There are many ideas revolving around sustainability, and it can be hard to narrow in on a singular definition. Many municipalities are trying to become more sustainable in their operations, and promote sustainability among their citizens.
How does sustainability and green practices relate to stormwater?
When you think of true stormwater sustainability, you think of what happens in nature. Nature has it’s own way of:
- Reducing runoff volumes by infiltration
- Reducing soil loss through vegetation
- Reducing pollutants in storm runoff by infiltration and biological uptake
What are we trying to achieve with stormwater sustainability?
Nature is a good example to follow as it works to reduce runoff volumes by infiltration, reduce soil loss through vegetation, enhance habitat, and reduce pollutants in storm runoff by infiltration and biological uptake.
When we develop land, we change the natural system. Often, we increase both the peak runoff rate (typically measured in cubic feet per second, CFS) as well as the volume of runoff (typically measured in cubic feet or acre feet). The increase in both developed runoff rate and volume can be harmful to downstream channels, resulting in degradation. This degradation has effects on habitat as well as water quality by increasing sediment loads.
In addition to increasing runoff, we also introduce new sediment loads and pollutants into the natural system through the development process. During construction, we can introduce new sediment loads by exposing previously vegetated soil. After development is completed, we often see a whole new set of pollutants in storm runoff.
The primary objectives of stormwater sustainability are to mitigate these changes to the natural system.
We can do this by:
- Incorporating Construction BMP’s
- Encouraging Stormwater Infiltration
- Incorporating Stormwater Infiltration
- Implementing Stormwater Settling Basins
1—Incorporate Contruction BMP’s
BMPs during construction should be implemented to reduce erosion, capture sediment, and contain hazardous materials.
Reduce erosion: Keep soil in place, treat soil as a resource.
Capture sediment: Some erosion will occur regardless of how good your erosion reduction measures are. The resulting sediment should be prevented from leaving the construction site.
Material containment: Measures should be implemented to avoid contamination of stormwater and downstream receiving waters through the leakage of hazardous materials.
2–Encourage Stormwater Infiltration
Increases in both runoff rate and volume resulting from land development can be harmful to downstream systems. By encouraging infiltration, we are seeking to mitigate the effects of land development.
Porous paver systems, for example, as seen in the picture to the right, can be utilized to enhance stormwater infiltration. In a porous paver system, the gaps between the pavers allow runoff to pass through to a gravel bed underneath the paver surface. Stormwater accumulates in this bed, and is allowed to slowly infiltrate into the underlying soil.
Infiltration can also improve water quality through filtration and biological uptake which occur during the infiltration process.
3—Incorporate Stormwater Filtration
Developed runoff can contain a number of pollutants such as suspended solids, trash, and oils. Filtration can be an effective means at removal of such pollutants.
Filtration can be designed in the form of rain gardens, like the one on the left; sand filtration beds; grass buffer strips; to list a few. Filtration systems can be installed within storm pipe systems as well. There are a variety of products available to capture solids and oils within storm manholes.
4—Implement Stormwater Settling Basins
Many of the pollutants in stormwater bind to suspended solids/sediment. By removing suspended solids, we can also reduce pollutants in stormwater. Additionally, removing suspended solids helps downstream channels and receiving waters by reducing sediment loading.
By incorporating stormwater settling basins, we can significantly reduce suspended solids carried by stormwater. Settling basins are also referred to as “Extended Detention” basins. “Water Quality Capture Volume” refers to the volume that a settling basin/extended detention basin is sized to hold.
A common design standard is to capture a certain amount of stormwater runoff (roughly the volume of a 1-year storm event) and release the water slowly over a 40-hour period. This holding period allows for the settling of suspended solids in turbid stormwater.
Photos: A. Cvar