Hidden Green Infrastructure: Reclaiming a Lost Infiltration Basin in the City of Torrance
In the 1950s, stormwater runoff from new housing developments in the City of Torrance was in many places routed to various sinks, where the runoff would be allowed to slowly seep into the soil, cleansing it of harmful pollutants, and recharging the groundwater aquifer used as a drinking water supply. But following storm drain improvements in the 1970s, runoff was rerouted to Machado Lake, and dry basins were left behind. Machado Lake is a freshwater reservoir approximately 40 acres in size within Ken Malloy Harbor Regional Park in the Harbor City neighborhood of Los Angeles. Currently there are impairments to Machado Lake’s designated beneficial uses caused by pollutants in stormwater runoff. Total Maximum Daily Loads (TMDLs) are required and/or have been established in Machado Lake for algae, ammonia, odor, polychlorinated biphenyls (PCBs), trash, eutrophic conditions, bacteria, and certain metals and pesticides.
The Walnut Basin project provides a way to reclaim one of these old basins and restore its original function as a natural wetland and infiltration basin while removing toxics and nutrients from runoff destined for Machado Lake. Walnut Basin, located in a residential neighborhood in southeast Torrance, is about two acres in size and has the capacity to store approximately 23 acre-feet of water. The Walnut Basin project called for the installation of a diversion structure in the 48-inch storm drain adjacent to the basin. The diversion structure was sized to take dry-weather runoff and stormwater runoff from storms up to the 85th percentile storm event, while allowing runoff from larger storm events to continue down the storm drain unimpeded. The diversion structure routes runoff to a hydrodynamic separator pre-treatment unit, where debris and sediments with medium-to-large grain sizes are removed. After passing through the pre-treatment unit, runoff enters a 48-inch reinforced concrete pipe, which was jacked into place in a narrow easement between two houses, that conveys water to Walnut Basin. Riprap was installed at the entrance to Walnut Basin to reduce the velocity of the runoff and prevent erosion.
The Walnut Basin project is designed for two phases. The first phase, which drains 56 acres of watershed within the cities of Torrance and Lomita, was constructed in the spring of 2020. The second phase will add an additional 722 acres from a storm drain north of Walnut Basin. Another diversion structure will be constructed in the second phase, and runoff from the second diversion structure will be pumped south to Walnut Basin. Implementation of both phases of the project will accomplish most of the required reduction from the critical baseline load for most pollutants, including for nutrients (72%), pesticides (79%), PCBs (78%), bacteria (100%), and lead (100% ).
An ongoing aspect of the project is a phenology study, or a study of water levels and plants, that will occur over the next year. The study will document the ponding areas and water levels within the basin, and the wetland plants that will return when the basin fills with water. Construction work within the basin was limited to designated areas to ensure compaction of soils would be kept to a minimum, thus allowing for greater future infiltration and plants to be established more readily.
The cost of the first phase of the Walnut Basin project was approximately $900,000, including both construction and design. The project was 50% funded with residual state grant funding from Proposition 13 (2000) and Proposition 40 (2002).