Low Impact Development: New and Continuing Applications

Beginning in 2000–01, the Puget Sound Action Team in Washington State began to carry out a strategy with regional partners to promote the low impact development (LID) approach to land development and stormwater management in the Puget Sound region. The Action Team identified barriers to implementation and undertook a series of activities to overcome barriers. Activities included raising awareness of LID and the limitations of conventional practices, improving knowledge regarding proper design, construction and maintenance of LID techniques, and removing local government regulatory barriers. Since 2001, the use of LID techniques has significantly increased in the region. This paper provides an overview of the Action Team's overall strategy to promote LID in the Puget Sound region, with emphasis on an innovative technical assistance project to help local governments remove regulatory barriers. The relative success of efforts will be examined, and next steps needed to continue progress will be suggested. By doing so, the author hopes to share lessons learned and provide ideas for others interested in promoting LID to potentially use.

The Berkeley-Charleston-Dorchester Council of Governments (BCDCOG) in South Carolina compared the benefits and costs of Low Impact Development (LID) versus conventional development methods for two developments (one single family and one multi-family residential) on the Cane Bay Plantation development site. Without reducing the proposed lot yield, BCDCOG — working with Tetra Tech — evaluated the potential for LID on these sites. Tetra Tech developed five LID scenarios that employed BMP treatment trains, preservation of undisturbed areas, and reduction of impervious surface. The best performing LID designs were estimated to reduce phosphorus loading by over 75 percent and total suspended solids loading by over 80 percent compared to the conventional load. The LID designs for the single family residential development are likely to cost between $2,000 and $11,000 per acre more than the cost of the conventional designs whereas the LID design for the multi-family residential development is likely to cost similar to or slightly less than the conventional design.

Like many cities around the world, Auckland is facing the significant challenge of accommodating a rapidly growing population while providing a high quality urban environment. A significant impact of urban development is the changes it makes to the natural hydrological cycle. Stormwater runoff from urban sources, including increased volumes and rates of runoff and contaminant loads, are clearly having impact on Auckland's aquatic receiving environments. The Auckland Regional Council (ARC) is responsible for establishing a policy direction, providing a regulatory framework and supporting non-statutory initiatives to manage the effects of land development and encourage behaviours and activities that minimise these effects. In exercising this role in the context of stormwater management, the ARC has identified the significant potential that Low Impact Design (LID) offers as both a functional stormwater solution and as a method to enhance the urban environment. The ARC has taken a multi-faceted approach to promoting LID and facilitating its implementation in the region. This has required a clear understanding of critical stormwater issues, identification of desired stormwater solutions, stakeholder engagement to identify barriers to the implementation of solutions and activities to inspire innovation and encourage implementation. Activities that cross policy, regulatory and non-statutory roles of the Council have been undertaken to progress this outcome and relationships with key external stakeholders have been critical in achieving progress. Key initiatives include interactive forums with a wide range of stakeholders, concept projects to illustrate how LID can be applied to the redevelopment and intensification of existing urban areas, a university student LID competition, exploration of incentives to encourage LID such as innovation funding and streamlining the regulatory process, development of LID guides and case studies and exploring opportunities to modify the policy framework to facilitate LID implementation. This paper discusses the development, delivery and contribution of these projects to facilitating the implementation of LID in Auckland, New Zealand.

This paper provides a general overview of the basic economic notions that underlie the benefits of low impact development (LID): willingness-to-pay (WTP) and willingness-to-accept compensation (WTA). It also summarizes the specific extra- and non-market estimation methods utilized by environmental and natural resource economists to estimate WTP and WTA values more holistically. Several examples of LID environmental economic benefits that have been estimated by OSU environmental economists are presented, selected methods are then applied to a specific case involving a proposed greenway on the Mill Creek River, and conclusions are drawn. Finally, implications for policy are suggested for a broad range of LID projects. This paper is intended as an introduction to and overview of non-market valuation and its relevance to LID, not as a comprehensive treatment of this large and expanding field.

