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BMP Descriptions <br />nearly continuous flow of stormwater through the practice. Although some infiltration does occur, the <br />primary form of treatment is the settling of pollutants as stormwater flows through the dense plant <br />community. <br />This practice was modeled to estimate the pollutant reduction capacity for TSS, TP, and stormwater <br />volume in medium density residential drainage areas ranging from 0.25 to 4 acres (Table 7). A 20' long <br />(parallel to roadway), 4' wide (perpendicular to roadway), and 3" deep bioswale was modeled with an <br />infiltration rate of 2.5"/hour. No underdrain was modeled with this practice as they are designed to be <br />flow -through systems with limited ponding (<_ 3"). Additional model inputs are noted in Appendix A. <br />Table 7: WinSLAMM model results for the boulevard bioswale with a 2.5"/hour infiltration rate. <br />Drainage <br />Area <br />(acres) <br />Standard Boulevard Bioswale <br />TP Removal <br />Ibs-TP <br />TSS Removal Volume Removal <br />Ibs-TSS % * ac-ft * % i <br />0.25 <br />0.5 <br />1 <br />2 <br />3 <br />4 <br />Infiltration Basin <br />0.07 <br />0.09 <br />0.08 <br />0.07 <br />0.08 <br />0.08 <br />33.3% <br />23.7% <br />13.0% <br />8.0% <br />6.8% <br />6.2% <br />43 <br />61 <br />53 <br />45 <br />47 <br />48 <br />38.0% <br />28.3% <br />15.6% <br />9.8% <br />8.6% <br />8.0% <br />0.058 <br />0.067 <br />0.074 <br />0.082 <br />0.087 <br />0.09 <br />21.9% <br />12.6% <br />7.0% <br />3.8% <br />2.7% <br />2.1% <br />Infiltration basins function identically to the curb -cut rain gardens previously described in this <br />bioretention section. However, these basins are proposed in locations where a large amount of space is <br />available. This presents an opportunity to construct a large-scale (i.e. > 500 sq-ft.) infiltration basin. This <br />would allow stormwater runoff to fill the basin and be filtered by the soil and vegetation. <br />Probable project cost includes installation of the project as well as promotion, administrative, and <br />design costs, all in 2016 dollars. A reduced construction cost (i.e. $15 to $20 per ft.2) relative to other <br />bioretention practices was proposed for the infiltration basin because of assumed cost savings with a <br />larger project. Furthermore, the large open spaces available at each of the proposed project locations <br />could allow the basins to be constructed without retaining walls, which would result in a significant cost <br />savings. Maintenance was assumed to be completed by city public works crews. Maintenance costs <br />were also included for rehabilitation of the basin every 10 years for the life of the project. <br />City of Ramsey Stormwater Retrofit Analysis <br />