Bioswale Designed by Urban Associates, Inc. to Treat Stormwater at Local Charter School

LOW IMPACT DEVELOPMENT (LID) is the use of a variety of nonstructural stormwater management practices to treat stormwater runoff and encourage rainwater infiltration into the ground to recharge groundwater aquifers and to reduce downstream flooding. The goal is to reduce stormwater runoff to match pre-development conditions without having to use costly pipes, CB’s and land consuming retention/detention basins. These structures will cost to construct in the hundreds of thousands of dollars and will have long term life cycle costs associated with them.


As projects are constructed permeable surfaces are replaced with impermeable roads, parking lots and rooftops. Years ago it was common practice to pipe surface runoff from impermeable surfaces without quality treatment directly to local waterways. This practice ultimately caused flash flooding, erosion and siltation of rivers/streams and the destruction of aquatic habitats.


In more recent years it has become common practice to direct surface runoff from impermeable surfaces via CB’s and pipes to a retention or detention basin. The idea is to treat the first flush dirty water in the basin and allow it to be filtered as it infiltrates into the ground. The remaining runoff is held in a basin large enough to hold the design storm and to release the stormwater at the pre development rate to adjacent waterways.


The problem with the large basin approach is that they require large areas of developable land. Basins are unsightly and are regularly fenced in to protect the health, safety and welfare of the public. Another problem with the basin approach is the cost of the drainage pipe and structures that are needed to transport the stormwater runoff from the impermeable surfaces to the basin. In addition, if the basin cannot be sized large enough to detain all of the stormwater it is common practice to install expensive exfiltration pipe or chambers under parking lots.


The low impact development method idea is to distribute the stormwater over the entire site and not just concentrate the stormwater in one place, in a basin. Low impact development methods include the construction of a treatment train of aesthetically pleasing landscaped bioswales, rain gardens, green roofs and green walls. Other low impact treatment methods include permeable pavements, the construction of French drains and the use of cisterns to store captured rainwater that can be used for irrigation and to flush toilets.


It is possible with the LID methodology to eliminate all of the impermeable surfaces on a development site to pre-development conditions, eliminate the unsightly basin, eliminate headwalls and end walls, eliminate catch basins and piping, all of which can provide construction costs savings in the hundreds of thousands of dollars. Based on the 20 year life cycle cost comparisons that we have performed using LID methods instead of structures there will be substantial long term cost savings for the life of the project.


It is important that the LID stormwater management design consultant either design the project or play a leading role in the site design process because it is critical that the LID treatment methods be incorporated into the site design early in the planning process.


If you have any questions about low impact development techniques, construction cost savings, EPA Case Studies, associated LEED credits and examples of LID projects that our firm has successfully managed and completed, please contact Leo Urban RLA, LEED AP BD+C at Urban Associates, Inc. For additional information on similar topics please link to our webpage blog.