Contech Stormwater experts discuss designing stormwater BMP’s, regulations, maintenance issues, and all things stormwater.
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.
EPA selected a removal standard of 80% total suspended solids (TSS) removal as the target pollutant of concern due to high TSS concentrations ubiquitous impact on water quality and degradation to aquatic habitat. Many other pollutants of concern are particle-bound, and TSS is thereby a surrogate for other pollutants. Testing methodologies for stormwater control measures (SCMs) in respects to TSS can vary greatly. There are many sediment characteristics that should be considered when evaluating a SCM for TSS removal performance to ensure apples and apples are being compared among removal efficiencies for SCMs.
The EPA selected a removal standard of 80% total suspended solids (TSS) removal as the target pollutant of concern due to high TSS concentrations impact on water quality and degradation to aquatic habitat. Many other pollutants of concern are particle-bound, and TSS is thereby a surrogate for other pollutants. Testing methodologies for stormwater control measures (SCMs) in respects to TSS can vary greatly. In part two, we’ll continue our look at stormwater sediment and discuss particle shape and density and their affect on TSS removal.
Rainwater harvesting (RWH) stores rainwater for reuse to supply non-potable uses like irrigation, wash water, toilet flushing, and laundry. During long dry periods the demand will drain the storage cistern down to a critical level where the pressurization pump(s) will need to shut down to prevent dry run damage. Make up water is typically a potable connection to the rainwater harvesting system to supply water, allowing the RWH system to continue to supply the non-potable end use applications until the next storm event refills the storage cistern.
It’s never surprising to see some type of fabric or geotextile called-out around an underground detention or infiltration system. The note is common across civil plans everywhere, but how is a geotextile selected as applicable for the particular underground system the detail was so aptly created? The answer to that question starts with one step back – why we even use filter fabric.
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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