1: Introduction to Low Impact Development

Traditionally, stormwater has been managed using large, structural practices installed at the low end of development sites—essentially as an afterthought—on land segments left over after subdividing property. This approach, sometimes referred to as end-of-pipe management, yields the apparent advantages of centralizing control and limiting expenditure of land. Unfortunately, end of pipe technology has been shown to have many economic and environmental limitations such as failure to meet receiving water protection goals, high construction, operation and maintenance costs, certain health and safety risks and limited use for urban retrofit. In response to these deficiencies an alternative technological approach has emerged that is generally more economical and potentially provides far better environmental protection. This new approach is referred to as LID.


In contrast to conventional centralized end-of-pipe management, LID uses numerous site design principles and small-scale treatment practices distributed throughout a site to manage runoff volume and water quality at the source. For new development, LID uses a planning process to employ site design techniques to first optimize conservation of natural hydrologic functions to prevent runoff.  If these conservation practices are insufficient to meet required stormwater goals then engineered at the source treatment practices are used to meet volume and water quality objectives.


LID’s distributed techniques provide retention, detention and filtration of runoff in a manner that more closely mimics the natural water balance (interception, interflow, infiltration and evapotranspiration). This is accomplished through the cumulative effects of using an array of runoff reduction techniques, small scale nonstructural or engineered practices to treat runoff.  Further the uniform distribution of controls throughout a site increases runoff time of travel and concentration dramatically reducing discharge flows and increasing opportunities for infiltration and filtration within landscape features.


With appropriate selection, application and design, LID principles and practices can be used in any land planning type, soils, climate or hydrologic regime. For example, in soils with high infiltration rates LID practices may heavily rely on infiltration. For high density urban or retrofit development infiltration may not be desirable or possible; therefore, filtration, detention and runoff capture-and-use practices would be more applicable. In cold climate filtration-infiltration practices must be designed to minimize freezing allowing treatment when needed. LID principles and practices are highly adaptable and can be customized for any development scenario or receiving water goal.


The creation of LID’s wide array of small-scale management principles and practices has led to the development of new tools to retrofit existing urban development. Small-scale practices can be easily integrated into existing green space, streetscapes and parking lots as part of the redevelopment process or through routine maintenance and repair of urban infrastructure.  As urban areas redeveloped with integrated LID techniques, over time it will be possible to dramatically reduce pollutant loads to receiving waters to restore impaired waters.


However, the use of LID practices does not necessarily supplant the need for end-of-pipe technology. Hybrid approaches, which incorporate both types of practices, may be needed to meet stringent water quality and flood control requirements. However, as LID’s decentralized practices can better reduce adverse environmental impact, Connecticut regulatory agencies will typically expect permit applicants first carefully consider all opportunities to use such practices prior to exploring end-of-pipe management. The use LID techniques alone or in combination with conventional techniques will not only reduce adverse water quality impact, but will help to restore vital ecological processes necessary to restore or sustain the ecological integrity and quality of our water resources.


LID represents an alternative approach to controlling stormwater runoff that provides effective new tools to restore or maintain a watershed’s hydrologic functions for both new and existing development. LID is still relatively new and rapidly evolving stormwater management technology.  It was first described in 1999 in the Prince George’s County, Maryland, Low-Impact Development Design Strategies: An Integrated Design Approach.  However, today due to LID’s many economic and environmental advantages over conventional end-of-pipe technology, it has been widely and rapidly adopted throughout the country.  This LID design guidance has been developed using the latest information and past lessons learned to provide the most up to date design guidance.


LID practices are commonly used on reparations made to current structures that have caused issues with stormwater runoff and the resulting water quality. However, LID practices can also be considered during new building construction and implementation. LID uses many decentralized small-scale management practices strategically located throughout a development to conserve and engineer the urban landscape in a manner that mimics predevelopment hydrologic conditions.  Ideally, these LID practices are seamless in the developed environment as all traditional site features are designed to be multifunctional. Residential, commercial, and industrial properties look the same but the landscape features are designed to provide water quality and hydrologic functions to storage, detain, filter, and infiltrate runoff.  Typical advantages of LID’s integrated approach over the conventional end-of-pipe approach include:


  • Reduced consumption of land for stormwater management – LID practices provide opportunities to integrated controls into all aspects of a site’s hardscape and landscape features.  This allows multifunctional use of the entire developed site for controls allowing the most cost effective use of land.  Less land is needed or consumed for end-of-pipe controls often allowing for more developable space.


  • LID does not dictate particular land-use controlsSince LID is a technological approach there is no need to change conventional zoning or subdivision codes accept to allow LID’s use. This means LID does not reduce development potential and with less land consumed for stormwater controls lot yields may increase.


  • Reduced construction costs – Traditional stormwater management requires significant storm sewering and earthwork. LID practices apply controls as close to sources of runoff as possible. Wherever practicable, conveyances incorporate natural flow paths and swales instead of pipes. Structures installed are small, thus reducing the need for excavation and construction materials.


  • Ease of maintenance –  LID landscape practices require limited maintenance or no increase in maintenance beyond typical landscape care. Much of the maintenance required can be accomplished by the average landowner.  Further many LID site planning, conservation, and grading techniques require no maintenance.


  • Takes advantage of site hydrology – Conservation of natural resources, topography, land cover, soils, and drainage features preserve the natural hydrologic functions allowing absorption of runoff from impervious surfaces. Runoff that is absorbed recharges groundwater and stream base flow and does not need to be managed or controlled by an end-of-pipe practice. Preserving and maintaining the natural hydrology also better protects streambank stability and riparian habitat.


  • Better quality of dischargeRecent research indicates conventional end-of-pipe controls are unable reduce pollutant concentrations below certain thresholds, which may exceed water quality standards. However, LID techniques have shown to be far more effective in reducing the annual pollutant loads through both volume reduction and filtration of runoff.  Use of natural landscape features and use of lot-level bioretention and swales may, in many cases, allow for retention all runoff from events smaller than the 2-year, 24-hour storm and significantly reduce peak discharges from larger storms.


  • More aesthetically pleasing developmentTraditional stormwater management tends to incorporate the use of large, unnatural looking practices such as detention ponds. When neglected, these practices may present drowning and mosquito breeding hazards. Nonstructural and upland practices optimize use of landscape features that are more aesthetically pleasing and fit well into the natural landscape.


  • Multiple benefitsLID has shown to provide multiple benefits such as reducing energy cost by using green roofs and proper location of trees for shading and water conservation by using rain water as a supplemental water supply.


  • Improved profit margin The advantages of nonstructural and upland management translate into the marketplace. The value added is significant. Several studies indicate that the cost of applying these nonstructural and upland stormwater management techniques is about half that of the traditional approach. The results of one example of such a study are summarized in Table 1.1 below (Schuler, 2000). Properties developed using nonstructural and upland stormwater practices tend to command higher sale prices.

Table 1.1 Cost Analysis for Convention and Alternative Development