implemented many forms of stormwater capture and rainwater harvesting. With cooperation from the Los Angeles County Flood Control District, spreading grounds located in the San Fernando Valley have provided significant opportunity for groundwater recharge, however demands on those centralized systems have increased. As well, there has been a movement to expand rainwater harvesting into the decentralized realm which includes green streets, Low Impact Development (LID), cisterns, rain barrels, and similar technologies. In addition to developing proposed enhancements to the Tujunga Spreading Grounds and other large facilities, the City has recently completed the implementation of its Pilot Rainwater Harvesting Program where residences were retrofitted for rainwater harvesting and on-site use. Results are being analyzed for potential full-scale City-wide implementation.

Water quality requirements for potable use are published and regularly updated. Lesser understood are water quality requirements for harvested rainwater for outdoor, non-potable uses. Proposed treatment standards for cisterns depends on the end use, and include: 1) pre-screening for drip and/or subsurface irrigation, dust control, and outdoor water features, 2) pre-screening and disinfection, chlorination, or equivalent for spray irrigation; and 3) pre-screening, retention/sedimentation, and disinfection, chlorination or equivalent for street sweeping. As of yet, no treatment standards are proposed for rain barrels under 150 gallons that feed hose irrigation and/or car washing.

Groundwater recharge within centralized spreading grounds is directly measured, whereas the water conservation benefits of rainwater harvesting may be estimated with rain barrel/cistern usage models. The stormwater quality effectiveness of rainwater harvesting may be estimated as a function of the fraction of average annual runoff managed. Overall, the effectiveness of rainwater harvesting is heavily dependent on the demand profile for harvested water; where sufficient on-site use exists to recover storage in a reasonable timeframe, rainwater harvesting systems can achieve high capture efficiency and therefore high effectiveness. Where sufficient on-site use does not exist, the storage volume provided in cisterns and rain barrels is less “valuable” for improving stormwater quality as tanks will overflow more frequently.

Conventional harvesting systems achieve only sub-optimal stormwater control effectiveness due to the passive nature of storage management associated with the typical system. This results in lost water capture opportunities associated with rain barrel and cistern overflows due to precipitation events occurring when the storage volume is partially full. Applying automated controls to harvesting system storage incorporate inexpensive, real-time system monitoring and predictive weather information enables storage volume management. This management dramatically increases system efficacy in comparison to traditional passive storage systems. Implementation of automated controls on harvesting systems can more than double stormwater control effectiveness while reducing impacts to downstream drainage infrastructure and water bodies due to reductions in wet weather overflow events.

Keywords: Automated Controls; Direct Use; Groundwater Recharge; Integrated Water Resources; QMRA; Rainwater Harvesting; Stormwater Capture; Sustainable Water Resources; Water Conservation; Water Quality; Water Supply

Document Type: Research Article

DOI: https://doi.org/10.2175/193864711802793173

Publication date: 2011-01-01

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