The incorporation of bioretention areas into a development site is a viable option for nutrient removal from storm water runoff. Large commercial developments often have surplus areas available for soft landscaping and bioretention.
An estimate of the bioretention area required to treat new developments is approximately 2% of the total development site (Concept Design Guidelines for Water Sensitive Urban Design: Healthy Waterways 2009).
The location of the treatment areas in a carpark can be incorporated into the grading design. Smaller sections of the carpark could be designed to grade into the multiple small bioretention areas to reduce the need for a single treatment area.
In cases where it is not possible to treat a segment of a site’s catchment, the option to oversize treatment areas can be used to make up for the untreated areas, but this option should be avoided due to inefficiencies.
Each bioretention area requires an inlet and outlet arrangement to suit the treatable flow and any surface bypass flow for the catchment. Inlet arrangements for carpark bioretention can be designed using flush kerb and cut-off walls for sediment diversion. Other possible inlet arrangements consist of a coarse sediment forebay to remove both large gross pollutants and coarse sediment. This option provides the added value of controlling the velocity of the incoming stormwater, preventing scour of the filter media.
Outlet arrangements can consist of a downstream discharge pit for the subsurface drainage and various overflow bypass structures.
Often however, the feasibility of bioretention in commercial developments is restricted by many factors including a lack of available space, minimal accessibility and even absence of direct sunlight.
Another potential constraint is that treatment areas have to be completely flat to provide consistent stormwater infiltration through the filter media.
There are a few worthy alternatives to treat stormwater that should be considered in these circumstances.
Proprietary stormwater treatment devices can be comparable in price to bioretention with regards to up-front and ongoing maintenance costs.
It is necessary to consider all forms of stormwater management and treatment, as each project will differ in its practical requirements. It is essential to strike a balance between the stormwater treatment objectives, the limitations of the site, and the cost effectiveness of the range of options, to ensure the best outcome is reached.