Sustainable Drainage: What are the Techniques for Protecting the Watershed?
Sustainable Drainage Systems are a collection of practices used to mimic natural processes of the hydrologic water cycle, which is the path of water as it moves around the earth and includes condensation, precipitation, infiltration, runoff, and evapotranspiration. These sustainable drainage systems can consist of natural features or man-made features made to look and act like natural features (bioretention facilities, rain gardens, vegetated rooftops). In the United States, Sustainable Drainage Systems are more commonly referred to as Best Management Practices (BMPs) or Low-Impact Development (LID).
What are Best Management Practices (BMPs) and Low-Impact Development (LID)?
The Environmental Protection Agency (EPA) defines Low-Impact Development as systems and practices that use or mimic natural processes that result in the infiltration, evapotranspiration, or use of stormwater to protect water quality and associated aquatic habitat. Why does this matter? As EPA notes, applied on a broad scale, LID can maintain or restore a watershed’s hydrologic and ecological functions.
Implementing LID practices allows the treatment of stormwater closer to the source using natural processes. Closer to the source means treating the water as close to where it reaches the earth’s surface as possible. For example, stormwater that sheet flows off a roadway and is collected in a swale then treated by an LID practice is treating the water closer to the source than if the stormwater were collected in a closed drainage system within the roadway and conveyed several hundred feet away to a larger detention pond. The swale (known as a level spreader) acts as a level swale that collects the stormwater and infiltrates it slowly into the ground, similar to what the water would do if the paved roadway were not present. For more significant flows that overtop the level spreader, a best management practice can provide some treatment of common pollutants, including total phosphorus (TP), total nitrogen (TN), and total suspended solids (TSS) (gravels in the stormwater) before the stormwater reaches a waterbody.
Best Management Practices (BMPs) are defined as methods that have been determined to be the most effective and practical means of preventing or reducing non-point source pollution to help achieve water quality goals. Non-point source pollution includes TP, TN, and TSS. Some BMP’s commonly used include bioretention facilities, rain gardens, vegetated rooftops, and tree box filters. Methods used to treat the stormwater include infiltration, filtration, detention, retention, and disconnection. Infiltration and filtration are similar in the way the water flows through a media which provides the treatment. The media used in infiltration practices it the natural soils which then convey the stormwater to the groundwater below. In filtration systems, the media is a man-made media, sometimes consisting of sand, sometimes a combination of sand/soil/and compost mixture, and sometimes a manufactured filter similar to filters you find in your house or car. Detention and retention are similar methods, they detain water. Detention practices detain water for a short time while retention practices typically have standing water at all times. Both treat the stormwater by allowing pollutants to settle out of the water over time. Disconnection methods includes conveying stormwater from an impervious surface (rooftop or pavement) to a pervious surface (grass) to allow the stormwater to naturally filter through the grass and into the soils prior to reaching surface water or groundwater sources.
Protecting our Watershed with These Methods
So how do BMPs or LID practices help with flooding or protecting the watershed?
One way these methods protect our watersheds are through the filtration. A rain garden is designed as a small depression in the ground that consists of various native plants planted on top of a filter media. The filter media allows the stormwater to be conveyed through it and also allows for uptake of the stormwater through the roots of the plants providing treatment of the stormwater and evapotranspiration of the stormwater (release of water to the atmosphere from soil and plant leaves). Stormwater that filters through the media can either infiltrate into the groundwater if the soils are conducive to that, or the treated stormwater can be collected in a pipe and conveyed to nearby surface waters. Treating the stormwater is important because untreated stormwater that reaches a waterbody (wetland, stream, pond) can affect the plant and animal life in that waterbody. This can lead to the degradation of ecosystems across the watershed.
Another way we can protect the watershed using LID is through flood control. Underground storage chambers can be used as flood control in areas where there is limited above-ground storage. These best management practices are common in urban developments, including shopping centers and stadiums. They are commonly placed beneath parking lots (such as this one pictured above, at Manchester-Boston Regional Airport) and act as a large storage facility for stormwater which can then be released at a controlled rate to nearby surface waters or infiltrated into the groundwater.
Our Recent Drainage Projects
In Massachusetts, we are working on two projects that are the same roadway and therefore have similar properties, although they are in two different towns. A large portion of these projects is within a water supply reservoir watershed. Its location means that treating the stormwater runoff is critical so that any contaminants in the runoff do not compromise the clean water in the reservoir. Massachusetts stormwater regulations are geared toward treating the stormwater at the source as opposed to collecting large volumes of stormwater and treating it in a larger detention basin somewhere down the road. To achieve compliance with the regulations and treat the stormwater, we have designed LID practices such as forebays, level spreaders, and grass swales at as many outlet pipe locations (outfalls) as allowable based on site constraints, including right-of-way and topography.
A town in Vermont is having erosion issues at the base of a steep, dead-end, gravel road. The erosion issues are due to the lack of stormwater conveyance practices and the lack of storage of more significant storm events. This section of town is upstream from a large wetland, however it is not hydrologically connected to it, which means stormwater does not directly get conveyed to the wetland. We were tasked by the regional planning commission to design two stormwater treatment practices that would convey the stormwater to the wetland area without creating additional erosion or flood control issues downstream. Both treatment practices included underground storage chambers for flood control of the larger storm events. More formal ditch lines and a closed drainage system were designed to collect the stormwater that currently flows over the gravel roadway.
The upstream BMP was designed to infiltrate the smaller storms. It was requested this BMP not permanently impact the adjacent Town Green area; therefore with the underground chambers, the BMP will not be visible from above, thus not impacting the character of the Town Green. It was requested the downstream BMP include a bioretention area (or rain garden) above the underground storage chambers. The town requested a more natural stormwater collection process and liked the visual aspect of what a rain garden offered. The soils beneath this BMP were not conducive to infiltration, therefore the flow out of the storage chambers was conveyed toward the downstream wetland via a pipe.
Sustainable Drainage Systems are everywhere – you have probably seen them and not even known it! Take a look around next time you are out and about and see what you can find. For more information about sustainable drainage, reach out to me!