Sustainable drainage
Introduction
Sustainable drains are landscape elements designed to concentrate or remove debris and pollution from surface runoff waters. They consist of a gently sloping drainage course (less than 6%) and filled with vegetation, compost and/or gravel.[1] The water flow path, along with the wide, shallow trench, is designed to maximize the time the water spends in the trench, which aids in the collection and removal of contaminants, silt and debris. Sustainable drains are also beneficial in groundwater recharge and are a great stormwater mitigation tool. Depending on the topography of the land, a sustainable drainage may have a meandering or nearly straight channel alignment. The composition of a sustainable drainage can be influenced by many different variables including: climate, rainfall patterns, site size, budget and available vegetation that can be planted.
It is important to maintain sustainable drainage to ensure the best possible efficiency and effectiveness in removing contaminants in stormwater drainage. Planning for these things is an important step, which may include introducing filters or large rocks to prevent clogging. Annual maintenance through soil testing, visual inspection and mechanical testing are also crucial to the health of sustainable drainage.
A common application is around parking lots, where substantial automotive pollution settles on the pavement and is then washed away by the first instance of rain, known as the first flush. Sustainable drains, or another type of biofilter, can be created around the edges of parking lots to capture and treat stormwater runoff before releasing it into the basin or sewer.
Treated contaminants
Sustainable drains work to remove contaminants through vegetation and soil. As stormwater runoff flows through sustainable drains, pollutants are captured and deposited by plant leaves and stems. The contaminants then enter the soil where they decompose or can be decomposed by bacteria in healthy soil.[2].
There are several kinds of water contaminants that can be collected or stopped with sustainable drainage. These fall into the categories of slime, inorganic contaminants, organic chemicals and pathogens.
Best locations
Sustainable drainage can be implemented in areas that require stormwater management to regulate runoff velocity and decontaminate runoff. Sustainable drains are created to handle the first discharge of pollutants during the rainfall event, therefore locations that have high areas of impervious surface such as roads, parking lots or rooftops can benefit from the addition of sustainable drains. They can also be integrated into road medians, curbs, sidewalks or any public space.[3].