Hydrology and Low Impact Development

About Hydrology and Low Impact Development

Welcome to the Hydrology and Low Impact Development (LID) web page. A team of Subject Matter experts will provide content to include guidance documents, case studies, best management practices and tools to assist the user in designing construction projects that conserve water and reduce the impact of the constructed environment on the surrounding natural resources. Careful planning and development can strategically create areas that reduce quantity of runoff from a site.

**JUNE 2018: UPDATED ARMY LID PLANNING AND COST TOOL POSTED IN DESIGN TOOLS SECTION FOR USE **

_ FEBRUARY 2018: NEW ADDITION! TWO WEBINARS ON THE LID PLANNING AND COST TOOLS UNDER TRAINING BELOW. ALSO UPDATED ARMY LID PLANNING AND COST TOOL UNDER DESIGN TOOLS BELOW _

** NOVEMBER 2017: UPDATED ARMY IMPLEMENTING GUIDANCE. SEE LINKS UNDER POLICY & GUIDANCE FOR 2017 IMPLEMENTING GUIDANCE ARMY STORMWATER MANAGEMENT USING LID (MEMO AND GUIDANCE DOCS) **

This site is intended to be a collaborative environment to share LID questions/concerns/lessons learned with the E&C community and to address latest successes, review and interpret new mandates or address shared challenges in LID.

In the Files sections below, you will find:

Policy and Guidance document - these are centered around the requirements of the Energy and Independence Security Act, Section 438 and include Federal, DoD, and Army policy and guidance documents Best Practices - this section includes the Army Low Impact Development Technical User Guide that is intended to educate users about stormwater compliance requirements, LID practices, site planning for LID, overview of implementing LID on a project site through using the Army LID Planning Tool, and information on construction and maintenance for LID practices. TechNotes, Technical Bulletins and other relevant information is also included in this section. Design Tools - the Army LID Planning and Cost Tools (in Access Database format) is included here for download and use on the users local computer. This section also include LID Plant Guidance factsheets by climatic region within the continental U.S. for use when selecting plants during the design phase. Articles & Case Studies - this section includes articles related to LID success and information from DPW Digests Training - this section includes powerpoint slides and webinars for users to educate themselves about stormwater compliance requirements, LID practices, site planning for LID, and use of the Army LID Planning and Cost Tools (coming soon!) Please check back regularly as we continue to update this website and develop new content. Feel free to share relevant documents or success stories to LID@usace.army.mil.

Also, check out the LID CXS Website on the HQUSACE Sustainability site: HQUSACE Sustainability Mission - LID CXS

Common practices used to manage stormwater at or near its source. Use of these practices can help achieve the low impact development goal of preserving a site's pre-development hydrology, as required by EISA Section 438. The four most common LID practices are:

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 1/2 acre to 5 acres. Micro-bioretention treats approximately 10,000 square feet up to 1/2 acre, and rain gardens treat drainage areas less than 10,000 square feet.

Permeable Pavement

Permeable pavement differs from conventional impervious pavement because runoff passes through it. Permeable pavement acts as a pervious surface where runoff filters through the surface layer and stone reservoir to the underlying soils below. This reduces runoff volume and rate when compared to traditional pavement surfaces. When underlying soils do not allow infiltration (e.g. 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. For porous asphalt and pervious concrete, their mix designs include void space to allow rainfall to pass through the surface. Permeable pavers are impervious concrete pavers with small spaces between the pavers at the joints. Typical application of reinforced turf is a plastic geogrid with soil and grass planted in the cells. All permeable pavement types 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 inches to 4 feet) capable of growing small trees and shrubs, and Extensive systems have shallow soil media sections (less than 6 inches) 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 runoff 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. In addition, green roof systems provide thermal insulation benefits, which reduce the building's heating demand during winter 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 running off to nearby surface waters. 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 reuse 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.

Research & Services

The LID CX continues to update this website and develop new materials. Currently underway is an update to the Army Technical User Guide, updated Training slides, and continual updates to the Army LID Planning and Cost Tools. Please visit this site regularly and contact us with any questions or to share success stories at LID@usace.army.mil.