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New Low Impact Design: Site Planning and Design Techniques for Stormwater Management
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Larry S. Coffman, Michael L. Clar, Neil Weinstein
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Author Info |
Abstract
Low impact development (LID) achieves stormwater management controls by fundamentally changing conventional site design to create an environmentally functional landscape that mimics natural watershed hydrologic functions (discharge, frequency, recharge and volume). This is accomplished in four ways. First, minimizing impacts to the extent practicable by reducing imperviousness, conserving natural resources /ecosystems, maintaining natural drainage courses, reducing use of pipes and minimizing clearing and grading. Secondly, recreate detention and retention storage dispersed throughout a site with the use of open swales, flatter slopes, rain gardens (bioretention) and rain barrels. Thirdly, maintain predevelopment time of concentration by strategically routing flows to maintain travel time. Fourthly, encourage property owners to use effective pollution prevention measures and to maintain management measures.
Introduction
This paper is only a brief overview of some major aspects of the Low Impact Design Manual developed by Prince George's County, Maryland's Department of Environmental Resources. The manual provides a detailed and comprehensive step by step site planning approach to use the LID technology as an alternative stormwater method. The manual explains LID principles, hydrologic analytical methodology, site planning techniques, best management practices, review processes and public outreach programs. A multi-agency task force developed the basic LID approach and philosophy over a two-year period. The LID design manual builds upon case studies, pilot projects, BMP technology developments and research produced by the County over the past ten years. The objective of LID is to provide an alternative lower cost effective environmentally sensitive method for stormwater management.
Rapidly urbanizing jurisdictions across the country are confronted with a growing major economic, environmental and public safety problem. How to effectively control stormwater runoff to reduce flooding and erosion, protect drinking water supplies, maintain the integrity of fisheries, provide safe water related recreational activities and to ensure the preservation of the ecological integrity of receiving waters, riparian corridors and associated wetlands. Whether in response to federal or state stormwater management requirements, wetland and natural resource conservation regulations or addressing public concern about development's adverse impacts on the quality and quality of runoff, local governments are striving to develop, improve or expand stormwater management programs in a cost effective manner. Typically, adverse stormwater impacts are mitigated (lessened) by conservation of natural resources (forests, streams, floodplains and wetlands); zoning restrictions to balance density with open space; and the use of structural control technologies (best management practices - BMP's) to treat or manage runoff quantity and quality.
Experience, monitoring and research data have shown that many conventional structural BMP's fail to provide desired environmental protection and have numerous limitations. For example, stormwater management ponds are typically designed to control specific peak runoff rates and improve water quality but they do not replicate predevelopment watershed hydrology. Furthermore, because ponds do not control the runoff volume, flooding can still occur in a watershed where peak discharges coincide in an additive fashion. Ponds also cause elevated water temperatures, have variable water quality benefits, can be a source of groundwater contamination, create public safety risks, have costly maintenance burdens and may accelerate stream erosion due to increased duration and frequency of runoff events. These limitations have direct and cumulative adverse impacts on ecosystem integrity, increase the economic burdens of maintaining BMP's and create an ongoing need for environmental restoration of aquatic ecosystems.
The LID approach presented in this paper combines resource conservation, a hydrologically functional site design with pollution prevention measures to reduce development impacts to better replicate natural watershed hydrology and water quality. LID controls discharge rates, runoff volume and runoff frequency to mimic predevelopment runoff conditions. Through a variety of on lot site design techniques stormwater is managed in small, cost effective landscape features disbursed throughout a developed site. This source control concept is quite different from conventional end of pipe treatment. Hydrologic functions such as runoff discharges volume, frequency and ground water recharge can be maintained by reducing impervious surfaces, functional grading, open channel sections, disconnection and utilization of runoff, and the use of bioretention or filtration landscape areas.
