Before development, San Francisco was composed of sand dunes, grassy hills and wetlands that absorbed rainwater and infiltrated most runoff into the soil to replenish groundwater and streams. Today, impervious surfaces such as buildings, streets, and parking lots have covered most of these areas and prevent rainfall infiltration. Runoff picks up pollutants like oil and debris that washes them into the sewer system or other receiving water bodies like the San Francisco Bay, Pacific Ocean, and lakes and creeks. Under heavy rain events, this runoff can even contribute to localized flooding and combined sewer discharges.
A Sustainable Approach - Low Impact Development
Cities around the world are taking advantage of green stormwater management technologies often called Best Management Practices (BMPs) and Low Impact Development (LID) approaches that can help mitigate the effects of urbanization on stormwater. These technologies and designs mimic natural watershed processes by replicating pre-existing hydrologic site conditions. LID directs runoff to natural vegetated systems, such as landscaped planters, swales and gardens that reduce, filter or slow stormwater runoff. Strategic placement of these vegetated systems help mitigate the impacts of impervious surfaces and in some cases increase the level of service provided by the traditional sewer pipes.
VIEW urban hydrology and LID models here
Multi-purpose Design
Green stormwater management technologies such as swales, rainwater gardens, stormwater planters, green roofs, pervious concrete and green streets can be incorporated into various urban designs such as tree planters, container gardens, traffic calming devices (such as bulb-outs and medians), parks and schools to create multi-purpose designs that beautify, increase pedestrian safety, and provide either passive or active recreational opportunities.
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Traditional Approach
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LID Approach
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Traditional roof
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Green rooftops intercept
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Benefits of Low Impact Development
- Improves the treatment efficiency of the combined sewers by reducing peak volumes of runoff entering the system
- Reduces combined sewer discharge volumes
- Removes pollutants close to their source
- Uses rainwater as a resource
- Increases vegetation in the city
- Provides educational opportunities
Examples of Low Impact Development
- Reducing Impervious Surfaces (e.g., rooftop gardens, narrower streets, pervious pavement)
- Conserving Open Space / Maintaining Groundwater Recharge Areas, Buffer Zones, and Drainage Courses
- Infiltration Swales, Grading Strategies, and Open Drainage Systems (e.g., bioretention)
- Reducing the Use of Pipes, Curbs and Gutters
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Urbanization dramatically changes the natural hydrologic cycle. Low Impact Design (LID), applied across the watershed, can partially repair the cities hydrologic cycle while beautifying the city. LID uses stormwater as a resource and reduces combined sewer overflows.
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