Case Study: Stafford County, Virginia
Low Impact Development Ordinance
Stafford County is one of the 29 counties included in the Virginia "Coastal Zone". The Virginia coastal zone includes about 30% of the land area of the State and includes 60% of the population.
In 1994, Stafford County adopted a Stormwater Management Ordinance (Chapter 21.5 of the County Code) to establish minimum stormwater management requirements which:
- protect the safety and welfare of Stafford County residents and businesses;
- reduce flood damage to property;
- minimize the impacts of increased stormwater runoff from new land development;
- maintain the adequacy of existing and proposed culverts, bridges, dams, and other structures;
- prevent, to the greatest extent feasible, an increase in nonpoint source pollution;
- maintain the integrity of stream channels for their biological functions and drainage;
- minimize the impact of development upon stream erosion;
- and preserve and protect water supply facilities from increased flood discharges, stream erosion and nonpoint source pollution.;
Low Impact Development Initiative
Low Impact Development (LID) is an innovative Stormwater Management approach with a basic principle that is modeled after nature: manage rainfall at the source using site design techniques that store, infiltrate, filter, evaporate and detain runoff. LID's goal is to mimic a site's predevelopment hydrology by using design techniques that infiltrate, filter, store, evaporate and detain runoff close to its source. A goal of LID is to use site and subdivision design techniques in coordination with Stormwater Management engineering to mimic the hydrologic conditions associated with an undeveloped site.
The Friends of the Rappahannock (FOR) is an advocacy group that is a strong supporter of LID concepts. In 2003 FOR developed a comprehensive CD-based tutorial and resource toolkit covering the concepts and practice of Low Impact Development. The CD (now in its 3rd revision) includes a narrated 1-hour slideshow that contains numerous pictures, design examples, and even sample calculations. Also on the disc are numerous text resources such as the national LID manuals, rain garden design specs, and example codes. Funding: Virginia Dept.of Conservation and Recreation.
The Friends of the Rappahannock led a number of educational and consensus building projects in Stafford County aimed at preparing the County staff and elected officials for the inclusion of Low Impact Development concepts in their Stormwater Management Ordinance.
In 2003, the Stafford Board of Supervisors passed a series of changes to the Subdivision Ordinance and the Stormwater Code to include LID as an option for complying with the water quality, stream channel erosion, and flooding technical criteria of the County Stormwater Management Ordinance. First numerous "roadblocks" in the subdivision code were removed. Then, the national Low Impact Development manual was adopted by reference into the County Ordinance providing clear technical guidelines for Low Impact Development design and implementation. Finally, the County adopted incentives for developers to choose the LID option. Specifically, developers that choose to construct new sites according to the LID guidelines were allowed to do special water-filtering swales into of curb and gutter. They also were allowed to do trails through common green space in lieu of sidewalks.
In 2004, the County went a step further and required the use of LID on new development projects to the "maximum extent practicable".
The primary goal of Low Impact Development methods is to mimic the predevelopment site hydrology by using site design techniques that store, infiltrate, evaporate, and detain runoff. Use of these techniques helps to reduce off-site runoff and ensure adequate groundwater recharge. Since every aspect of site development affects the hydrologic response of the site, LID control techniques focus mainly on site hydrology.
There is a wide array of impact reduction and site design techniques that allow the site planner/engineer to create stormwater control mechanisms that function in a manner similar to that of natural control mechanisms. If LID techniques can be used for a
particular site, the net result will be to more closely mimic the watershed's natural hydrologic functions or the water balance between runoff, infiltration, storage, groundwater recharge, and evapotranspiration.
With the LID approach, receiving waters may experience fewer negative impacts in the volume, frequency, and quality of runoff, so as to maintain base flows and more closely approximate predevelopment runoff conditions.
The list below highlights some of the main goals and principles of LID:
- Provide an improved technology for environmental protection of receiving waters.
- Provide economic incentives that encourage environmentally sensitive development.
- Develop the full potential of environmentally sensitive site planning and design.
- Encourage public education and participation in environmental protection.
- Help build communities based on environmental stewardship.
- Reduce construction and maintenance costs of the stormwater infrastructure.
- Introduce new concepts, technologies, and objectives for stormwater management such as micromanagement and multifunctional landscape features (bioretention areas, swales, and conservation areas); mimic or replicate hydrologic functions; and maintain the ecological/biological integrity of receiving streams.
- Encourage flexibility in regulations that allows innovative engineering and site planning to promote .smart growth. principles.
- Encourage debate on the economic, environmental, and technical viability and applicability of current stormwater practices and alternative approaches.
LID is a comprehensive technology-based approach to managing urban stormwater. Stormwater is managed in small, cost-effective landscape features located on each lot rather than being conveyed and managed in large, costly pond facilities located at the bottom of drainage areas. The source control concept is quite different from conventional treatment (pipe and pond stormwater management site design). Hydrologic functions such as infiltration, frequency and volume of discharges, and groundwater recharge can be maintained with the use of reduced impervious surfaces, functional grading, open channel sections, disconnection of hydrologic flowpaths, and the use of bioretention/filtration landscape areas. LID also incorporates multifunctional site design elements into the stormwater management plan. Such alternative stormwater management practices as on-lot microstorage, functional landscaping, open drainage swales, reduced imperviousness, flatter grades, increased runoff travel time, and depression storage can be integrated into a multifunctional site design.
Specific LID controls called Integrated Management Practices (IMPs) can reduce runoff by integrating stormwater controls throughout the site in many small, discrete units. IMPs are distributed in a small portion of each lot, near the source of impacts, virtually eliminating the need for a centralized best management practice (BMP) facility such as a stormwater management pond. By this process, a developed site can be designed as an integral part of the environment maintaining predevelopment hydrologic functions through the careful use of LID control measures.
LID designs can also significantly reduce development costs through smart site design by:
- Reducing impervious surfaces (roadways), curb, and gutters
- Decreasing the use of storm drain piping, inlet structures, and
- Eliminating or decreasing the size of large stormwater ponds.
The focus of conventional stormwater management is to dispose of the runoff in a rapid fashion. The magnitude of hydrologic changes (increases in volume, frequency, and rate of discharge) are amplified as natural storage is lost, the amount of impervious surfaces is increased, the time of concentration is decreased, runoff travel times are decreased, and the degree of hydraulic connection is increased. Typical conventional site design results in developments devoid of natural features that decrease travel times and that detain or infiltrate runoff. Lack of these features often adversely affects the ecosystem.
In contrast, the principal goal of low-impact development is to ensure maximum protection of the ecological integrity of the receiving waters by maintaining the watershed's hydrologic regime. This goal is accomplished by creatively designing hydrologic functions into the site design with the intent of replicating the predevelopment hydrology and thus having a significant positive effect on stream stability, habitat structure, base flows, and water quality. It is well documented that some conventional stormwater control measures can effectively remove pollutants from runoff. Water quality, however, is only one of several factors that affect aquatic biota or the ecological integrity of receiving streams. Fish macroinvertebrate surveys have demonstrated that good water quality is not the only determinant of biological integrity. In fact, the poor condition of the biological communities is usually attributed to poor habitat structure (cover, substrate, or sedimentation) or hydrology (inadequate base flow, thermal fluxes, or flashy hydrology). Stormwater pond technology is limited in its ability to protect the watershed and cannot reproduce predevelopment hydrological functions. With this in mind, LID can be a way to bridge this gap in protecting aquatic biota and provide good water quality as well.