Urbanization and urban sprawl are converting more and more natural land (e.g. forest) into impervious surfaces - roadways, driveways, parking lots, rooftops, and sidewalks. When it rains, stormwater flows over these impervious surfaces, captured by conventional drainage system and discharged into rivers. To avoid flooding, every feature of a conventional drainage system is carefully planned to quickly convey runoff to gutters to stormwater drains and finally to stream discharge.
However, conventional drainage systems change the water balance both in quality and quantity (Anastas 2003). Before an area is developed, stormwater flows on natural land and slowly infiltrates into pervious surface, recharging groundwater, which eventually connects to surface water bodies (i.e. rivers and lakes). During the process, sediments are filtered by soil particles and nutrients are absorbed by plant roots. After developed into an impervious surface, stormwater is charged directly into rivers with minimal treatment (i.e. screening), taking with it sediments, nutrients and surface pollutants (e.g. petroleum products and heavy metals), causing non-point source pollution. In addition, since the stormwater is retained for much shorter time than in pre-development landscape, increased runoff peak flow will increase stream erosion, further deteriorating water quality of rivers. In cities with combined sewer systems, the capacity of sewer systems is often exceeded by heavy rainfall or snowmelt. Excess wastewater is discharged directly to nearby water bodies, causing Combined Sewer Overflow (CSO).
To tackle these problems, Low Impact Development (LID) strategies, which aims at maintaining or replicating the pre-development hydrologic condition through the use of design techniques that mimic the previous hydrologic landscape, caught people’s attention in the late 1990s (US EPA 2000a).
This report reviews the concept, principles and common practices of Low Impact Development (LID). To get a picture of current development status of LID, federal and state governments’ policies and regulations, as well as local governance and public participation, are reviewed. Finally, the institutional, technical, and market barriers of LID implementation are discussed.
The philosophy of LID is based on several principles: conservation of existing natural features, minimization of land disturbances and impervious surfaces, hydraulic disconnects, disbursement of runoff and phytoremediation(US EPA 2000; Davis and McCuen 2005). Bioretention areas or rain gardens, grass swales or channels, vegetated roofs, rain barrels and permeable pavements are some common practices of LID.
Biorention is a vegetated management practice designed to collect, store, infiltrate, and treat runoff (Dietz 2007). Bioretention facilities are depressed areas several centimeters deep in the landscape. They are commonly used in a parking island or linearly along a roadway with curb-cut inlets, where water will flow in as a result of gravity. Overflow drains leading to adjacent traditional drainage systems are usually installed to handle large flows exceeding the depths of the bioretention facilities. Larger facilities are generally designed with underdrain, which is typically a perforated plastic pipe that lies below the soil (Anastas 2003). During a storm event, the runoff will pond in the bioretention facility, slowly infiltrate through layers of sand, soil and gravel, and reach the pipe. Researches show that bioretention facilities are effective in retaining storm flow, removing heavy metals, petroleum hydrocarbon pollutants and suspended solid (Roseen et al. 1984; Davis et al. 2003). However, some researchers also reported the problem of phosphorous export (Dietz and Clausen 2005) and low retention of certain forms of nitrogen (Davis et al. 2001). In terms of maintenance, since the Cation Exchange Capacity of the soil depletes along the time, it is necessary to change soil 5 -10 years after construction (US EPA 2000a). Regular replacement of mulch layer, removal of dead vegetation, and treatment of diseased vegetations are also important to avoid clogging.
Grass swale or grass channel is a simpler LID practice that is suitable for drainage areas with mild slope along residential streets and highways. Grass swales can extend the hydraulic length of a drainage area, provide hydraulic resistance through vegetation to slow down flows, provide infiltration opportunities, and contribute to delay and attenuation of peak flow at the drainage outlets (Shuster, Morrison, and Webb 2008). Where grass swales are used, road curbs are either eliminated or specially designed to have multiple outlets adjacent to the swales. Therefore, proper design of the road is necessary to assure its stability. The maintenance of grass swales involves periodically mowing and removal of sediments.
Vegetated green roof is a common LID practice in Europe, especially Germany, where about 14% of the roofs are green (Getter and Rowe 2006). A green roof generally consists of multiple layers of materials, including vegetative layer, growing substrate, a geotextile layer, and a synthetic drain layer (US EPA 2000a). Green roofs can be classified as extensive, semi-intensive, or intensive. Extensive green roofs have 6 inches or less of growing substrate, while intensive green roofs have greater than 6 inches of substrate. Semi-intensive green roofs are a hybrid of the two, but at least 25% of the roof area should be above or below the 6 inch threshold (US EPA 2005). Extensive green roofs are cheaper than intensive ones in terms of capital investment and maintenance, but they are less accessible. Therefore, developers, property owners, and municipalities are usually more interested in intensive green roofs, where they can be an amenity to customers, employees, or the general public. Through a comprehensive review on the roles of green roofs in urban environment, Getter and Rowe (2006) concluded that by replacing impervious surfaces into vegetated areas, green roofs can reduce the volume of stormwater runoff through soil absorption and plant transpiration. They can also delay stormwater runoff between 95 min to 4 hr and therefore reduce the risk of sewer overflowing (Getter and Rowe 2006). This is a particularly important function for old cities with outdated sewer infrastructure. A point worth noting is that green roofs can effectively reduce buildings’ ambient temperature and mitigate the Urban Heat Island effect. They can also extend the lifespan of roof structures due to reduced solar exposure. For maintenance, extensive green roofs need minimal irrigation and other maintenance, while intensive green roofs need maintenance similar to a garden at ground level (Oberndorfer et al. 2007).