The Land Use Department of New Castle County, Delaware, under an ordinance referred to as "Environment First" has become one of the first local government agencies to promote the integrated use of low impact development (LID) technology and the "Conservation Design" approach. Conservation design incorporates a broad range of planning and open space concepts into the land development design process. One of the features of this approach includes a minimum requirement for open space for every project. In New Castle County, this requirement has been set at 50 percent. This requirement for open space offers many opportunities to use LID techniques and to incorporate stormwater management practices into the landscape. This paper provides an overview of how these two methods can work together to result in ecologically sensitive land development scenarios. Case studies of the successful integration of these two concepts are presented.

Significant advances have been made in the use of spatial and hydrologic models to quantify the impact of BMP/LID practices on water quality, but little research has focused on calculating the implementation costs associated with these BMP's when integrated with a decision support system (DSS). This research project had three phases. The first was a review and selection of a public domain water quality model. Hydrologic Simulation Program in FORTRAN (HSPF), an unsteady flow model, was selected as the hydrologic and water quality program. The second phase assessed the potential to link the model to a desktop Geographic Information System (GIS). The third phase focused on identifying BMP's that are often included in low impact development strategies, including implementation, operation, and maintenance cost data. This information was collected from several national sites and loaded into a database, which was later linked to the site's individual BMP's housed in the GIS. This allowed development costs for different combinations or configurations of BMP's to be calculated in real time.

Stormwater managers use infiltration to promote groundwater recharge in urban areas and to reduce the volume and flow rate of stormwater runoff created by post development conditions. However, the potential for groundwater contamination exists. Studies have shown that some areas are more prone to groundwater contamination than others. As a result, methods for easily predicting contamination potential beneath infiltration devices are being developed. One such method involves the use of computer programs, such as SESOIL — a one-dimensional vadose zone model. SESOIL was selected because of the multitude of processes available in the program for predicting pollutant removal. SESOIL was first evaluated using a full factorial design, considering six potential factors that were thought to influence the predictions (rainfall, concentration, vadose zone thickness, intrinsic permeability, soil pH, and soil organic matter). The results showed that, at the concentrations typical of urban stormwater runoff, only rainfall, concentration and intrinsic permeability were important. Difficulties of application were encountered due to a lack of available input values and data sets. Therefore, laboratory research on partitioning factors between the water and the soil were undertaken. The results showed that the partitioning coefficients calculated from the removal of pollutants from a multicomponent mix at typical stormwater concentrations were similar to the low range found in the literature. However, the literature did not have values for sparingly-sorbable compounds such as the nutrients. This research is using a vadose zone model, such as SESOIL which incorporates these numerous chemical reactions in the calculations, as a design tool for infiltration devices with both engineered and natural soils. This research will support the development of guidance for protecting groundwater from unintentional contamination by stormwater runoff passing through an infiltration structure.

This paper will give an overview of a new manual to meet a new set of stormwater management requirements in the Etowah River watershed of North Georgia. The background issues that drove the creation of the new manual will be discussed. The paper will describe the infiltration BMPs that were recommended. It will give an overview of the specifications created for the infiltration best management practices (BMPs) on design, installation, and maintenance. The Etowah Habitat Conservation Plan was developed to prevent further degradation of aquatic habitat for several endangered species of darter fish native to the Etowah River. One of the unique requirements established for stormwater management is the runoff limit program which requires retaining and infiltrating stormwater on-site. The runoff limit program was designed to create post construction stormwater responses that would mimic pre-development and pre-agriculture hydrology conditions. To assist local officials and developers in meeting the runoff limit criteria, guidelines and specifications for infiltration BMPs were developed and included in a Manual on how to meet the runoff limit criteria for managing stormwater on-site. This manual and trainings on design, installation and maintenance of the infiltration BMP will provide options for developers and guidance for local officials to meet the runoff limit criteria.

A Bioinfiltration best management practice (BMP) was constructed within a parking area serving a dormitory and parking complex on the west campus of Villanova University in 2001 Figure 1). It was retrofitted into an existing mounded turf traffic island serving as a traffic lane divider. The BMP was constructed by excavating six feet below the existing grade; backfilling with a sand-soil mixture; grading a bowl-shaped depression for a temporary pond; and planting with salt tolerant grasses and shrubs native to the Atlantic shore. The drainage area for the BMP is 1.5 acres of which one-half is impervious. The BMP has performed well for the five years it has been in service and has infiltrated approximately 80% of the precipitation delivered to the watershed. The modeling approach developed for this site is based on HEC-HMS. Green and Ampt is used to represent the losses on the pervious areas. The infiltrating basin is modeled using stage-storage routing with diversion elements representing two physical components of the infiltration: the rapid infiltration that occurs during precipitation generated inflow and the recession infiltration that occurs continuously over the wetted area of the pond. Ongoing calibration has brought the modeled results into close alignment with the flows observed during continuous monitoring and has revealed a definite and consistent seasonal pattern to the infiltration effects. Research initiatives following on this work emphasize a better understanding of how the observed and modeled infiltrations relates to the properties of the soil; the unsaturated flow conditions; and the groundwater. It is expected that the finding will be useful in continuing to develop design criteria.

