San Diego’s Stormwater Pilot Program: Designing, Monitoring, and Maintaining a Non-Proprietary Subsurface Filter
In 2007 the City of San Diego (City) formulated a strategy for progressively implementing pilot stormwater best management practices (BMPs) across six watersheds over a period of five years. Major drivers of this strategy were existing and developing TMDLs which require measurable reductions in pollutant discharges within a period of 10 to 20 years for indicator bacteria, metals, and nutrients. One of these pilot study sites, the 43rd Street and Logan Avenue pilot project, investigated the efficacy of implementing a non-proprietary, under-sidewalk filtration units (curbside filter) and a surface biofiltration facility (basin) as standard approaches for addressing stormwater quality management needs in urban environments where typical green infrastructure is not feasible. The curbside filter featured a specialized high-flow-rate filter media of rhyolite, zeolite, granular activated carbon, and peat moss for targeted removal of dissolved metals; the specialized filter media performance was validated through university laboratory column tests with synthetic stormwater prior to field placement. The characteristics of the curbside filter media enabled treatment of relatively high flows from a 3-acre tributary drainage area (compared to the smaller 0.7-acre drainage area to the basin), but this also warranted robust pretreatment design to accommodate design flow rates with heavy solids loading and to prolong the design life of the costly media. Alternative pretreatment configurations were tested in 2013 at a university setting using full-scale simulated runoff events performed at a range of synthetic stormwater flow rates and quality, consistent with site characteristics. Modes of failure were forensically determined and the most effective pretreatment alternative was selected for incorporation into the pilot study site based on metrics linked to anticipated maintenance frequency.
The City received grant funding in 2014 to monitor the field effectiveness of the pilot BMPs over a two-year period, with measurements of flow, infiltration, stormwater quality, and soil moisture and chemistry. Seven storm events were monitored during this period, ranging from 0.25 inches to 1.4 inches. Preliminary results from the first monitored water year (2015) indicated unexpectedly high stormwater volume retention despite the BMPs’ impermeable liners and underdrains (volume reductions ranged from 11-29% and 31-82% on storm-event-bases for the curbside filter and basin, respectively). Both BMPs also consistently reduced loading of zinc to receiving waters, although preliminary nutrient and bacteria water quality results substantiate several concurrent studies that indicate deficient removals due to soil media composition. Continuous soil moisture data will demonstrate the utility of stormwater as a resource to reduce irrigation demand of BMPs. Preliminary results will be finalized in May of 2016 after analysis of the Water Year 2016 storm events, and will provide valuable information for an area generally lacking long-term BMP monitoring data.
Results from the two-year monitoring study and lessons learned from project planning will be summarized for a series of public workshops during summer of 2016 to solicit necessary feedback on system design and maximizing ancillary benefits from BMPs in the public right-of-way. Workshop feedback will be summarized to provide stormwater program managers and practitioners with clear, concise outcomes and recommendations for future projects, including trash management and pretreatment standards, strategies to optimize BMPs for site-specific conditions, lessons and constraints from building within the transportation right-of-way, methods for prescribing targeted maintenance, and designing stormwater systems for ease of monitorability. Stormwater managers will be engaged with videos and photos of system performance and with provocative monitoring results that are placed into concise, relevant context.