Post Construction Turbidity - How a Skimmer Can Be Used to Meet Water Quality Requirements

Historically, post-construction stormwater management has focused on meeting the water quantity requirements set by local regulatory agencies. However, these agencies are increasingly targeting water quality goals as well. As a result, property owners are now required to implement various types of stormwater control measures (SCMs) to meet these new standards. While detention basins typically achieve an approximate 60% reduction in total suspended solids (TSS), adding a skimmer can enhance this reduction to over 90%. This improvement can eliminate the need for additional SCMs, thereby reducing the overall cost of installation and maintenance throughout the project's lifespan.

Full Abstract: Many local and state entities mandate the use of post-construction stormwater management practices to achieve stormwater quantity and quality goals. These requirements are enforced through Municipal Separate Storm Sewer System (MS4) permits, which aim to minimize the hydromodification impacts on natural receiving waterbodies due to urbanization and development. MS4 permits typically require peak flow management to control discharge rates, ensuring they mimic pre-development conditions. This necessitates treating any additional impervious areas that contribute to increased discharge and flow rates, typically through detention, infiltration, evapotranspiration, and/or water reuse.

Stormwater quality goals primarily focus on total suspended solids (TSS), usually requiring the removal of 80% of TSS from the inflow concentration of the water quality volume (WQV). The WQV is the volume of runoff targeted for treatment. Most stormwater detention practices for post-construction runoff management use a combination of orifices and weirs to manage inflows, regulate discharge, and detain stormwater, allowing for some sedimentation. These orifices and weirs are often staggered within the water column so that as depth increases, additional outlets are utilized until peak discharge is reached.

Although detention allows for some sedimentation, it is often not the primary focus of detention basins. Therefore, other stormwater quality improvement practices are typically used in conjunction with these basins to meet local jurisdictional water quality requirements. For instance, the Georgia Drainage Design Manual for Highways estimates a 60% reduction in TSS for dry detention basins. To achieve the minimum 80% reduction, additional practices need to be included, potentially increasing land acquisition and maintenance requirements.

Dry detention basins are one of the most common post-construction stormwater practices due to their ability to store substantial volumes and mitigate post-development hydrology. However, their 60% TSS treatment efficiency necessitates the use of additional practices to achieve the 80% removal goal. This is typically accomplished by implementing a grassed channel upstream of the basin. The 60% TSS removal efficiency for detention basins is reported by the International Stormwater BMP Database and is based on data from 32 basins monitored throughout the U.S.

This presentation will summarize the findings from large-scale testing of using a Faircloth Skimmer as a post-construction water quality improvement discharge device at the Auburn University - Stormwater Research Facility (AU-SRF). Three triplicate installations were tested using the same runoff and sediment introduction conditions. All tests demonstrated an improvement in water quality, with discharge TSS decreasing by an average of 91.7%.