The use of permeable pavements is another effective means of reducing impervious surface in a watershed. In highly urbanized places like Somerville (MA), unpaving low traffic areas, such as parking lots and sidewalks, has great potential in stormwater management. US Environmental Protection Agency (US EPA) has conducted a series of research on effectiveness of different types of permeable pavements. It was found that the use of permeable pavement dramatically reduced surface runoff volume and attenuated the peak discharge by infiltration. Some permeable pavements are integrated with vegetation, which provides additional removal of nutrients through plant roots (US EPA 2000b).
The use of rain barrels is the most cost effective strategy in stormwater management. It involves connecting rain gutters of a house to a rain barrel or cistern for later use in irrigating lawns and gardens.
III. Federal and State Regulations and Policies
The 1987 amendments to the Clean Water Act required US EPA to apply the National Pollutant Discharge Elimination System (NPDES) program in municipal stormwater system to address nonpoint source pollution (Murphy 2010). NPDES has two phases. Phase I was initiated in 1990, targeting operators of “medium” and “large” Municipal Separate Storm Sewer Systems (MS4s), which serve populations of 100,000 or greater. And Phase I applied effluent standards as a regulatory tool. Phase II was extended in 1999 to address smaller operators of MS4s that serve population under 100,000. Different from Phase I, Phase II applied a new regulatory approach that requires Best Management Practices (BMPs) to be implemented instead of using effluent standards. Each regulated MS4 is required to develop and implement a stormwater management program that addresses six minimum control measures (US EPA 2000c). Implementing these minimum control measures typically requires the application of one or more BMPs. An important point to note is that LID practices are currently included in EPA BMPs Menu as innovative measures.
The Section 438 of the 2007 Energy Independence and Security Act (EISA) established strict stormwater runoff requirement for federal development and redevelopment projects to maintain or restore pre-development hydrology. According to EPA, “[f]ederal agencies can comply with Section 438 by using a variety of stormwater management practices often referred to as ‘green infrastructure’ or ‘low impact development’ practices (US EPA 2009).” The purpose of Section 438 is to demonstrate federal leadership in stormwater management, particularly in LID.
From the above, we can find that US EPA as a federal level agency has not mandated state and local governments to adopt LID, although it encourages LID as one of the BMPs options and has mandated federal development projects to install LID. At state level, no mandatory requirements have been made, although many states have been encouraging the adoption of LID by local governments and private sector. Wisconsin, for example, developed technical standards for bioretention facilities and swales for local governments and private sector to follow (Wisconsin Department of Natural Resources 2006). Massachusetts created a Low Impact Development Tool Kit, which provides a checklist to local governments on how to modify their codes to encourage builders and property owners applying LID (Metropolitan Area Planning Council).
IV. Local Governance & Community Participation
Although the state and federal level governments can provide guidance via demonstration projects and guidelines, the threat of stormwater is “born locally, felt locally, and most effectively addressed locally” (Denzin 2008). Successful implementation of LID needs great momentum from local governments and public participation at community level.
At municipal level, City of Portland (OR) is in the forefront of LID practices. The Bureau of Environmental Services initiated the Willamette Stormwater Control Program, providing technical and financial assistance to pilot LID projects (National Resources Defense Council). In addition, City of Portland removed all minimum parking requirements in the downtown commercial district to promote public transit and, at the same time, allow more open space for LID. It also provides Floor-to-Area Ratio (FAR) Bonuses to developers who install green roofs. The most significant program initiated by Portland is the Green Street Program, which installed LID techniques along municipal streets (Denzin 2008).
City of Lincoln (NE) initiated the Holmes Lake Watershed Improvement Program, a community-based program for public participation and water quality education. It provided a pilot incentive of up to 90% of the cost of rain gardens and free rain barrels to homeowners. More than a hundred homeowners have a rain garden and over a thousand have a rain barrel (Meder and Kouma 2010).
Shuster et al (Shuster, Morrison, and Webb 2008) conducted research in Cincinnati Ohio, where they installed parcel-scale LID techniques (50 rain gardens and 100 rain barrels) on properties of individual landowners. The landowners participated by reversely bidding for compensation to their loss of landscape and the maintenance cost of LID 3 years. About 60% of bids were zero dollars, meaning that large portion of landowners didn’t consider having LID installed on their property an additional cost to them. Some landlords who did not participated in the previous bidding were desired to have another bidding process. The authors concluded from the case study that decentralized management through guided public participation and local partnerships is necessary to the success of LID.