One of the general benefits of the Low Impact Development (LID) often advocated in LID literature is that some LID practices could increase the time of concentration (T_c) of the drainage area and thereby decrease the runoff peak flow rate (Q_p). However, the relative impact of increasing T_c on Q_p has been neither experimentally nor theoretically adequately quantified. This paper presents the findings of a runoff evaluation exercise aimed at assessing the relative impact of T_c elongation on Q_p. Sample calculations were performed for Fairfax County, Virginia and the results showed that the effectiveness of elongating T_c increases with the Curve Number (CN) of the drainage area. The relative impact is insignificant for CNs smaller than 60, modest for CNs between 60 and 80, and significant for CNs greater than 80. Moreover, T_c elongation is more effective for larger infrequent storms than for smaller more common events. The impact is almost insignificant for a 2-year event but becomes significant for the 25-year and larger events. Under typical LID design conditions, elongating T_c by a few minutes may not be an efficient way of reducing Q_p.

Since its initial development and application in 1993 (Clar, et al., 1993), the Bioretention system also referred to as "rain gardens" has rapidly become one of the most versatile and widely used BMPs throughout the US and many parts of the world. It has recently become identified as a preferred site practice for green building design and LEEDS certification. A recent review of Bioretention design guidance being used throughout the US revealed that with some relatively minor changes, the design concepts contained in the original design manual remain unchanged. This paper provides a critical review of some elements of the original design concepts and introduces new directions for the evolution of this BMP. The design elements examined in this paper include: 1) The allowable ponding depth, 2) The minimum width and length parameters, 3) The depth and type of the soil/filter media, 4) The design of bioretention as an infiltration practice, 5) Plant materials.

Water quality drawdown regulations require release of the water quality volume over an extended period of time to minimize stream erosion. The requirement for a bioretention basin is commonly 40 hours. Drawdown methods often analyze how fast the water quality volume would drain from the surface of the bioretention cell. The high permeability of bioretention soils make it difficult to prove that the basin is able to hold the water quality volume for extended periods of time without minimizing the surface area and lowering the permeability. The methodology presented shifts the focus to the volume released to the stream. Volume lost to evapotranspiration and infiltration can be as much as 50% of the water quality volume. If 50% of the volume does not discharge directly to the stream, the two design points on the drawdown curve are achieved independent of how fast the volume drains from the surface of the basin. Also provided is a method for meeting the drawdown requirements if the volume discharged to the stream is over 50%.

Bioretention systems are stormwater treatment devices installed to remove both dissolved and particulate pollutants. As stormwater percolates through the bioretention system, dissolved pollutants are removed from solution by chemical and biological processes. In our study, 20 bioretention mesocosms (10 with loamy sand and 10 with sandy loam, half with and half without vegetation) were used to investigate hydraulic behavior and dissolved nutrient retention. The mesocosms were dosed with 120L to 160L synthetic stormwater comprising 0.8 mg-l^–1 TP and 4.8 mg-l^–1 TN over 3h to 5h. The effluent was collected in 150L containers. Average TP retention in the vegetated loam mesocosms was over 90%, compared to 75% in non-vegetated (barren) mesocosms. TP retention in the sand was very high (average 90%), regardless of treatment. TN retention increased to 80% in the vegetated loam mesocosms from 43% in barren loam mesocosms. TN retention in the sand was less overall, with retention increasing to 41% in the vegetated loam mesocosms from 26% in barren mesocosms. Increasing retention time in the sand mesocosms increased retention of nitrogen to 61%, a nearly 50% increase in retention performance. Nutrient retention persisted even under very high nutrient loads. While the loam is a less effective media by itself, the presence of vegetation substantially improves TP and TN retention. TN retention performance is improved by increasing retention time.