The effective use of LID site design techniques can significantly reduce the cost of providing stormwater management. Savings are achieved by eliminating the use of stormwater management ponds, reducing pipes, inlet structures, curbs and gutters, less roadway paving, less grading and clearing. Where LID techniques are applicable and depending on the type of development and site constraints, stormwater and site development costs can be reduced by 10 to 25 % compared to conventional approaches.
LID allows for the same or in some cases higher lot yields compared to conventional approaches. Since stormwater management is control on each lot using multifunctional landscape, that portion of the buildable area that would have been used for stormwater ponds can now be recovered and used for building, parking lots, open space or habitat enhancements.
Hydrologic Impacts of Conventional Systems and BMP's
Conventional stormwater conveyance systems are designed to collect, convey and discharge runoff as efficiently as possible. The developed landscape is designed and constructed (cleared, graded, piped and paved) with the intent to create a highly efficient drainage system to prevent on lot drainage problems, promote good drainage and quickly convey runoff to a BMP or stream. This efficient drainage system increases runoff volume, decreases ground water recharge and changes the timing, frequency and rate of discharge. Conventional site developed because of efficient site drainage causes flooding, water quality degradation, stream erosion and the need to construct BMP's. Typical use of BMP's as an end of pipe treatment device reinforces and perpetuates the design of the efficient runoff collection and conveyance system.
Conventional designs typically locate BMP's at the most downstream point of the site for end of pipe control. The stormwater management requirement is usually to maintain the peak runoff discharge rate at predevelopment levels for a particular design storm event. The discharge using conventional BMP's is set to match the predevelopment peak rate. However, this approach only controls the rate of runoff allowing significant increases in runoff volume, frequency and duration of runoff from the predevelopment conditions. These hydrologic alterations provide the mechanisms for further degradation of receiving waters. Conventional application of site and BMP designs can only lessen hydrologic and water quality impacts and can not restore natural hydrological functions.
Hydrologic Response of Low Impact Development
The objective of LID site design is to minimize, detain and retain the post development runoff volumes close to the source to simulate predevelopment hydrologic functions. Management of both runoff volume and peak runoff rate is included in the design. This is in contrast to conventional end of pipe treatment.
The LID design approach is to leave as much undisturbed area as practical and optimize infiltration, detention and interception to reduce runoff volume and discharge. LID management controls are integrated throughout the site to compensate for the hydrologic alterations of development. The approach of conserving and utilizing areas with high infiltration and low runoff potential in combination with small on lot stormwater retention / detention facilities creates a "hydrologically functional landscape". This functional landscape can also enhance a site's aesthetic and habitat value by encouraging greater conservation of trees and use of more plant landscape materials.
Wide spread use and uniform dispersion of on lot retention and/or detention to control both runoff discharge volume and rate is key to better replication of predevelopment hydrology. The frequency and duration of runoff are also much closer to the existing condition than can be achieved by typical application of conventional BMP's.
LID Hydrologic Analysis
The LID site analysis and design approach focuses on four major hydrologically based planning elements. These fundamental factors affect hydrology and are introduced below.
- Curve Number (CN)- A factor that accounts for the affects of soils and land cover on amount runoff generated. Minimizing the change in the post development CN by reducing impervious areas and preserving more trees and meadows to reduce runoff storage requirements to maintain the predevelopment runoff volume.
- Time of Concentration (Tc) - This is related to the time runoff travels through the watershed. Maintaining the predevelopment Tc reduces peak runoff rates after development by lengthening flow paths and reducing the use of pipe conveyance systems.
- Permanent storage areas (Retention) - Retention storage is needed for volume and peak control, as well as water quality control and to maintain the same CN as the predevelopment condition.
- Temporary storage areas (Detention) - Detention storage may be needed to maintain the peak runoff rate and/or prevent flooding.
