Low Impact Development by Claire Caudill
Low Impact Development (LID) is a concept in smart urban planning which aims to protect watersheds in densely developed urban areas.
Its goal is to manage storm water runoff and its pollutants with an approach modeled after nature, taking care of rainfall at its source using decentralized management techniques.
Municipalities have been dealing with sewage and storm water management for years, using conventional engineering and large-scale centralized facilities. LID offers a more natural approach, cleaning and managing the rain water where it falls.
How does Low Impact Development accomplish its goal?
Low impact design techniques infiltrate, filter, store, evaporate and detain runoff close to its source.
LID slows down the generation of runoff by reducing the impervious footprint of a site, thereby reducing the amount of water needed to be treated.
It directs storm water runoff to on-site areas that can absorb and use the water, instead of piping it to a central holding station to be treated and released back into the system at a huge expense. In many areas conventional stormwater management sends untreated runoff directly into the receiving body of water.
LID cleans the runoff water through natural filtering processes.
How does Low Impact Development work with nature?
LID incorporates the framework of the existing landscape
discourages severe grading of the land, which upsets the natural topography
discourages clear-cutting/bulldozing of land and vegetation. This destroys nature's built-in filter system.
uses simple, nonstructural, low-cost, low tech methods
swales and natural low-lying areas are used to retain and filter rain water
By applying the principles of low impact development we can use nature to clean our water supply, control storm water runoff, enhance our urban environment, and keep development costs low.
Bioretention
When rain falls on impervious surfaces like rooftops, roads, and parking lots, rain does not soak into the ground, and storm water runoff is created. Stormwater runoff picks up pollutants such as fertilizer, pesticides, sediment, motor oil, litter, and pet and yard waste and delivers these pollutants to local streams and rivers. Impervious surfaces and traditional stormwater collection infrastructure also increase the speed at which rain water arrives at the receiving water body. In heavy storm events, the greater volume of runoff can cause flooding, and the excess speed and volume can cause bank erosion. On the other hand, when rain falls on natural areas, such as a forest or meadow, it is slowed down, filtered by soil and plants, and allowed to soak back into the ground. LID development seeks to mimic the natural process by using soil and plant based bioretention cells, often called rain gardens, to remove pollutants, reduce runoff volume, increase groundwater recharge and reduce the temperature of runoff from impervious surfaces. In addition, these gardens provide beneficial wildlife habitat. Rain gardens are placed between stormwater runoff sources (roofs, driveways, parking lots) and runoff destinations (storm drains, streets, and streams).
Permeable Paving
Most of the 'paving over' in developed areas is due to roads and parking lots, which play a major role in transporting increased and faster stormwater runoff and pollutant loads to receiving waters. Alternative paving materials can be used to locally infiltrate rainwater and reduce both the amount and temperature of runoff leaving a site. Permeable pavers and pervious concrete and also improve the quality of the runoff by filtering out pollutants. Use of these materials can also eliminate problems with standing water, provide for groundwater recharge, control erosion of streambeds and riverbanks, and provide for a more aesthetically pleasing site.
Laboratory research has found that permeable pavers made of interlocking concrete blocks can significantly reduce runoff loads of nitrates, phosphates, heavy metals and ammonium and can reduce surface runoff temperatures by 2 to 4 degrees Celsius. The ingredients mixture for pervious concrete has very little sand, which creates a high void content, giving it the ability to allow water to pass through it and into the ground at the rate of 3 - 8 gallons per minute per square foot.
Claire Caudill and Evelyn Born Shanley
testing permeable concrete
Green Roofs
Green roofs are a natural way to mitigate the effects of urbanization on water quality by filtering, absorbing and detaining rainfall. They are constructed of multiple layers, including a lightweight soil mixture, drainage and water storage layers, insulation and an impermeable membrane system. The soil is planted with a mix of plants that can grow in the hot, dry conditions of the roof and as well as withstand short periods of heavy rainfall during storms.
Green roofs are an important stormwater management tool. The plant and soil complex processes airborne pollutants and prevents them from entering the storm sewer system. They also reduce water volume and decrease the velocity of water flow into the drainage system. In addition to being attractive components of the urban landscape, they reduce the "heat island" effect, absorb carbon dioxide, reduce the temperature of the roof in summer and insulate the roof in winter, and increase the longevity of the roof structure by 2 to 3 times.
Green roofs provide stormwater management benefits by:
Utilizing the biological, physical, and chemical processes found in the plant and soil complex to prevent airborne pollutants from entering the storm drain system.
Reducing the runoff volume and peak discharge rate by holding back and slowing down the water that would otherwise flow quickly into the storm drain system.
Green roofs are not only aesthetically pleasing, but they also:
Reduce city "heat island" effect
Reduce CO2 impact
Reduce summer air conditioning cost
Reduce winter heat demand
Potentially lengthen roof life 2 to 3 times
Treat nitrogen pollution in rain
Negate acid rain effect
Help reduce volume and peak rates of stormwater
The hydrologic processes that can be influenced by design choices and aid in the management of stormwater include:
Interception of rainfall by foliage, and subsequent evaporation
Reduction in the velocity of runoff
Infiltration
Percolation
Shallow subterranean flow, through the soil
Root zone moisture uptake and evapotranspiration
What are the Low Impact Development benefits?
LID is cost effective, using simple techniques close to the storm water site
It is appealing to developers because of the low cost and the simple techniques
On conventional development sites, ponds must be set aside for storm water collection
When LID is incorporated, a retention pond is not necessary, thereby leaving the developer more land for profitable development
Less storm water is channeled into the municipal sewage facility or conventional stormwater channels reducing the need to expand this expensive and environmentally damaging infrastructure as the city grows
The natural landscape is preserved, enhancing the beauty of the community and providing wildlife habitat
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