Low Impact Development is a comprehensive land planning and engineering design approach with the goal of maintaining and enhancing storm water runoff of developing watersheds. LID systems utilize practices that create or mimic natural processes that result in infiltration, evapotranspiration or use of storm water in order to protect water quality and associated watersheds.
Principles of LID
- Preserve and recreate natural landscape features
- Minimize effect of development to site hydrology (imperviousness)
- Scatter integrated management practices throughout site development
- Maintain runoff rate and duration from a developed site
- Implement pollution prevention, proper maintenance, and public education
Why is LID Needed?
As land is developed, it is covered by structures, streets, driveways, and parking lots. These hard surfaces are commonly referred to as impervious surfaces. Rainwater cannot soak into impervious surfaces and the result is less water soaking into the ground (infiltration) and more water being transported into ditches and stormdrains that lead to the nearest waterway. This lack of infiltration and increase in flow rates into waterways can cause:
- Lower Base Flow – The amount of water in a stream is sustained between rain events by groundwater seeping into the bed and banks of a stream. Without infiltration to replenish groundwater levels, stream levels are lower.
- In-stream Erosion – Streams that have unnaturally large amounts of water flow due to impervious runoff, have their banks eroded or “deconstructed” at a much faster rate than what happens naturally. This can cause property damage.
- Aquatic Habitat – The powerful flow of water and the sediment it carries quickly removes or fills in the “homes” and hiding places for all types of aquatic creatures. The preservation of this habitat means healthier ecosystems.
- Water Quality Impairment – Stormwater runoff picks up pollutants from roads and other impervious surfaces. LID practices decrease runoff and help improve water quality.
- Localized Flooding – As rainwater quickly travels to the nearest stream through the stormwater system, the amount of water can overwhelm the capacity of the infrastructure and the capacity of the receiving waterbody causing flooding.
Economic & Environmental Benefits of LID
- Reduced Building and Maintenance Costs – With less volume of water leaving sites, the need for the construction of curbs, gutters, and other conventional stormwater infrastructure is reduced thus decreasing cost. Construction costs can also be reduced by leaving more of the natural area untouched thus decreasing the cost of grading. Natural areas also have less construction and maintenance cost compared to landscaping a newly cleared area. Overall preserving natural areas has shown to increase the value of residential lots while decreasing the construction costs overall.
- Added Value to Communities – The presence of more natural surroundings and improved aesthetics means increased marketing potential and desired properties. Access to open space and more possibilities for recreational opportunities all increase the desirability and value of property.
- Reduced Property and Infrastructure Repair Costs – By decreasing the volume and speed of water entering a local waterway, the number of flooding events will be reduced. Th.is can cause less damage to structures and roads meaning less money spent on rebuilding or repairing property and infrastructure.
- Improved Water Quality and Reduced Water Treatment Costs – Since stormwater flows to the nearest waterbody untreated, it can transport oil, bacteria, sediment, metals, hydrocarbons, and nutrients. Using LID practices will reduce the amount of stormwater reaching local waterways thus improving the water quality. In areas that use surface waters such as lakes and rivers as sources for drinking water, good water quality means a reduction in water treatment costs.
Common LID Techniques
- Rain Gardens (Do-It-Yourself Example). A garden that takes advantage of rainfall and storm water runoff in its design and plant selection. Two basic types include underdrained and self-contained. Both are designed to improve water quality, reduce runoff volumes and facilitate infiltration of clean water. Water in a rain garden should fully drain within four hours of a one inch rain event.
- Bioretention/Bioswale. Designed to provide an element of water quality control while also allowing quantity control by infiltrating and temporarily storing water runoff. Often used in development sites, the main goal is to minimize, detain and retain post construction runoff uniformly and mimic pre-development hydrology. In a swale design, water is often spread along a shallow lengthy graded ditch to maximize stormwater duration in the swale to allow for greater infiltration and trapping of contaminants and silt.
- Rain Barrel or Cistern (Do-It-Yourself Example). A waterproof container for holding liquids, built to catch and store rainwater. Distinguished from wells by their waterproof lining; rain barrels are often used to capture rainwater from rooftop gutters. The captured water can be utilized to irrigate the surrounding landscape and reduce stormwater runoff of developed property.
- Vegetated Roof (Green Roof). A roof of a building designed to be partially or completely covered in vegetation. Plant materials are layered over a waterproof membrane and aid in the absorption of rainwater. Green roofs also provide insulation, mitigate a heat island effect in urban areas, provide a pleasing aesthetic and create wildlife habitat.
- Permeable Pavers/Pavement. Specifically designed materials in conjunction with a base and subbase that allow the movement of storm water through the surface. Permeable pavers or pavement designs can reduce runoff, trap suspended solids and filter pollutants. The porous surface may require specific clean out maintenance based on design; but can be utilized for parking areas, sidewalks, paths, lawns, bikelanes and driveways.