Minimizing the Change in CN
Calculation of the low-impact development CN is based on a detailed evaluation of the existing and proposed land cover so that an accurate representation of the potential for runoff can be
obtained. This calculation requires the engineer/ planner to investigate the following key parameters associated with LID; 1) land cover type, 2) percentage of and connectivity of impervious cover, 3) hydrologic soils group (HSG), and 4) hydrologic conditions (average moisture or runoff conditions).
The following LID site planing practices can be utilized to achieve a substantial reduction in the change of the calculated CN: 1) narrower driveways and roads (minimizing impervious areas), 2) maximized tree preservation and/or afforestation, 3) site finger printing (minimal disturbance), open drainage swales, 4) preservation of soils with high infiltration rates to reduce CN, 5) location of BMP's on high-infiltration soils and, 6) construction of impervious features on soils with low infiltration rates. Reducing the change in CN alone will reduce both the post development peak discharge rate and volume.
Maintaining the Predevelopment Tc
The LID hydrologic evaluation requires that the post development Tc be close to the predevelopment Tc. This is important because LID is based on a homogenous land cover and distributed retention and detention BMP's. The following site planning techniques can be used to maintain the existing Tc; 1) maintain predevelopment flow path length by dispersing and redirecting flows using open swales and natural or vegetated drainage patterns, 2) increasing surface roughness (e.g., preserving woodlands, vegetated swales); 3) detaining flows (e.g., open swales, rain gardens), 4) minimizing disturbances (minimizing compaction and changes to existing vegetation), 5) flattening grades in impacted areas, 6) disconnecting impervious areas (e.g., eliminating curb/gutter and redirecting down spouts) and 7) connecting pervious and vegetated areas (e.g., reforestation, afforestation).
Combined use of these techniques, and those to reduced the change in the CN can modify runoff characteristics to effectively shift the post development peak runoff time to that of the predevelopment condition and lower the peak runoff rate.
Maintaining the Pedevelopment Curve Number and Runoff Volume
Once the post development Tc is maintained at the predevelopment conditions and the impact of CN is minimized, any additional reductions in runoff volume must be accomplished through distributed on site stormwater management techniques. The goal is to select the appropriate combination of management techniques that simulate the hydrologic functions of the predevelopment condition to maintain the existing CN and corresponding runoff volume. LID sites use retention practices distributed throughout the site to provide the required volume controls at the source.
Retention storage allows for a reduction in the post development volume and the peak runoff rate. The increased storage and infiltration capacity of retention BMP's allow the predevelopment volume to be maintained. The most appropriate retention BMP's include; 1) bioretention cells (rain gardens), 2) infiltration trenches and 3) rain barrels. Other possible retention BMP's include retention ponds, roof top storage, cisterns and irrigation ponds but it may be more difficult to distribute these types of controls throughout a development site.
As retention storage volume of LID BMP's is increased there is a corresponding decrease in the peak runoff rate in addition to runoff volume reduction. If a sufficient amount of runoff is stored, the peak runoff rate may be reduced to a level at or below the predevelopment runoff rate. This storage may be all that is necessary to control the peak runoff rate when there is a small change in CN. However, when there is a large change in CN, it may be less practical to achieve flow control using volume control only.
Potential Requirement for Additional Detention Storage
In some cases where large changes in CN can not be avoided, retention storage practices alone may be either insufficient to maintain the predevelopment runoff volume or peak discharge rates or require too much space to represent a viable solution. In these cases, additional detention storage will be needed to maintain the predevelopment peak runoff rates. A number of traditional detention storage techniques are available that can be integrated into the site planning and design process for a LID site. These techniques include: 1) swales with check dams, restricted drainage pipes, and inlet / entrance controls, 2) wide low gradient swales, 3) rain barrels, 4) rooftop storage and 5) shallow parking lot storage. These detention practices can easily be integrated into the site design features.