Two case studies will be reviewed, highlighting the merits of storm water infiltration in the Charlotte area. Within the geologic region of the "Charlotte Belt," soils are typically high in clay. These two project sites are underlain by soils in the Cecil series, classified by the NRCS as well drained, moderately permeable soils. The third project site, included for comparison, is underlain by poorly drained Monacan and Iredell soils.

Filterra is the latest advancement in Bioretention treatment technology for urban stormwater runoff. Americast, a Division of Valley Blox, Inc., working with the University of Virginia's Civil Engineering Department has optimized the treatment capacity of this innovative best management practice (BMP). Filterra relies on a specially-engineered high flow rate treatment system to provide exceptional pollutant removal. Monitoring data shows Filterra can treat over 90% of the total annual volume of rainfall with maximum pollutant removal rates reaching 95% for total suspended solids, 82% total phosphorus, 76% total nitrogen and 91% heavy metals (measured as Cu). The high pollutant removal efficiency is primarily due to the multiple treatment systems inherent in its unique plant / soil / microbe treatment media. Its unique design and use of typical landscape plants also provides many added values such as low maintenance costs, enhanced aesthetics, improved habitat value, and easy / safe inspection. The "at-the-source" treatment strategy is highly adaptable for any urban setting to achieve multiple stormwater management water quality and quantity goals including combined sewer overflow control.

Swales are "engineered vegetated ditches" that provide stable routing for stormwater runoff. They provide a low-cost drainage option for highways, farms, industrial, and commercial areas. It is reported in the literature that swales mitigate the runoff-carried pollutants, reduce runoff volume, and reduce peak stormwater runoff rate that can damage low-order streams. EPA recognizes the reported capabilities of swales to reduce the environmental footprint while meeting the practical need to manage stormwater runoff. This research will provide a more complete understanding of how swales operate, thereby enabling guidance on the design, construction, maintenance, and monitoring to reduce the pollutants reaching receiving waters. Swales are one management practice that can help watershed managers better assure that receiving waters meet the "fishable and swimmable" goals that Congress outlined in the Clean Water Act, meet designated uses, and better assure the continuing supply of high-quality, potable water needed for human life and economic growth.

The Department of Civil and Environmental Engineering at the University of Alabama conducted research on the effectiveness of grass swales for stormwater sediment transport to quantify swale hydraulic and sediment transport under relatively small flows. This research was supported by the Water Environment Research Foundation (Johnson et al., 2003) and the University Transportation Center of Alabama (Nara and Pitt, 2005). Grass swales are vegetated open channels that collect and transport stormwater runoff that can be used as beneficial alternatives to concrete gutters for stormwater management due to their advantages of infiltration and filtration of stormwater.

Level Spreaders are commonly used in combination with riparian buffers as a storm water Best Management Practice (BMP) in many parts of the United States. These systems have not been extensively studied in urban environments to determine if they can provide a long term water quality benefit. In Winter, 2005, 24 level spreaders were evaluated in the Piedmont of North Carolina. Detailed observations were made at 20 of these locations. The results of the study indicate that level spreaders may not be the versatile structure they are perceived to be. No level spreader — riparian buffer system was able to provide diffuse flow through the riparian buffer from the level spreader to the stream. Common causes for failure to maintain diffuse flow included: lack of maintenance (12 cases), poor design (11), riparian topography / content (11), level spreader lip not level (7), built with easily eroded materials (6), poor construction methods (3), and human interference (2). This field evaluation indicates that level spreader systems may need design revisions, construction guidance, and maintenance requirements before they continue to be used en masse.

