November 18, 2009

Managing stormwater runoff at Lake Merced

Plan view of Sunset Circle parking lot

Trees, vegetated swales, slopes, and infiltration basins: four elements of the stormwater runoff management landscape at the Sunset Circle parking lot off Lake Merced Boulevard. The project was designed by the San Francisco Public Utilities Commission and the Department of Public Works. The management system was designed to slow and to clean stormwater runoff before it enters the lake. The landscape is an example of Low Impact Design (LID).

Vegetated swale

Vegetated swales were designed to capture stormwater runoff from the parking lot paving. Source: SF Public Utilities Commission Sunset Swales Project Summary

Main infiltration basin

The infiltration basins reduce and delay stormwater flows by capturing and holding runoff. They also allow stormwater to percolate into the soil, recharging groundwater and postponing or completely bypassing drainage into on-site catch basins. Source: SF Public Utilities Commission Sunset Swales Project Summary

Two central swales, planted with coast live oaks and native herbaceous plants, shrubs, and grasses such as Berkeley sedge (Carex tumilicola), capture and direct stormwater from the parking lot surface to a series of swales located at the edge of the parking lot and connected to a central infiltration basin (there are a total of three basins). Cutouts in the concrete lips of the swales allow stormwater to flow into the swales. The exterior swales are connected to each other by concrete tunnels on top of which pedestrian walks were constructed. Click here to see the swales on a rainy day.

The oaks are not faring well; 7 of 18 are dead or in poor condition (0-50% live canopy). I did not find any information about the causes of tree death; perhaps the oaks are less tolerant of contaminants in the stormwater runoff than the other vegetation.

Question: Are swales being used to manage stormwater runoff in your city?

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