The following site design elements should be considered in low impact development.
(a) Reduce Limits of Clearing and Grading. The limits of clearing and grading refer to the site area to which development is directed. This development area includes all impervious areas (roads, sidewalks, and rooftops) and pervious areas (graded lawn areas and open drainage systems).
(1) To minimize hydrologic impacts on the existing site, the land cover area of development should be located:
A. In less sensitive areas or areas with lower value in terms of hydrologic function (e.g. developing barren clay soils will have less hydrologic impact than development of forested sandy soils).
B. Outside of sensitive area buffers such as streams, floodways, floodplains, wetlands, and steep slopes.
C. Outside of areas with soils which have high infiltration rates to reduce net hydrologic site impacts.
(2) Minimum disturbance techniques may be employed to further reduce the limits of clearing and grading, by restricting ground disturbance by identifying the smallest possible area and clearly delineating it on the site.. These techniques include:
A. Reduce paving and compaction of highly permeable soils;
B. Minimizing the size of construction easements and material storage areas during the construction phase of a development;
C. Avoid removal of existing trees where possible, and specifically those trees over 18 inches in diameter;
1. Minimizing imperviousness by reducing the total area of paved surfaces;
2. Disconnecting as much impervious area as possible to increase opportunities for infiltration and reduce water runoff flow;
3. Maintaining existing topography and associated drainage divides to encourage natural dispersed flow paths.
(b) Drainage as a Design Element. To reduce impacts created by land development, site planning should incorporate drainage by carefully conducting hydrologic evaluations and reviewing spatial site layout options. These procedures should be incorporated into the site planning process early on to understand and take advantage of site conditions. Hydrologic evaluation procedures can be used to minimize runoff potential and to maintain the predevelopment time of concentration. Open drainage systems, for example should be designed within natural landforms and land uses to become major design elements and amenities of a site plan or development plan. The stormwater management drainage system can suggest pathway alignment, optimum locations for open space, and potential building sites. The drainage system helps to integrate urban forms, giving the development an integral, more aesthetically pleasing relationship to the natural features of the site.
(c) Minimize Impervious Surfaces. The entire traffic distribution network, (roadways, sidewalks, driveways, and parking areas), are the greatest source of impervious area. Changes in the impervious area alter runoff, recharge values, and site hydrology. Managing the imperviousness contributed by road and parking area pavement is an important component of the site planning and design process. An appropriate strategy may avoid problems from runoff and water table depletion, by reducing such surfaces that prevent natural filtration. Methods that can be used to achieve a reduction in the total runoff volume from impervious surfaces are presented below:
(1) Narrow road sections. Reduced width road sections can be used to reduce total site imperviousness as well as clearing and grading impacts. By using a more rural road section the width of the street surface may be reduced. A more rural design also eliminates the use of concrete curb and gutter and allows for vegetated roadside swales.
(2) Reduced application of sidewalks to one side of primary roads. Total site imperviousness can also be reduced by limiting sidewalks to one side of primary roads.
(3) Reduced on-street parking. Reducing on-street parking requirements to one side, or even elimination of on-street parking altogether, has the potential to reduce road surfaces and therefore overall site imperviousness. Two-sided parking requirements may be unnecessary to provide adequate parking facilities for each lot.
(4) Vegetative roof systems. Moss, grass, herbs, wildflowers, and native plants may be used to create a lightweight and aesthetically pleasing permeable vegetative surface on an impervious roof area.
(5) Driveways. Driveways are another element of the site plan that can be planned to reduce the total site imperviousness. Some techniques that can be used include:
A. Using shared driveways whenever possible, but especially in sensitive areas.
B. Limiting driveway width (for both single and shared driveways).
C. Minimizing building setbacks to reduce driveway length.
D. Using driveway and parking area materials which reduce runoff and increase travel times such as pervious pavers or gravel.
(6) Permeable pavement surfaces. A variety of materials ranging from traditional asphalt, and concrete, gravel or pavers may be used to construct these surfaces. These roadways or parking areas must allow water to flow through, replenishing the soil areas directly beneath. The subbase underneath these permeable pavements must be engineered to accommodate temporary water storage and filtration.
(7) Minimize Directly Connected Impervious Areas. Additional environmental benefits can be achieved and hydrologic impacts reduced by disconnecting unavoidable impervious areas. Strategies for accomplishing this include:
A. Disconnecting roof drains and directing flows to vegetated detention areas.
B. Directing flows from impervious (paved) areas to stabilized vegetated areas.
C. Breaking up flow directions from large paved surfaces.
D. Encouraging sheet flow through vegetated areas.
E. Carefully locating impervious areas so that they drain to natural systems, vegetated buffers, natural resource areas, or infiltratable soils.
(8) Modify drainage flow paths. The time of concentration, in conjunction with hydrologic site conditions, determines the peak discharge rate for a storm event. Site and infrastructure components such as: travel distance (flow path); slope of the ground surface and/or water surface; surface roughness; and channel shape, pattern, and material components can affect the time of concentration. Techniques that can affect and control the time of concentration can be incorporated into site design by managing flow and conveyance systems within the development site:
A. Maximize overland sheet flow;
B. Increase and lengthen flow paths;
C. Lengthen and flatten site and lot slopes;
D. Maximize use of open swale systems;
E. Increase and augment site and lot vegetation.
(9) In order to reduce the volume of stormwater runoff and decentralize flows, a basic strategy incorporating the following low impact development practices and techniques should be integrated in the overall site design.
A. Open Swales. These may serve as alternatives to curb and gutter systems. Grass or other vegetation should be used to reduce runoff velocity and allow filtration, while channeling high volume flows safely away.
1. Plantings, checkdams, and other similar features may be incorporated to further reduce velocity and increase filtration;
2. Walkways shall be separated from roadways by such swales or relocated to another area;
3. Plant species used shall be selected for their tolerance to salt.
B. Rain Gardens. These areas provide storage for excess storm water to collect and filter into the soil. Typical components of these gardens include grass buffers, sand beds, a ponding area for excess runoff storage, organic layers, and planting soil and vegetation.
1. They shall be located on site away from any structures and/or roadways;
2. Downspouts should be directed towards such rain gardens;
3. Permanent ponds may be incorporated into the design of the garden;
4. Temporary storage areas without ponds may be used;
5. Such areas shall be landscaped with native plants and grasses;
6. Plantings shall be selected according to their ability to tolerate pollutants;
7. Annual maintenance guarantees must be provided for these areas in the site plan or development plan.
C. Filter Strips. These areas are designed to collect flow from large impervious surfaces such as parking lots. They may direct water into vegetated detention areas or special sand filters that capture pollutants and gradually discharge the water.
D. Cisterns/Rain Barrels. Cisterns are designed to store stormwater for irrigation during dry periods, rather than channeling it away. Cistern collection systems may be designed to be installed beneath permeable pavement areas allowing for maximum storage capacity. Rain barrels are smaller and often designed to collect roof drainage from small buildings.