As Low Impact Development (LID) becomes a mainstream design approach for water quality and quantity management, the use of natural solutions such as bioinfiltration, rain gardens, biofiltration, ecoroofs, etc. will continue to increase. Sometimes viewed as soft solutions vs "hard" engineered solutions, specifications for soils and soil amendments are often vague and can frequently lead to early failure of BMP or the addition of pollutants from leaching or degradation of these materials. The objective of this rather simple paper is to alert the design community to a potential issue associated with the generic specification of composts with particular reference to composts being used as a soil amendment to designs which are part of a stormwater quality management plan or facility. Compost is used as a soil amendment to promote soil tilth, moisture retention, gas exchange, and as a nutrient source. Applications of compost as an amendment for a growth medium in stormwater applications such as green roofs, swales or infiltration facilities adds new challenges. Properties such as nutrient content, soil moisture holding capacity, metals uptake capacity, shrink/swell, product maturity, pathogen and weed seed content require a high level of scrutiny to insure the appropriate amendments are being used. There are many feedstock sources of compost and methods of composting which yield very different end products. Feed stock such as mixed yard debris, manure, fallen deciduous leaves offer substantial differences in nutrient content, presence of residual chemicals such as pesticides and herbicides, woody material and relative percent differences in cellulose or lignistic materials. Methods of processing also have a significant impact on the quality of the compost such as maturity, content of foreign materials, and biological contaminants such as pathogens, weed seed viability. This paper provides an overview of what designers should be looking for and considering for both design and construction specifications.

The City of Seattle is committed to preserving and protecting the natural environment that surrounds its neighborhoods including the creeks and streams that support wildlife and fish habitat. Through the Seattle Public Utilities' (SPU) Comprehensive Drainage Plan, SPU created the Thornton Creek Watershed Action Agenda to improve water quality and to provide an environmental benefit for Thornton Creek. Thornton Creek, located in the Seattle neighborhood of Northgate, is an urban creek system that meanders through surrounding commercial and residential developments. In conjunction with evolving development plans in the Northgate area of Seattle, SPU assembled a design team to prepare plans for an off-line water quality treatment channel at the headwaters of Thornton Creek. The team includes Gaynor and Associates, concept design; SvR Design Company, civil engineering and landscape architecture; Herrera Environmental, hydraulic modeling and monitoring services; HDR Inc, structural and electrical engineering; and Associated Earth Sciences Inc., geotechnical engineering. Through a community process SPU identified several goals for the Thornton Creek Water Quality Channel (TCWQC) project. These include: 1) Enhancement of stormwater quality entering Thornton Creek by treating flows through an open vegetated channel and accommodation of creek base flow. 2) Provide a public open space that connects the Northgate urban village pedestrian streetscape. 3) Increase wildlife habitat through planting of native species. The water quality goal of the project was achieved by diverting a portion of the water from the creek headwaters, maintaining the existing piped storm drainage system for high flow bypass and designing a treatment channel based on wet bioswale design method. The open space goal was achieved by developing a pedestrian system of pathways, plazas and bridges which allow public access into and along the channel. This was accomplished by coordinating the channel design with new mixed-use development adjacent to the site, and negotiating the construction sequencing. The technical challenges were significant; developing a new strategy for water quality treatment design, reducing flooding risks to the upstream drain system caused by diverting flows, and managing site and channel grading within a narrow and steep project space.

Many factors are at play in the choice of stormwater BMPs as part of a stormwater management plan. Factors such as costs for land area and construction and the effectiveness of BMPs are important variables to consider when implementing low impact development strategies. A study was conducted to briefly compare stormwater BMP costs and benefits while focusing primarily on the effectiveness of green roofs to mediate stormwater using computer simulation and field study. Simulations were run using HYDRUS-1D for 24-hour design storms to determine peak flow, retention and detention time for runoff. Storm data collected at an Athens, GA green roof study were used to validate HYDRUS-simulated runoff. The study site was atop a utility room for University of Georgia Science Library and consisted of a 37 m² (400 ft²) modular block green roof containing engineered soil and several Sedum species. The study revealed that rainfall depth per storm strongly influences the performance of green roofs for stormwater mitigation, providing complete retention of small storms (< 2.54 cm) and detention for larger storms.

