By way of background, here’s a diagram of a typical “water budget” for a piece of land in its natural state. Of all the rain that falls, half of it goes into the ground, and only 10% runs off directly into streams and rivers.
After development, however, pavement and storm sewers work together to collect stormwater and send it into rivers and streams as fast as possible. As a result, more than half of the stormwater runs off into waterways, and only 15% returns to the ground through infiltration.
Looking at these photos, you can see why those runoff and infiltration numbers change so much—lots of asphalt! This is what most current zoning codes require: expansive parking lots and excessively wide streets. How do you know if your street is too wide? If teenagers are doing donuts right in the roadway—see the skid marks in the picture on the left. The places here are designed primarily for the automobile, and as a result they are not particularly friendly to pedestrians, bicyclists, or wildlife. The goal of Low Impact Development is to create multifunctional places are designed for people, wildlife, and cars.
It’s also important to recognize the incredible investment that goes into conventional site work and stormwater management systems: asphalt curbs, catch basins, storm sewers, and detention ponds consume a large proportion of the development budget for most projects. Unfortunately, “end-of-pipe” solutions have a limited effectiveness, because they have to treat huge amounts of runoff. Through good site design and low impact development techniques that deal with stormwater close to the source, developers can improve the environmental impact of their projects while also saving money, resulting in more profit and increased potential for affordable housing.
Now let’s go through six key Low Impact Development Principles. The first and most important principle is to work with the landscape. Rather than imposing a predetermined development formula or cookie cutter approach, LID uses the existing landscape as the framework for site planning. Good site design begins with careful attention to good land use planning at the community level as well as the watershed level, so that the right uses go in the right places with regard to the watershed. In general, a site design should identify important environmental resources and should define a development envelope where new housing or business can be placed with minimal impact on those resources. Those resources might include legally protected wetlands, aesthetically important features such as mature trees or good vistas, and hydrologic features such as permeable soils and drainageways.
Our second key principle is to focus on prevention, not mitigation. The goal is to work with the site characteristics to maintain hydrologic functions and processes rather than attempt to mitigate impacts of alteration. For example, avoiding the disturbance and grading of vegetated areas can significantly reduce the need for stormwater controls and will help to recharge groundwater. Designers can also reduce the amount of impervious surface and associated runoff by reducing road widths, clustering buildings and using permeable surfaces for parking. Road alignment is important as well—designers should consider ways to reduce the total road length, and to place the roads to minimize the amount of cut and fill necessary. Green rooftops can also be used to reduce the amount of runoff from buildings. The photo on the slide shows a residential site where a forested area has been preserved, roof runoff is directed to vegetated areas where it will infiltrate, and the road is placed close to the house to minimize the length of the driveway.
Conventional stormwater systems treat stormwater at the “end of the pipe” in large detention ponds design to treat large volumes of runoff. In contrast, Low Impact development strategies use small, decentralized techniques to treat stormwater runoff close to where it is created. The site is designed and graded to create many small drainage areas, and the runoff from each of these subwatersheds is treated through nearby LID structures. LID techniques are commonly designed in series; for example, runoff might flow over a filter strip, into a vegetated swale, which leads to a bioretention cell. This “treatment train” approach provides excellent stormwater management because each of the techniques helps to remove pollutants, allow infiltration, and slow down the runoff to reduce peak discharge rates. In contrast to the conveyance goals of storm sewers, site designs that create long flow paths and low slopes help to reduce runoff rates and increase groundwater infiltration. Finally, it is important to remember that LID techniques are excellent strategies for controlling peak discharge rates from frequent, low-intensity storms, and they can usually be sized to accommodate the entire “water quality volume”—the first inch or half-inch that must be treated according to the Massachusetts Stormwater Policy. However, on sites with large amounts of impervious surfaces, it is likely that additional controls may be needed to manage the runoff from very large storms, such as the 25-year or 100-year storm. However, these controls will be considerably smaller than they would be otherwise.
While LID strategies must be carefully selected and implemented, it is important to remember that many of these techniques are very simple, almost common sense approaches to dealing with stormwater. So-called “open drainage systems” such as vegetated swales cost considerably less than conventional curb and catch basin designs, but they provide better water quality treatment and improve the appearance of a site. Property owners and developers can also reduce the amount of runoff in the stormwater system by directing rooftop runoff to vegetated areas where it will infiltrate or into low-cost rain barrels and cisterns where it can be used for infiltration, saving money on water bills. In the photo here, a rooftop downspout is directed to a vegetated area where it will infiltrate into the ground. One of the most effective methods of improving water quality is frequent street sweeping and parking lot sweeping. Finally, it is very important to reduce site disturbance during construction. The clearing, grading, and compaction that occurs during development can severely reduce the capacity of soils to hold and infiltrate stormwater. Developers should define a disturbance envelope that is as small as possible, and when construction is complete they should try to restore disturbed areas. Builders can help to restore the storage and infiltration capacity of compacted soils by roto-tillling them before seeding. Even better, disturbed areas should be planted so that they become meadows or forested areas, rather than conventional lawn, which generally does a poor job of absorbing rainwater. Lawns also require irrigation, do not provide wildlife habitat, and are usually treated with pesticides, herbicides, and fertilizers, all of which contribute to poor water quality.
For more information, you can visit the Low Impact Development Toolkit website created by MAPC, where you can find fact sheets, the local codes checklist, a stormwater bylaw summary, and various other resources. MAPC also created three different brochures about LID: one for citizens and regulators, one for developers, and one for engineers. You can download these from the website.
These practices can be integrated into every part of site design to create a hydrologically functional lot. Trees and native vegetation provide nature’s storage of rainfall. Permeable pavement, grass swales along roads, rain gardens and amended soils store, filter and infiltrate runoff. Adding rain barrels and other cisterns provide additional water storage.
3/26/10 SuAsCo Municipal Stormwater Workshop: Stormwater Management Bylaw
Presentation Overview :
Low Impact Development
Why a Bylaw is needed
Summary of Sterling Bylaw
Low Impact Development Principles, Techniques, and Implementation Slide show prepared by: In coordination with: