Westside Park

A US$2.9 million Proposition O project for the City of Los Angeles Bureau of Engineering. A city park takes its irrigation water from the drainage channel that runs beneath it. The project targets runoff from 3,700 acres beside South Fairfax Avenue, which drains to Ballona Creek and Santa Monica Bay.

Flow in the channel is intercepted and partly diverted to a pump lift station. The water is screened for floating waste and heavy sediment, then fed to a two-acre network of EPIC pipes in 13 in of sand over 2 in of gravel. When channel flow exceeds the park's demand, normal drainage carries on. The system is unpressurised and runs by gravity. It is laid in 15 sections of 300 to 500 ft on laser-levelled benches that follow the park's slope, each with one inlet and one outlet.

The chambers sit on Firestone EPDM liner with all sides folded up to hold water in cells. Moisture reaches the surface by capillary action along a winding flow path from high ground to low. Excess filtered stormwater is stored in underground chambers and, once they are full, is designed to discharge towards a dry creek and back into the storm drain. An existing tree was kept, with its root mass buried to finished grade and watered by capillary movement over the liner wall.

Wallace Laboratories analysed soil and water samples taken on 24 August 2011. The soil is sand with pH 7.77, and roots were 4 to 5 in deep at two months. The September 2011 site report found the pump lift station running non-stop, so the profile never rested and breathed. It recommended a 36-hour irrigation cycle once a week, noting that roots grow beyond 10 in when a profile is cycled correctly. The report concluded that the stormwater can be used for irrigation, with extra fertiliser needed.