What goes up on a roof can come down ...... but it will cost you. Understanding the long-term design intent of a green roof project can thwart future expense when a change is desired. The installation costs of green roof media vary. There are many factors to consider. A ground-level intensive installation can be much less expensive compared to an extensive ten stories up. $150.00 per cubic yard sounds like a high price to install the material on a high-rise; however you can double that figure when it must be removed because it did not perform to specifications. If the crane is gone and the building is occupied, a wheel barrow up and down an elevator on a Sunday may be the only option. When selecting a growing media for a green roof system, it must be understood that the performance of the plant material is affected by the micro-climate. Media for sedums and other succulents for the green roof are limited to the species of plants that can tolerate the drying conditions. Plants that are not drought tolerant cannot survive the summer heat growing in the typical extensive green roof media designed only for sedums. Installing irrigation will allow some seasonal plants to thrive but this defeat's the purpose for what we define as an "extensive green roof". I have seen sedums do very well in Intensive green roof media; however during long periods of wet weather, especially during the winter months, some species may succumb to root rot. During the past few years, the performance of green roof media has been evaluated; and it was determined that the most crucial physical property the media should have is good drainage. This is made more challenging with the desire for green roof systems to retain additional water to not only reduce irrigation needs but also reduce and cleanse runoff in urban areas. Less water and fertilizer means slower initial growth but better sustainability for sedums and less weed growth. When the client desires total plant coverage, many designers use seasonal color (annuals) plants to fill in the gaps; however water and fertilizers are needed for this most filler plantings to look their best. This paper will discuss methods of installing media and review studies and plant trials to determine the proper media to maximize sustainability while meeting the expectations of the design intent.

The departments of Agricultural and Biological Engineering and Horticulture at the Pennsylvania State University have combined efforts to quantify the stormwater attenuation capabilities of extensive green-roof systems. This green-roof system consisted of a roof with a conventional flat-roof covering, a 0.5-in thick Enka-drainage layer, 3.5 in of porous medium, and Sedum spurium planted 3.0 in on center. The combined layers of this green roof had a maximum retention of 1.5 in and a saturated hydraulic conductivity of 0.43 in/s. The green roof system was modeled using a checkbook approach with daily rainfall depth as an input and ET and runoff as the outputs. The AGRR model was applied to 28 years (1976–2003) of rainfall data in Raleigh, NC, and showed that 45% of the annual rainfall volume (depth) can be retained on the green roof. Increasing the volume of storage does not improve the roofs ability to retain rain water. Providing only 0.125 in of roof storage will still cause over 30% of the annual rain depth to be retained on the roof.

Green roofs are a best management practice for urban areas where land for traditional stormwater practices is unavailable. The two primary research objectives are the following: (1) To develop an effective media for green roofs that will improve roof runoff quality while maintaining the known water retention benefits of green roofs, and (2) To demonstrate that green roofing will generate lesser pollutant loadings into urban runoff than traditional roofing materials. In Phase I (currently ongoing), several green roof media (formulated from commonly-used expanded minerals, stormwater filter media, and organic matter) are being evaluated for their abilities to retain the pollutants from a synthetic acid rain. The samples are being analyzed for metals, nutrients, pH, and conductivity. The hypothesis is that one media will be "better" at pollutant removal and permanent retention. In Phase II, once the optimized media has been selected, it will be field-tested on a green roof. The water quality of the runoff from the green roof will be compared with the runoff quality from a traditional roof. Phase I results on the mineral portion of the future media mix demonstrates all media were able to neutralize the acid rain. Comparing the media for pollutant removal and retention, the expanded shale was best able to retain phosphorus, ammonia, and metals from the synthetic acid rain. Phase I results on the mineral and additive combinations show the expanded shale and granulated activated carbon mix to be the most effective at pollutant removal and retention.

In North Carolina and several other U.S. states, all permeable pavements are currently considered to have similar capabilities in reducing runoff, but are not credited with improving water quality. Previous research conducted by the North Carolina State University on one particular permeable pavement type has shown that when compared to runoff from an adjoining asphalt lot, permeable pavement exfiltrate contained significantly lower concentrations of phosphorous and zinc, as well as reductions in total nitrogen. To further test various permeable pavement designs, a parking lot consisting of four different types of permeable pavements and standard asphalt was constructed in Kinston, NC. The permeable pavement sections consist of porous concrete (PC), permeable interlocking concrete pavers (PICP) with 8.5 % void space, PICP with 12.9 % void space, and concrete grid pavers (CGP), each covering a 1200 sq. ft. area with a 10 in. gravel storage layer. The purpose of this study is to evaluate and compare the effects of each pavement type on water quality and runoff reduction. Conclusions on the difference in reduction between each pavement type have not previously been determined. Site analyses on every rainfall event began in March, 2006, and will continue for one year. Preliminary results indicate significant peak flow and volume reductions in all permeable pavement exfiltrates. Additionally, there has been little to no runoff observed from any of the pervious sections. As a result of the Kinston study, it is expected that the state of North Carolina will be able to make an informed judgment on how much pollutant removal credit permeable pavements should receive when implemented as stormwater best management practices. Also, this study can be used to determine whether or not stormwater credit should vary based on pavement type.

