A pilot bioretention basin for storm water treatment was constructed by the California Department of Transportation (Caltrans) in Orange County California in winter of 2005. The basin was designed to accommodate both storm water quality treatment and flood control for the adjacent State Route 73. The pilot was a retrofit of an existing detention basin.The Bioretention basin has several components: vegetation of several species, a ponding area, a bypass structure, a mulch layer on the surface, a planting soil layer, a gravel layer, an underdrain system and a geomembrane liner to prevent infiltration (for monitoring purposes). The basin was also designed with two pretreatment devices, a bioswale and a gross solids removal device. Design of the basin followed the most recent guidance provided by Prince George's County, Maryland as closely as possible.This system was designed as an on-line device. The basin has a drainage area of approximately 4 acres.Construction of the basin took approximately 60 days and was driven by the nesting season of the California Knatcatcher, an endangered species. Due to the critical habitat area, vegetation selection for the bioretention area was crucial to both to water quality treatment performance and to provide habitat for the Knatcatcher.The planting plan for the basin was developed by native plant specialists from Cal Poly, San Luis Obispo. The basin was divided into three zones: the treatment area, the bioswale pretreatment area and the basin side slopes. Species seeded included Creeping Wildrye (Leymus triticoides), Salt Grass (Distichlis spicata), Clustered Field Sedge (Carex praegracilis), Tall Umbrella Sedge (Cyperus eragrostis Lam.), Mexican Rush (Juncus mexicanus Willd.), and Brown-headed Rush (Juncus phaeocephalus Engelm.). An irrigation plan was also developed including overhead sprinklers during the plant establishment period. Plug planting was also added.Operational monitoring occurred during the first wet season. The subsequent two wet seasons will be monitored for water quality treatment performance. Automated sampling equipment will be used. The presentation will present a lessons learned discussion on designing a bioretention basin for storm water treatment, developing a planting plan, developing an irrigation plan, the plant establishment period and operational monitoring in the Southern California climate. The first year of monitoring will also be discussed.
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