Current stormwater design guidance typically recommends starting with preservation of the natural landscape and hydrology wherever feasible. But, even with preservation, new stormwater management facilities are likely to be required to capture and clean runoff from pollution generating surfaces. These new stormwater facilities are likely to include imported soil or soil amendments that add permeability and/or optimize soil structure for vegetative growth. For many years, the industry has characterized media in vegetated facilities generically as soil, sand, compost, etc.
Not done with siting issues yet, maybe this becomes five parts? One issue on siting and design is the hydraulic grade lines. Recall from your road drainage days the equations that were used to space catch pits and throat openings? The equations allowed for you to estimate gutter efficiency and top width for specified design storms. Well, these equations still apply, and I am thinking maybe even more considerations for very low flows.
As a volume based stormwater control measure, bioretention systems are providing beneficial use in that they reduce runoff volumes and peak flows. In areas where combined sewers are an issue bioretention can reduce CSO frequency while increasing evapotranspiration and helping with groundwater recharge via infiltration processes. Common design criteria include storage volume and a design infiltration rate of the media and the underlying native soils. These criteria are tied to the site characteristics and statistical hydrology, for example, design the storage volume such that 95% of the mean annual runoff volume is retained. In addition to these sizing criteria we also need to design with these other factors in mind.
As bioretention becomes more popular, many types of designs are being deployed throughout the U.S. Though relatively simple in concept, many are finding that the devil is in the details with respect to maintenance and performance. These issues are driving newer designs and improving criteria for use. Over my next few posts, I will be sharing some of the experiences and lessons learned with bioretention design.
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