Determination of Design Storm Event
The hydrologic approach of LID is to retain the same amount of rainfall within the development site as that retained prior to any development (e.g., woods or meadow in good condition) and then releases excess runoff as the woods or meadow would have. By doing so, it is possible to mimic, to the greatest extent practical, the predevelopment hydrologic regime to maximize protection to aquatic ecosystems and ground water recharge. This approach allows the determination of a design storm volume that is tailored to the unique soils, vegetation and topographic characteristics of the watershed. This approach is particularly important in watersheds that are critical for ground water recharge to protect stream / wetland bases flow and ground or surface water supplies.
LID Applicability
LID combines of a wide variety of site design techniques to provide stormwater management. Controls are integrated and disbursed throughout the site to micro-manage runoff at the source. The approach can be used for residential, industrial and commercial developments. However, its feasibility and desirability is dependent on several key planning considerations shown below.
- Need for conservation of the hydrologic regime. How important is it to maintain runoff volume, peak discharges, frequency and infiltration to protect receiving waters and aquatic ecosystems?
- Reasonable and practical use of the property. Is there is space available for both LID controls and the normal use of property?
- Conflicts with existing site planning and design regulations. Will clustering and construction of narrow streets to reduce impervious surfaces be allowed?
- Importance of aesthetic and habitat considerations. Can the LID landscaping techniques be used for multiple purposes to enhance property values and habitat functions?
- Prevention of flooding problems. Are there critical flooding problems that would require more conventional controls?
- Property owner acceptance. Is there a market for green development and will property owners properly maintain on lot LID controls measures?
- Time delays to approve innovative site designs. Will any additional review time or studies be needed to demonstrate the effectiveness of the LID approach?
- Development density must allow space for LID controls. Is the density of the development such that there is no available landscape for significant use of on lot LID management practices or conservation of natural resources?
LID Basic Site Planning Strategies
The goal of LID is to design the site in a way that reproduces hydrologic functions. The first step is to minimize the generation of runoff (reduce the change in the CN). In many respects, this step is very similar to traditional techniques of maximizing natural resource conservation, limited disturbance and reducing impervious areas. The major difference is with LID you must carefully consider how best to make use of the hydrologic soil groups and site topography to help reduce runoff. These considerations would include; 1) maintain natural drainage patterns, topography and depressions, 2) preserve as much existing vegetation as possible in hydrological soil groups A and B, 3) locate BMP's in hydrologic soil groups A and B, 4) where feasible direct impervious areas to soil groups C and D, 5) disconnect impervious surfaces to direct and disburse runoff to soil groups A and B, 6) flatten slopes within cleared areas to facilitate on lot storage and infiltration and 7) revegetate cleared and graded areas.
LID BMP's
There are a wide variety of BMP's available. The site design techniques and BMP's can be organized into three major categories as follows; 1) runoff prevention measures designed to minimize impacts and changes in predevelopment CN and Tc, 2) retention facilities that store runoff for infiltration, exfiltration or evaporation and 3) detention facilities that temporarily store runoff and release through a measured outlet.
Table 1 below lists some BMP types and their primary functions. Placing these BMP's in series and uniformly dispersing them throughout the site provides the maximum benefits for hydrologic controls.
Table 1. Examples of LID BMP's and Primary Functions
| BMP
| Runoff Prevention
| Detention
| Retention
| Conveyance
| Water Quality
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| Bioretention
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X |
X |
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X |
| Infiltration Trench
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X |
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X |
| Dry Wells
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| Roof Top Storage
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X |
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| Vegetative Filter Strips
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X |
X |
| Rain Barrels
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X |
X |
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| Swale and Small Culverts
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X |
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X |
X |
| Swales
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X |
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X |
X |
| Infiltration Swale
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X |
X |
X |
X |
| Reduce Imperviousness
| X |
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| Strategic Clearing / Grading
| X |
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| Engineered Landscape
| X |
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| Eliminate Curb and Gutter
| X |
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X |
| Vegetative Buffers
| X |
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X |
The application of LID BM's is demonstrate in the attached figures. Figures 1and 2 show how tree conservation, bioretention and open drainage systems can be used to control runoff for single family residential lots. Figure 3 shows typical open drainage road sections with wider infiltration swales. Figure 4 shows how the landscape in a commercial / industrial site can be used to control runoff.
