Low Impact Development

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Services: Low Impact Development (LID)

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Low Impact Development is a land development approach aimed at managing stormwater runoff as close to its source as possible. Projects which make use of Low Impact Development practices strive to preserve or recreate natural landscape features, reduce and break up large areas of impervious surfaces, and promote the natural movement of water through a watershed. As municipalities and land use regulatory agencies begin to adopt Low Impact Development (LID) regulations, guidelines, and strategies, stormwater management provisions may begin to shift from a single traditionally large centralized drainage system that concentrates runoff to multiple smaller decentralized systems located closer to the runoff source in order to promote ground infiltration or sheet flow.

Low Impact Development stormwater measures include the use of bioretention facilities (rain gardens), vegetative practices (filter strips, swales, buffers), infiltration measures (drywells, leaching trenches), rainwater harvesting techniques (rain barrels), vegetated rooftops, and permeable pavements to treat stormwater runoff closer to the source. A description of each of these measures can be found below:

Bioretention Facilities/ Rain Gardens

Bioretention is a stormwater practice that manages and treats stormwater runoff through the use of a specially designed soil bed with specified plant materials to filter runoff stored in a shallow depression. Bioretention facilities make use of the physical, chemical, and biological properties of plants, microbes, and soils, which include adsorption, filtration, plant uptake, microbial activity, decomposition, sedimentation, and volatilization, to remove pollutants from stormwater runoff.

Rain gardens are a small scale form of bioretention that can be incorporated into a variety of new and existing residential, commercial, roadway, and parking projects. Rain gardens serve as functional landscape elements that consist of shrubs, grasses, and flowering perennials planted in a depression that provides a temporary pool of water following a rain storm. The soil matrix within the rain garden stores and absorbs the rainwater and provides nourishment for the plantings in the rain garden. Rain gardens a cost effective means of reducing stormwater runoff, providing groundwater recharge, and removing pollutants.

Vegetated Swales, Filter Strips, and Buffers

Vegetative practices, such as vegetated (water quality) swales, buffers, and filter strips, can be used to cost effectively provide water quality and quantity control for a variety of new and retrofit projects. These measures, which are flexible in design and layout, can promote the filtration, infiltration, and adsorption of the stormwater being directed to them.

vegetated swale

Water Quality Swales are vegetated open channels designed to provide both water quantity and quality control by promoting stormwater infiltration, filtration, and adsorption. Vegetated Water Quality Swales provide significantly higher pollutant removal than conventional grass drainage channels which are intended primarily for stormwater conveyance rather than providing water quality treatment. Water Quality Swales are typically characterized as either Wet or Dry Swales. Water quality swales can be used in place of curbs, gutters, and storm drain systems on residential and commercial sites to enhance pollutant removal and provide groundwater recharge, flood control, and channel protection benefits.

Vegetated Buffers are strips of natural or planted vegetation that are provided around sensitive resources such as wetlands, watercourses, or areas prone to erosion. These buffers, which consist of a strip of land with permanent undisturbed vegetation, provide protection (during and after construction activities) to sensitive natural resources that are located adjacent to a development. Vegetated buffers act to filter pollutants from runoff, protect receiving resource water quality and temperatures, provide wildlife habitat, and aesthetically screen development.

Vegetated Filter Strips are uniformly graded, vegetated areas located between pollutant source areas (parking areas, roadways) and downstream sensitive natural resources such as wetlands or watercourses. Vegetated filter strips are typically planted with grass or other close-growing type vegetation to serve as a means of pre-treatment prior to discharge to other filtering or bioretention practices. Vegetated filter strips typically treat stormwater sheet flow directly from adjacent impervious surfaces, or small concentrated flows that can be distributed along the width of the strip using a level spreader. Vegetated filter strips are commonly designed to slow runoff velocities, trap sediment, and promote infiltration.

Infiltration Measures

Infiltration measures, such as stone drywells and leaching trenches, are small excavated pits or trenches filled with stone aggregate that promote groundwater recharge and reduce the amount of runoff from the improvements they are intended to treat. Drywells and leaching trenches treat stormwater runoff through soil infiltration, adsorption, trapping, filtering, and bacterial degradation. The use of dry wells and leaching trenches is applicable for runoff from building rooftops and small drainage areas with low sediment or pollutant loadings. In addition, the soils surrounding infiltration measures must have have adequate infiltration rates and a groundwater table that is low enough to support the the use of infiltration.