A field monitoring study of an interlocking block permeable pavement system was initiated in Auckland, New Zealand to assess performance in achieving LID goals. Automatic sampling and continuous hydrologic monitoring occurred for one year on an active carriageway and bus lane. Concurrent testing of a conventional asphalt surface and the permeable pavement system indicate that on average, the permeable system delays the onset of runoff by 2.4 hrs (40 events), and decreases peak flow by 83% (18 events). Peak flow during a 10-yr event was reduced by 70%. Clay subsoils prevent volume control. While flows exceed modelled predevelopment conditions, the permeable system contributes significantly to overall hydrologic control. Water quality sampling (6 storms) indicates that the permeable system discharges lower pollutant mass by 58 ±15% for TSS, 58% ±10% and 71±10% for total and dissolved Cu, respectively, and 86 ±8% and 96 ±2% for total and dissolved Zn, respectively. Differences between asphalt surface and permeable system EMCs are statistically evident. Permeable system pollutant discharge is less than or comparable to effluent from most BMPs reported by the International Stormwater BMP Database. Incorporating hydrologic routing in the design process would likely further increase hydrologic and water quality control. Surface infiltration testing for four permeable pavement sites is used to assess maintenance needs. The primary study site shows no loss in permeability and all tests achieve at least the 1200 mm/hr design objective. Other sites showed that fines and leaf litter can reduce permeability by an order of magnitude or more.

The Friday Center Park and Ride Lot represents a unique initiative in which subsurface stormwater detention is combined with two types of porous pavement to produce the largest porous pavement parking lot in the State of North Carolina. The site demonstrates that pervious pavement can be utilized as a beneficial best management practice in the piedmont soils of North Carolina by controlling both volume and peak such that post development (paved) total site runoff is less than predevelopment (wooded) total site runoff. This project represents the first known use in the state of North Carolina Department of Transportation (NCDOT) of open grade friction course asphalt mix with polymer and fiber additives (Superpave) in a parking lot. Laboratory test data and results regarding anticipated asphalt performance are provided. Design elements include the development of a stepped bed system to accommodate existing site topography with underground cells to achieve the mitigation for stormwater volume and peak flow control. The Portland Cement Concrete Association spreadsheet model was utilized to demonstrate expected hydraulic performance and maximum water surface elevations beneath the pavement surface.

Seattle Public Utilities' (SPU) natural drainage systems program takes an innovative approach to street design in order to reduce the negative impacts of stormwater runoff on our creeks, lakes, and the Puget Sound. SPU began the program in 1999 and has since completed five projects ranging in scale from the retrofit of a single residential block to the complete redevelopment of 129 acres of mixed-income housing. New projects in development include a Natural Drainage System (NDS) retrofit in north Seattle and a downtown ultra-urban application using components of the NDS design approach. Natural drainage systems infiltrate or filter runoff in distributed areas spread throughout the landscape, maximizing the soil and vegetations's ability to absorb and clean stormwater. All the projects are diverse in their size and form, but they all use a combination extensive tree and shrub and wetland plantings, and skinnier streets to meet one or more of meet following objectives: reduced flooding in neighborhoods, improved water quality, reduced stormwater flow rates and reduced stormwater volumes. Additional benefits achieved through the project are traffic calming, increased pedestrian safety and enhanced aesthetics. With each project built, we learn new lessons about LID design, installation, and efficacy.

The Pembroke Woods Subdivision (Figure 1) is a ½-acre residential development located in northern Frederick County, Maryland, near the state line with Adams County, PA, and is not only the first Low Impact Development (LID) subdivision permitted in Frederick County, but also the first subdivision designed and under construction using the LID Design Manual developed by Prince George's County, Maryland (P.G., 1997; EPA, 2000a,b; Clar, 2001; 2002a,b). LID is an innovative approach to stormwater management (SWM) that attempts to mimic the pre-development hydrology of a development site thus maintaining the hydrologic functions of the site and precluding the environmental impacts, especially the water quality degradation of receiving streams, lakes and estuaries that are normally associated with land development activities. Because LID is a relatively new technology that substantially changes a number of the design paradigms used for stormwater management, one can anticipate a substantial learning curve for stormwater practitioners to understand and apply the technology. The Pembroke project site, being one of the first subdivisions designed and built with this technology, provides a valuable case study related to the issues that will be encountered in the application of this technology.

The City of Rockville Department of Public Works (City), working with the College Gardens community, Biohabitats, Inc. and the Center for Watershed Protection (CWP), embarked on an effort to demonstrate the potential for low impact development (LID) stormwater management approaches and identify implementation opportunities in the 160-acre College Gardens subwatershed of Watts Branch in Rockville, Maryland. Specific project objectives included: 1) providing a technical basis for innovative strategies and stormwater demonstration projects that can be applied to this existing urban neighborhood; 2) placing priorities on techniques that reduce nutrient loads, slow runoff rates, improve water quality, and utilize drainage area capacity to absorb runoff and recharge groundwater; and 3) providing the basis for technology transfer of similar approaches to other similar neighborhoods. Field reconnaissance and data collection utilized the Unified Subwatershed and Site Reconnaissance (USSR) protocol developed by CWP and a residential characterization and awareness survey. The subwatershed was divided into 9 catchments based on drainage patterns and land use to identify priority locations and types of LID practices to be implemented throughout the study area. A modeling effort using the Watershed Treatment Model (WTM) (CWP, 2001) was conducted to predict the nutrient load reduction potential of recommended LID practices. The priority LID practices identified and sited included bioretention, rain barrels, raingardens, and filters. The suite of priority practices were estimated to be able to provide partial or full water quality treatment for a total combined drainage area of about 30 acres. This includes a rain barrel and raingarden pilot program on residential lots, which has resulted in the installation of 110 rain barrels and 9 raingardens as of the end of 2005. This study suggests that implementing low impact stormwater management measures is feasible and beneficial with regard to the project goals. However, because this subwatershed is highly developed (50% impervious cover) and space for siting practices is limited, opportunities to provide more substantial management of runoff volume (i.e., channel protection storage) are limited. In fact, a more conventional retrofit of an ornamental pond located near the lower end of the drainage area is viewed as a necessary supplement to the LID practices to provide sufficient storage capacity to slow runoff rates and reduce the erosive velocities of larger storm events in this urbanized subwatershed.

Studies conducted by the Massachusetts Water Resources Commission classified the Ipswich River as "highly stressed" due to water withdrawals from tributary streams and polluted stormwater runoff from increasing impervious areas associated with land development. As part of a comprehensive restoration project for the Ipswich River Watershed, the Town of Wilmington hired Geosyntec Consultants to develop stormwater treatment designs for the Silver Lake town beach parking lot and three stormwater outfalls that discharge to the lake. The Silver Lake project involved installation of several Low Impact Development (LID) practices to treat stormwater in the Silver Lake watershed, including porous pavement surfaces, bioretention cells, vegetated swales, and raingardens. In conjunction with the BMP installation, both pre-construction and post-construction runoff volume and water quality measurements are being taken for 3–4 years by the USGS to evaluate the benefits and potential hazards of these new stormwater treatment technologies.

This paper describes the site evaluation, design, permitting, and construction of a Low Impact Development stormwater management system for a 259-unit residential development on 23.9 hectares (59 acres) in Lebanon County, PA. Stormwater management is achieved by the use of 124 storage/infiltration elements distributed throughout the site. These elements are integrated into the built landscape and include vegetated infiltration beds, pervious concrete sidewalks, vegetated infiltration swales, rain gardens, infiltration beneath standard driveway parking areas, porous asphalt pathways, and similar landscape/stormwater elements. These elements are designed to reduce runoff volume and maintain the natural hydrologic balance. The system is designed such that there is no increase in the volume of runoff after development for the 2-year frequency, 24-hour duration storm event, and no increase in peak flow rates for the 1-year through 100-year storms. One benefit of LID is that projects often incorporate many distributed BMPs to manage stormwater "close to the source." The challenges of developing calculations with traditional engineering, methodologies for a site that includes many volume-control BMPs will also be discussed.