17290_Authority_February_2024

municipalauthorities.org | 7 These basins consist of a depression or embankment with an outlet structure that detains runoff for an extended period, typically 24 to 48 hours, before it is released downstream at a controlled rate. The basins are referred to as "dry" because water is not left standing between storm events. The basin bottoms can maintain a native grass mix since the facilities only experience inundation during storm events. Dry extended detention basins work well with urbanized drainage areas of all sizes. Basin slopes are gentle, with maximum side slopes of 3:1. A forebay is constructed at the inlet(s) to capture sediment and debris in a controlled area and prevent it from causing further issues downstream. Longer facilities help increase treatment flow path. The outlet structure is a critical feature, controlling discharge rates. It generally contains a small orifice or other flow control element to attenuate peak flows. The small orifice helps extend detention time in the facility, allowing sediment to deposit, as well as significantly reduce discharge rates and downstream flooding for more frequent rainfall events. Large rainfall events may release discharge through a combination of other orifices/weirs as well as the primary spillways in order to safely control the release of stormwater and prevent potential damage to the facility embankments. Dry extended detention basins provide numerous stormwater management benefits. Their primary advantage is peak flow rate reduction downstream, which mitigates flooding and erosion. They also remove sediment and particulate pollutants through settlement while water is stored. In addition, they are relatively low maintenance, since they are dry most of the time. These facilities are aesthetically pleasing with proper landscaping and establishment of native grass mixes. Additional water quality benefits occur through saturation of the soil in the BMPs, minor infiltration, evapotranspiration, and the filtering of runoff through native plantings. With appropriate design considering site topography and soil conditions, dry extended detention basins offer an efficient, multipurpose stormwater BMP. Their flow control capabilities combined with secondary water quality benefits make them a popular choice for developers, municipalities and engineers. Constructed Wetlands Constructed wetlands are an effective BMP for improving stormwater runoff quality. As the name suggests, constructed wetlands mimic the features of natural wetlands to leverage their innate water treatment abilities. These systems use aquatic plants, soils, microbes and shallow pools to remove sediments, metals, nutrients and other pollutants from stormwater runoff. Constructed wetlands contain shallow (6-24 inch) pools, forebays, and micropools filled with wetland vegetation like cattails, bulrushes and reeds. Stormwater runoff enters these BMPs into sediment forebays before discharging across the wetland for filtration. The runoff slowly flows through vegetated pools and channels, providing time for pollutant removal through physical and biochemical processes. Plants trap particulate matter while microbes in the soil and plant roots break down organic material. Constructed wetlands are designed to maintain consistent base water level, ensuring sufficient water for plants Continued on page 32. Table X shows the calculated sediment (TSS), phosphorus, and nitrogen removal of each BMP in pounds per year. These loads are based on a combination of drainage area, land cover, pollutant loading rates, and BMP effectiveness. Table X