Leaching Trenches are typically linear trenches that are dug and filled with stone aggregate that provides temporary stormwater storage in the void space between stones until the water is able to be infiltrated into a sites underlying soils. Often times, leaching trenches may sometimes include perforated pipes, vaults, or other form of modular stormwater infiltration system that will increase water storage within a trench. Leaching trenches are commonly used for managing roof or pavement runoff where there is little to no risk of pollutants.

Drywells commonly consist of a pit that is dug and filled with stone or a struture surrounded by stone that provides temporary stormwater storage until the water is able to be infiltrated into the underlying soils. Structural drywells are typically made of reinforced concrete and surrounded by stone. Drywells are typically used to manage roof or pavement runoff that is free of pollutants.

Rainwater Harvesting Techniques

rain barrelRainwater Harvesting techniques can be used to supply water for washing, irrigation, landscaping, and even some interior building uses as part of a greywater recovery system. The main goal of rainwater harvesting is to collect and retain a portion of water from a rain event and then used the collected water during periods of little to no rainfall. Typically, rainwater is harvested from building rooftops where it is directed to rain barrels or cisterns via pipe and gutter systems. Roof runoff is typically used since it typically free of sediment and other pollutants.

Rain Barrels are typically used to store small runoff volumes to provide water supply for landscaping and gardening. They are applicable for use on residential, commercial, and industrial sites and can be incorporated as part of a projects landscaping plans. Multiple rain barrels may be used to provide storage for larger runoff volumes. Rain barrels are usually found above ground and provide water supply via gravity or small pump.

Cisterns are typically much larger than rain barrels and may be located above or below ground. Cisterns are also applicable for residential, commercial, and industrial sites. Pre-manufactured cisterns come in a variety of sizes ranging from 100 to 10,000 gallons. They can also be constructed on site for large industrial, commercial, and public uses.

Vegetated Rooftops

Vegetated roof covers or “green roofs” are an effective means of reducing urban stormwater runoff by replacing impervious rooftop surfaces with permeable, vegetated surfaces. In a green roof design, growing media and plants take the place of bare membrane, gravel ballast, shingles, or tiles. The number and placement of layers that make up a green roof vary from system to system. All green roof systems include a single to multi-ply waterproofing layer, drainage system, growing media, and plant materials. Green roofs are separated into two primary groups which are termed intensive and extensive. A variety of factors, which include roof loading capacity, roof slope, budget, and use, are all used to a select an appropriate green roof design

Extensive green roofs (also called eco-roof or low-profile) are constructed when the primary desire is for an ecological roof with limited human access. An extensive greenroof consists of less layers and are thinner, lighter, and less expensive than an intensive greenroof. An extensive greenroof also usually requires less maintenance than an intensive one. Extensive greenroofs can be constructed on slopes up to 30°, and steeper ones can be installed with raised grids or laths to hold plants and soil media in place. The growing media or soil substrate depth ranges from 2 ½” to 6”. The engineered soil media usually consists of 20-30% organic material. Low growing, horizontally spreading root ground covers with maximum plant heights ranging from 16” to 24” are typically used. Common plants usually include sedums and other succulents, flowering herbs, and certain grasses and mosses. Alpine-type plants are often used due to their tolerance for heat, wind, frost, and drought. An extensive greenroof does require watering during the first year after planting. After that first year, the plants, if properly selected, should be self sufficient with regard to water except during extreme conditions.

roof garden planting plan

Intensive greenroof: An intensive greenroof (also called high-profile) is constructed when the primary desire is for human recreation and the encouragement of human/nature interaction. An extensive greenroof consists of a wider variety of vegetation with thicker, heavier layers than an extensive greenroof. An extensive greenroof is usually more expensive than an intensive greenroof and usually requires more maintenance than an extensive one. Extensive greenroofs are usually constructed on flat slopes. The growing media or soil substrate depth typically ranges from 8” to 12”. The engineered soil media usually consists of 45-50% organic material. An intensive greenroof will require regular watering due to the variety of plant materials. Often times irrigation systems are incorporated into an intensive greenroof design to provide water to the plants. Architectural accents such as waterfalls, ponds, or gazebos are also sometimes incorporated. An intensive greenroof differs from a roof garden in the fact that plant material is planted in soil layers that are spread upon the roof deck. A roof garden usually consists of plant material that is planted in pots or containers.

Greenroofs do require periodic maintenance depending on plant selection and greenroof type. Soils for a greenroof should be engineered to be lightweight, to provide proper drainage, and to minimize the amount of undesirable and unexpected weeds, pathogens, and insects. A root-resistant layer of dense, inorganic material, such as polyethylene, should always be provided to prevent root penetration into the roofing membrane. It is important to plant nonflammable succulents or other types of plantings that store water in their stems to reduce the risk of fire. It is also important to provide a 12”-24” perimeter free of vegetation to serve as a fire break and allow firefighters a sure footing in the event of a fire.

