The Benefits of Low Impact Development
Hydrology and Low Impact Development can help military installations and communities reduce the impact of the constructed environment on surrounding natural resources.
By Patricia Donohue, P.E., PMP, CEM, CBCP
Surface water is the latest focus area for conservation as droughts and floods alternately impact the regional hydrologic cycle. The emergence of Low Impact Development (LID), which utilizes new construction techniques to manage and conserve stormwater on site, is helping communities and installations manage stormwater and reduce the impact of the constructed environment on the surrounding natural resources.
A rapidly-growing concept in stormwater management that began in Prince George’s County, Md., in the early 1990s, LID was developed to address runoff issues associated with new residential, commercial, and industrial suburban development to protect natural resources and restore the natural groundwater recharge cycles. LID presents a paradigm shift from treating stormwater as something to be quickly drained from the site to managing it onsite as a valuable resource.
Hydrology is an organizing principle that is integrated into site planning and even installation-level planning. Development increases impervious areas and impacts natural hydrology, causing erosion, sedimentation, habitat loss, and water quality degradation. Managing stormwater through the use of integrated small-scale LID Best Management Practices (BMPs) provides for infiltration, filtration, evaporation, detention and storage to control stormwater runoff discharge, volume, frequency; maintain water temperatures; and provide pollution prevention opportunities. The primary goal of LID is to mimic pre-development hydrology by managing stormwater close to its source. LID integrated across a project site, and especially into installation-level planning, brings about a holistic approach to the management of stormwater runoff.
The mission of the U.S. Army Corps of Engineers (USACE) Hydrology and Low Impact Development Center of Expertise in Sustainability is to provide guidance and technical support for the design and construction of LID features to manage stormwater and reduce the impact of the constructed environment on the surrounding natural resources.
IMPLEMENTATION IN CONSTRUCTION
Incorporation of LID BMPs into the Army’s construction program is the methodology used to implement requirements regarding stormwater management. The design objective of LID is to maintain or restore the predevelopment (pre-project) hydrology of the property with regard to the temperature, rate, volume, and duration of flow.
Section 438 of the Energy Independence and Security Act of 2007 (EISA) requires that any development or redevelopment project involving a Federal facility with a footprint that exceeds 5,000-ft² shall use site planning, design, construction, and maintenance strategies for the property to maintain or restore, to the maximum extent technically feasible, the predevelopment hydrology. Careful planning and development can strategically create areas that reduce quantity of runoff from a site. USACE, in collaboration with the Army (Assistant Chief of Staff for Installation Management), has developed the Army Low Impact Development Technical User Guide to assist Department of Defense planners, engineers, and stormwater managers in meeting the requirements of the EISA Section 438 for their facilities. This technical guide, along with photos of successful LID projects, can be found at the USACE sustainability web page for Hydrology and Low Impact Development.
Several common practices are used to manage stormwater at or near its source. These practices can help achieve the low impact development goal of preserving a site's pre-development hydrology, as required by EISA Section 438.
Bioretention. Bioretention is a vegetated depression with a cross section of gravel, engineered soil media, and plants that filter, evapotranspire, and infiltrate runoff. During rain events, runoff enters the bioretention and temporarily ponds above the filter bed, then slowly filters through the layers. The soil media removes pollutants as runoff passes through it, and plant roots uptake water. Depending on the in situ soil conditions, the bioretention may have an underdrain at the bottom. Without an underdrain, runoff infiltrates into underlying soils.
Bioretentions are classified into three types, depending on the size of their contributing drainage area. Bioretentions treat approximately 0.5-acre to 5-acres. Micro-bioretention treats approximately 10,000-ft² up to 0.5-acre, and rain gardens treat drainage areas less than 10,000-ft².
Permeable Pavement. Permeable pavement differs from conventional impervious pavement because stormwater is able to pass through it into a gravel reservoir and ultimately the underlying soils below. This reduces runoff volume and rate when compared to traditional pavement surfaces. When underlying soils do not allow infiltration (such as clay or karst topography), an underdrain can be used to convey runoff to the storm drain system.
The four types of permeable pavement are porous asphalt, pervious concrete, permeable concrete pavers, and reinforced turf. The mix designs of porous asphalt and pervious concrete include void space to allow rainfall to pass through the surface. Many permeable pavement products have a surface layer that can accommodate vehicle loads with open graded aggregate layers below.
Green Roof. A green roof is a vegetative system installed over a conventional roof. The vegetation uptakes and transpires rainfall and also removes pollutants, which improves water quality. Green roof types are classified by section depth. Intensive green roofs have deeper soil sections (6-in to 4-ft) capable of growing small trees and shrubs, and Extensive systems have shallow soil media sections (less than 6-in) typically seeded with grasses, groundcover or succulents. A typical green roof system includes (from the bottom up): waterproof membrane, drainage materials, growing media, and landscaping. The dead load of the saturated green roof must be included in the structural engineer's design.
Green roofs provide partial retention of stormwater and filter the remaining runoff prior to the roof drain system. Runoff volume and velocity is reduced when compared to a conventional impervious roof. Green roofs reduce the heat island effect, which reduces the building's cooling energy load during summer months.
Rainwater Harvesting. Rainwater harvesting is the collection, storage, and reuse of rainwater. Runoff from pavement or building roofs can be collected and reused instead of being discharged into local waterways. Rainwater harvesting systems reduce runoff rate and volume and can decrease a site's water demand.
Rainwater harvesting can be as simple as a rain barrel at the bottom of a downspout, with a hose spigot connected to it for irrigating plants. More complex systems may include large cisterns, filters, water quality treatment, and pumps to circulate rainwater for use inside buildings. Uses for non-potable harvested rainwater include flushing toilets, irrigation, exterior washing, and fire suppression systems. The reuse function determines filtering and treatment requirements in the design.
TOOLS AND SUPPORT
A primary goal of the Army LID Technical User Guide is to refocus typical planning and stormwater construction practices through a paradigm shift to LID BMPs as solutions. To accomplish this, the Army LID Technical User Guide includes a description of legislative drivers, descriptions and appropriate uses of non-structural and structural LID BMPs and simulation techniques to determine the volume of stormwater required to be managed to comply with EISA Section 438, facility case studies, and design and construction specifications.
The simulation chapter provides practical methods for the planning and selection of LID BMPs to include step-by-step examples on how to plan, select and size practices that are consistent with EISA Section 438. The Army LID Planning Tool is a desktop application designed for use by planners and engineers to estimate the stormwater runoff volume required to be managed for post-construction land use and identify potential types and sizes of LID BMPs.
Additional support on the website includes the Low Impact Development UFC 3-210-10 and Environmental Protection Agency Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under EISA Section 438. Other professionally recognized industry standards and guidance can be used for planning and design criteria not available from the Army, Department of Defense) or Environmental Protection Agency.