In undeveloped areas, very little rainwater or snowmelt runs off the land like it does in cities. Trees, plants and soil capture much of the precipitation, and some of it evaporates back into the air. Most of the precipitation that doesn’t evaporate or get captured by vegetation soaks into the ground where soil and microbes remove pollutants naturally. The water slowly recharges streams, wetlands and groundwater. Very little runs off, except in very large storms.
This natural hydrologic cycle is radically changed when land is developed in the way it has been for decades. Typical development clears the land of vegetation and covers it with hard surfaces such as roads, parking lots and rooftops. Construction compacts soils, so that even landscaped areas can generate unnaturally high runoff volumes. Storm drains are installed to get water out of the way by sending it into local streams or injecting it underground without treatment. Development dramatically increases runoff volumes which, even when controlled by detention basins, causes flooding, damages fish and wildlife habitat, and delivers urban pollutants such as oils and pesticides to local waterways. The decreased infiltration results in less cool, clean groundwater to recharge streams in the dry summer months.
We use the term low-impact development (LID) to mean a combination of practices that conserve natural resource areas and use existing natural site features with distributed, small-scale stormwater management practices to capture and treat runoff with vegetation and soil similar to a well-vegetated undeveloped landscape. Examples of such practices include street trees, bioretention areas (bioswales, rain gardens, etc.), pervious pavement, vegetated roofs, and soil amendments.
LID practices may be incorporated into existing as well as newly built developments in a community. They increase groundwater supplies and reduce the negative water quality impacts to streams and fish habitat, flooding, and in many cases the cost of stormwater treatment and infrastructure. They are aesthetically pleasing and have been shown to increase real estate values.