From SDCK Watershed Wiki
San Diego Region Low Impact Development(LID) Inventory
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What is Low Impact Development?
Low Impact Development is a stormwater management approach that applies the basic principle of modeling natural hydrologic function in accordance with the specific geotechnical opportunities and constraints of the site or development under consideration.
Low impact development is a system of practices intended to reduce the quantity of stormwater runoff produced and improve the quality of the remaining runoff by controlling pollutants at their sources and utilizing or mimicking the hydrologic functioning of natural vegetation and soil in designing drainage systems, including the practices:
- Source control best management practices—minimizing pollutants; isolating pollutants from contact with rainfall or runoff by segregating, covering, containing, and/or enclosing pollutant-generating materials, wastes, and activities; conserving water to reduce non-stormwater discharges;
- Conserving natural areas including existing trees, other vegetation, and soils;
- Minimizing soil excavation and compaction and vegetation disturbance;
- Minimizing impervious rooftops and building footprints;
- Constructing streets, driveways, sidewalks, and parking lot aisles to the minimum widths necessary, provided that public safety and a walkable environment for pedestrians are not compromised;
- Constructing low-traffic areas with permeable surfaces such as porous asphalt, open-graded Portland cement concrete, coarse granular materials, concrete or plastic unit pavers, and plastic grid systems (areas that should be considered for permeable surfaces include, but are not limited to, driveways, patio slabs, walkways and sidewalks, trails, alleys, and overflow or otherwise lightly-used parking lots);
- Draining runoff from roofs, pavements, and other impervious areas into one or more of the following natural drainage systems (* signifies with compost-amended soils as needed to maximize soil storage and infiltration)—bioretention area*, also known as a rain garden; vegetated swale*; vegetated filter strip*; infiltration trench; roof rainwater collection cistern; vegetated roof;
- Maintaining natural drainage patterns (e.g., depressions, natural swales) as much as possible, and design drainage paths to increase the time before runoff leaves the site by—emphasizing sheet instead of concentrated flow; increasing the number and lengths of flow paths; maximizing non-hardened drainage conveyances; and maximizing vegetation in areas that generate and convey runoff.
LID is different from LEED and Smart Growth although many of the principles overlap.
Bioswales are storm water runoff conveyance systems that provide an alternative to storm sewers. They can absorb low flows or carry runoff from heavy rains to storm sewer inlets or directly to surface waters. Bioswales improve water quality by infiltrating the first flush of storm water runoff and filtering the large storm flows they convey.
A key LID technique in a setting with soils relatively restrictive to infiltration is water harvesting, which can be applied at smaller residential scales using rain barrels or cisterns at larger scales in commercial and light industrial developments. Harvesting has been successful in reducing runoff discharged to the storm drain system and conserving water in applications at all scales.
Cisterns and rain barrel capture roof runoff from the roof downspout and provide an effective way to store and slowly release runoff into the soil. A cistern is an above ground storage vessel with either a manually operated valve or a permanently open outlet. If the cistern has an operable valve, the valve can be closed to store stormwater for irrigation or infiltration between storms. This system requires continual monitoring by the resident or grounds crews, but provides greater flexibility in water storage and metering. If a cistern is provided with an operable valve and water is stored inside for long periods, the cistern must be covered to prevent mosquitoes from breeding. A cistern system with a permanently open outlet can also provide for metering stormwater runoff. If the cistern outlet is significantly smaller than the size of the downspout inlet (say 1/4 to 1/2 inch diameter), runoff will build up inside the cistern during storms, and will empty out slowly after peak intensities subside. This is a feasible way to mitigate the peak flow increases caused by rooftop impervious land coverage, especially for small storms. Cisterns can be incorporated into the aesthetics of the building and garden. The cistern must be designed and maintained to minimize clogging by leaves and other debris. In the dryer regions of the County, cisterns and rain barrels may only fill up a couple times a year and may be more practical when the system is supplemented with graywater from a County Permitted Graywater System
Vegetated roofs (also known as green roofs and eco-roofs) offer a number of benefits in the urban landscape including: increased energy efficiency, improved air quality, reduced temperatures in urban areas, noise reduction, improved aesthetics, extended life of the roof, and most importantly improved stormwater management. Stormwater benefits include: reduction of stormwater run-off, reduce quantity of industrial effluent, extend lifetime of infrastructure, reduce CSO events, and reduce flooding potential
LID Database form
Sources : San Diego County LID Handbook