For many years, civil engineers have approached stormwater management with the idea that the best solution is to eliminate the rainwater as quickly as possible. This approach was rooted in the need to get water out of the medieval streets so you were not constantly slogging through puddles. A great deal of energy has been spent studying runoff coefficients and sizing pipes to obtain maximum efficiency in stormwater removal. Significant consequences of this historic “rapid runoff” design approach are being felt in modern urban societies. As stormwater volumes increase downstream, flooding can result. Furthermore, contaminants washed off of pavements and other hard constructed surfaces convey the water away from the property, which can concentrate downstream.

A more sustainable way to manage stormwater is taking hold with many civil engineers: Low-Impact Development (LID). Low-impact site development is an alternative approach that seeks to maintain the natural hydrogeologic character of the property or region. This is accomplished by both alternative site design and specific best water flow management practices. A key goal of LID is to control the volume of stormwater runoff such that stormwater leaving the site after development is approximately the same volume of stormwater that left the site before development.

The LID Tool Kit
LID design considerations focus on several key initiatives: 1) reducing the rate of water runoff by increasing the retention time on the property, and 2) enhancing water infiltration into the soils on the property.

Several tools to accomplish these initiatives include the use of:

• Swales rather than just pipes for stormwater conveyance which results in a longer retention time on the property
• Bioretention areas (e.g., rain gardens) to enhance infiltration
• Porous pavement to enhance infiltration
• Soft surfaces (rather than increased hard surfaces) which results in longer retention time and enhanced infiltration
• More greenery to enhance evapotranspiration of stormwater absorbed into plants
• Underground storage systems which result in longer retention time

Sustainable Benefits of LID
Maintaining the natural hydrogeologic character of a property can have increased benefits for long-term sustainable development. Increased infiltration can aid the replenishment of local ground water levels, which, by a rapid runoff design approach, may have become depleted due to water being transported downstream. Maintaining stormwater on site longer can reduce potential downstream flooding, while also reducing the social and economic costs associated with flooding. More naturalized design can decrease the carbon footprint of the property. Maintaining storm-water on site may tend to redistribute or treat small quantities of contaminants on a localized basis, which reduces the potential for concentration of contaminants within the environment (e.g., contaminants that bioaccumulate in river sediments).

Sample of LID design for a community

Costs
Many of the tools discussed have been used by civil engineers for years and have proven quite effective. Thus, the challenge is to apply these tools to a site-specific development in different ways. When this technology is applied consistently by experienced LID engineers, the costs should not significantly change from the historic rapid runoff approach. Finally, it is important to note that there may be hurdles to overcome with existing regulations and ordinances that may lack the flexibility needed to apply LID approaches.

So, for your next property, stop and consider the sustainable benefits of Low-Impact Development and put a LID on your project!