Public Outreach and Pollution Prevention
Pollution prevention and maintenance of on lot BMP's are two key elements in the overall comprehensive approach. Effective pollution prevention measures can reduce the introduction of pollutants to LID BMP's thereby enhancing their ability to reduce pollutant levels and extend the life of the facilities. Public education is essential to successful pollution prevention and BMP maintenance. Not only will effective public education complement and enhance BMP effectiveness, it can also be used as a marketing tool to attract environmentally conscious buyers, promote citizen stewardship, awareness and participation in environmental protection programs and help to build a greater sense of community based on common environmental objectives and the unique character of LID designs.
Road Blocks to LID
In the development and acceptance of LID site planning approach, a number of roadblocks had to be over come. Regulating agencies, the development community and the public all had numerous concerns about the use of new technology. The LID design manual represents the culmination of four years of work to address all of these concerns and issues. Some of the major components of the LID approach which address the many concerns include: 1) development of a hydrologic analytical methodology to demonstrate the equivalence of LID to conventional approaches; 2) development of new road standards which allow for narrow roads, open drainage and cluster techniques; 3) streamlining the review process for innovative LID designs which allow easy modification of site, subdivision, road and stormwater requirements; 4) development of a public education process which informs property owners on how to prevent pollution and maintain on lot BMP's; 5) develop legal and educational mechanisms to ensure BMP's are maintained; 6) demonstrate the marketability of green development; 7) demonstrate the cost benefits of the LID approach; 8) provide training for regulators, consultants, public and political leaders; and, 9) conduct research to demonstrate the effectiveness of bioretention BMP's.
Summary
LID is a viable cost effective alternative approach to stormwater management and the protection of natural resources. LID is designed to provide tangible incentives to a developer to save more natural areas and reduce stormwater and roadway infrastructure costs. LID can achieve greater natural conservation not by regulating protection of more areas but by using conservation as a stormwater BMP. As more natural areas are saved less runoff is generated and stormwater management costs are reduced. This allows multiple use and benefits (environmental and economical) of the resource.
Additionally, developers have incentives to reduce infrastructure costs by reducing impervious areas, eliminate curbs / gutters and stormwater ponds to achieve LID stormwater controls. Reduction of the infrastructure also reduces infrastructure maintenance burdens making the LID development more economically sustainable. Superior protection of aquatic and riparian ecosystems can be achieved since the LID developed watershed functions in a hydrologically similar manner as predevelopment conditions. Recreating the predevelopment hydrological regime is a better way to protect the receiving waters as opposed to conventional mitigation approaches.
LID promotes public awareness, education and participation in environmental protection. As every property owner's landscape functions as part of the watershed, they must be educated on the benefits and the need for maintenance of the landscape and pollution prevention. LID developments can be designed in a very environmentally sensitive manner to protect streams, wetlands, forest habitat and save energy. The unique character of a LID green development can create a greater sense of community pride based on environmental stewardship.
References
Prince George's County, Maryland. Department of Environmental Resources. Low-Impact Development Design Manual. (PGC, 1997)
Prince George's County, Maryland. Department of Environmental Resources. Low-Impact Development Guidance Manual. (PGC, 1997).
Prince George's County, Maryland. Design Manual for use of Bioretention in Stormwater Management. (PGC, 1993)
Prince George's County, Maryland. Maryland National Capital Planning Commission. A Technical Manual for Woodland Conservation with Development. (Darr, 1990).
Larry S. Coffman
Associate Director
Department of Environmental Resources
Prince George's County, Maryland
Michael L. Clar
Executive Vice President
Engineering Technology, Inc.
Ellicott City, Maryland
Neil Weinstein
Senior Scientist / Engineer
Tetra Tech Inc.
Fairfax, Virginia
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