Permeable Pavements

Permeable pavements, such as porous asphalt, porous concrete, pervious pavers, and grass pavers, are all designed to allow rain and snowmelt water to pass through them. Allowing rainfall to penetrate a porous surface minimizes runoff potential, promotes groundwater recharge, provides some stormwater pollutant removal, and allows stormwater to follow a more natural path through a watershed. These porous materials also reduce the need for road salt and sand since areas of standing water are minimized resulting in a lower icing potential.

Porous Asphalt is similar to traditional asphalt in every way but the mix specification. Porous asphalt does not include the finer size particles used in traditional asphalt. By leaving out these finer particles, gaps are formed within the asphalt profile that allow water to flow through the pavement, rather than over the pavement. A typical porous pavement has an open-graded surface over an underlying stone recharge bed. Rain and snow melt water drains through the porous asphalt and into the stone bed, then, slowly, infiltrates into the soil. If contaminants were on the surface at the time of the storm, they are swept along with the rainfall through the stone bed. From there they infiltrate into the sub-base so that they are subjected to the natural processes that cleanse water. CLICK HERE for our new section on Porous Asphalt!

Porous Concrete is a structural concrete pavement with a large volume (15 to 35 percent) of interconnected voids. Like conventional concrete, it’s made from a mixture of cement, coarse aggregates, and water. Porous concrete differs from conventional concrete in that it contains little to no sand, resulting in a porous open-cell structure that allows water to readily pass through. Most porous concrete sections provide an underlying gravel recharge bed that provides temporary storage of rain or snow melt water until it is able to be infiltrated into the underlying site soils. Since the pavement acts as a retention area, it helps to minimize the amount of polluted runoff that normally occurs with traditional impervious pavements. In addition, the aerobic bacterial processes and act of water percolating through the open cells of the pavement, gravel, and underlying soils helps to promote water filtration and the break down of harmful pollutants and chemicals.

Pervious pavers are commonly made of pre-cast concrete, brick, stone, or cobbles. Pervious pavers are usually installed to form interlocking patterns and are placed within a rigid frame on top of a sand bed or an under drain system. Sand or gravel is placed in the gaps between pavers which allows water to pass to the underlying subgrade where it is infiltrated into the ground. Pervious pavers are available in a variety of colors, shapes, sizes, and textures, and can support heavy traffic loads and weights. They can replace conventional asphalt or concrete paving in parking lots, roads, outdoor recreational spaces, and sidewalks. By infiltrating precipitation, pervious pavers reduce stormwater runoff flow rates, volumes, and temperatures, and can filter pollutants.

picture of grass pavers

Grass pavers are a type of open-cell unit paver in which the cells are filled with soil and planted with turf. The pavers, which are made of concrete or synthetic material, distribute the weight of traffic and prevent compression of the underlying soil. Traffic volume, vehicle loads, and need for snow plowing can all limit application. Grass pavers also promote infiltration of precipitation and also aid in the removal of pollutants from precipitation. Due to the turf which grows in each of the open cells, grass pavers provide a more natural look and feel as compared to traditional hardscape.

There is currently great debate regarding the use of pervious pavements in cold weather climates. The debate which primarily focuses around the ability of pervious pavements to infiltrate precipitation when the ground is frozen and to withstand repeated freeze/thaw cycles. As more and more research is being conducted, it is being found that pervious pavement, if designed properly, can operate properly and withstand environmental conditions associated with cold weather climates. The University of New Hampshire Stormwater Center is currently conducting ongoing research into pervious paving materials and offers a wealth of knowledge in the field of Stormwater Quality Best Management Practices.

Be a Trendsetter

The goal of Low Impact Development (LID) is to maintain and mimic pre-development hydrology through the use of small scale controls integrated throughout the site. The practical use of Low Impact Development (LID) measures is largely dependent on a site’s conditions. Soil permeability, site slopes, and existence of an elevated water table or bedrock are all site characteristics that may limit the use of Low Impact Development (LID) practices. Local land use regulations, public perception, and breaking old habits also present obstacles to the implementation of Low Impact Development (LID) practices. If you are interested in developing a site using Low Impact Development (LID) techniques, contact us at (860) 354-9346 so that we can discuss how Arthur H. Howland & Associates, P.C. can provide you with cost effective, site specific, “green” solutions that you can feel good about.

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