V. Barriers to Implementation of LID
Although the benefits of LID have been reported in numerous literatures, the implementation of LID is slow in the U.S (Bowman and Thompson 2009). The barriers hindering the paradigm shift of stormwater management are multi-facets.
Institutional Barriers: There is a discrepancy between federal or state requirements and local obligations. Clean Water Act stormwater permits issued by state governments generally set uniform requirements for entire states. To obtain the permits, municipalities are required to use local stormwater management plans and ordinances to reduce pollution to the “maximum extent practicable” (Denzin 2008). However, the local officials often consider stormwater management a federal or state level issue, while they are not obliged to spend additional efforts to revise zoning codes and ordinances other than obtaining MS4 permits. The blur boundary between requirements and mandate makes no accountability for implementation.
A series of community workshops conducted in three Oregon communities (Godwin, Chan, and Burris 2008) found that some municipalities felt a lack of leadership and funding from federal and state governments. As described by Holland, “federal government increases its policy demands on state and local governments without providing either funding support or the means to raise new revenues to meet the new requirements imposed” (Holland et al. 2007). Many of municipalities are lack of the necessary resources to build demonstrative LID projects, initiate incentive schemes, and provide trainings. Some local officials considered it unreasonable to expect them to deviate from normal practices without significant support from superiors.
Technical Barriers: The Oregon community workshops (Godwin, Chan, and Burris 2008) also identified that lack of basic understanding of LID and their consequences is another barrier to LID implementation. Neither the public nor local officials grasp the effects that individual planning decisions will have on infrastructure capacity and water quality. Moreover, the effectiveness of LIDs is very site specific. The same LID technique adopted under different soil and climate condition will produce very different result. There is no one-size-fits-all solution. Strecker questioned that whether LID is really low or just lower, since many researches asserting the effectiveness of LID were not substantiated by proper hydrological design procedures (Strecker 2001).
Moreover, the long-term effectiveness of LID techniques largely depends on maintenance, which is done either by municipalities or by communities and individual property owners. Indifference or misperception in maintenance is another technical barrier. For instance, the export of phosphorous caused by misuse of soil media or over fertilization is commonly found in green roofs, bioretention, and grass swales (Dietz 2007). The phosphorous drained into surface water bodies will deteriorate the water quality. Another problem resulted from failed maintenance is clogging. It happens when the sediments and dead plants are not removed timely in bioretention facilities and bioswales. Clogging will drastically reduce the infiltration rate of stormwater, or even worse, cause flooding. Therefore, without necessary resources and proper education, the municipalities, communities, and individual property owners may not be competent in maintaining the LID features.
Market Barriers: Many developers think LID means greater costs for both permit approval time and site development (Godwin, Chan, and Burris 2008). Moreover, they perceive consumer indifference in maintenance and lack of willingness to pay for open spaces in residential design are the most important barriers (Bowman and Thompson 2009).
However, a thorough report prepared by ECONorthwest reviewed both tangible and intangible costs and benefits of LID techniques from numerous literatures, indicating that the construction costs of LID could be lower if LID techniques are planned early in design process (MacMullan 2007). According to their review, the costs of LID techniques can be site specific and will vary depending on the LID technique used. Grass swales, for example, can produce a net saving by avoiding gutters and curbs. In addition, many benefits of the LIDs, such as reduced flooding, improved water quality, increased ground water recharge, reduced public expenditure on stormwater infrastructure, and enhanced aesthetics, are intangible. Because developers do not benefit directly from these benefits (so called “the split of interests”), they usually focus only on the construction cost of LID (MacMullan 2007).
And sometimes, developers simply do not realize the direct benefits from LID – the increase of property value. Bowman and Thompson conducted a survey in Iowa to understand the perceptions of developers and residents (Bowman and Thompson 2009). They found residents are actually willing to pay more for open space, through which some of the LID strategies can be installed. The application of intensive green roof in office buildings, for example, can increase property value by making use of unused space and providing additional amenity (Peck and Kuhn 1999). Even though, developers do not perceive the demand of LID and they do not actively pursue information about people’s preferences.
In view of the above barriers, it may be still too early for federal and state governments to mandate LID practices in stormwater management. A lot more efforts need to be paid in both directions - top down in terms of federal and state regulations and policies, and bottom up in terms of municipal engagement and public participation. Recommendations in promoting wider adoption of LID practices will be made in the policy brief assignment.
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 Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe.
 The same strategies are named Sustainable Urban Drainage System (SUDS) in the United Kingdom and Water Sensitive Urban Design (WSUD) in Australia.
 Phytoremediation refers to the practice of using plants for pollutants removal.
 Cation Exchange Capacity is the ability of soil to adsorb pollutants via ion attraction.
 Urban heat island describes built up areas that are hotter than nearby rural areas.
 Six minimum control measures include: public education and outreach, public participation/involvement, illicit discharge detection and elimination, construction site runoff control, post-construction runoff control, and pollution prevention / good housekeeping (US EPA 2000c).