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Low Impact Development (LID) is a more sustainable land development approach that begins with a site planning process that first identifies critical natural resource areas for preservation. Then, once the building envelope is established, LID techniques, such as maintaining natural drainage flow paths, minimizing land clearance, clustering buildings, and reducing impervious surfaces are incorporated into the project design. A series of small stormwater best management practices (BMPs) that preserve the natural features and hydrology of the land are used instead of the conventional methods of collecting, conveying, and piping away runoff.
Development patterns based on conventional zoning codes in Massachusetts often result in "sprawl" with its associated large impervious areas (such as parking lots), loss of natural resources and habitat, increase in nonpoint source pollution, and alteration of hydrologic systems. Conventional developments often start with clearing and leveling of the entire parcel. Then the construction of wide, paved roads and over-designed large parking lots follow.
These sprawling impervious areas eliminate vegetation (nature’s natural pollution filters), and prevent water from infiltrating into the ground (which normally replenishes groundwater supplies and supports nearby wetlands and streams with base flow). The result is the conveying of polluted runoff to water bodies In order to deal with stormwater that runs off of these sites, structural controls such as catch basins, pipes, and detention ponds are used. Instead of “greenscapes”, conventional landscaping of these developments brings additional concerns including the introduction of non-native plants, use of herbicides, pesticides and fertilizers, and excessive water consumption.
Besides loss of natural resource areas, habitat corridors, and buffers to wetlands and stream, conventional development (right hand graphic) creates large areas of impervious surfaces which prevent the infiltration of rainwater. Under natural (pre-development) conditions (left hand graphic), rain infiltrates through soils and percolates downward to the underlying water table, where it recharges the groundwater. Throughout the more permeable areas of Massachusetts approximately 50% of the annual precipitation infiltrates and recharges. Groundwater serves as drinking water supply and provides base flow to streams and wetlands. This base flow is critical to habitat quality for fish and other aquatic ecosystems.
The LID approach provides opportunities to build the homes and businesses that we need, while conserving natural areas and drainage patterns. LID is accomplished as a two-step process
Thoughtful site planning begins with the identification of critical site features such as wetlands, habitat areas, or drinking water protection areas that should be set aside as protected open space. Natural features, such as vegetated buffers and view sheds, will also play an integral role in any LID planning exercise. After the critical open space areas are identified and set aside, sustainable development areas are then identified as "building envelopes".
Within the delineated building envelopes, a broad range of design techniques or BMPs, such as shared driveways, permeable pavers, and bioretention are used to reduce the level of impervious cover and improve the quantity and quality of stormwater drainage. Other LID design techniques include:
Through these techniques, natural drainage pathways are conserved, open space is preserved, and the overall impact from development is significantly reduced.
Often LID techniques provide benefits beyond those related to water and drainage. For example, green roofs also:
These four examples of rain gardens look like conventional landscaped areas but are actually LID features that capture, treat, and slowly release rainfall to replenish local groundwater resources while beautifying parking lots, buildings, and roadways.
LID techniques implement several basic tenets of the Sustainable Development Principles including:
LID can increase property values. Converting or designing normally unused roof areas into green roofs, can increase your property value by reclaiming that elevation of a building and make it an amenity to be used by the buildings occupants.
LID results in reduced energy costs. Green roofs reduce heating and cooling costs for the building between 30 - 50%, compared to buildings with conventional roofs
There is a perception that LID projects may have to sacrifice good access to achieve the goal of reduced impact on natural systems. This is a real issue, but one that planners, engineers, public safety officials, and regulators can manage with careful design and common sense. Key issues and concerns include:
Key Design Elements:
Some critics of LID have raised the question of whether it costs more to design, implement, and maintain LID practices than it is does for conventional practices. While this is a very simple question to ask, the answer can be quite complex. First, the term "cost" must consider all costs, including: planning, design, and capital costs; long term maintenance costs; land values; and potential environmental impacts. One cannot simply assert that a practice costs less if it is less expensive to construct, since it may cost more to maintain, may result in environmental degradation, or lead to decreased property values in the long run.
This being said, costs for LID practices as well as for the conventional approach vary as a result of many factors, and generic costs cannot be easily quantified. Typical factors affecting cost include:
The table below is offered to help planners, engineers, regulators, and developers compare the costs and benefits of LID with a more conventional land development approach.
Qualitative Cost Comparison – How LID Practices Compare with Conventional Practices