Agenda - Parks and Recreation Commission - 05/12/2016

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170th Trail NW Rain Garden January 7, 2016 Page 2 home is constructed in the sub -watershed. We assumed that for a full build out condition, the remaining 3 developable lots will have a similar sized home. Under full build conditions, approximately 0.48 acres of pervious area and 0.23 acres of impervious area (approximately 32% impervious) will ultimately discharge to the proposed rain garden. The 1-inch, 1-year, 2-year, 5-year and 10-year return interval rainfall events were analyzed assuming a total storm duration of 24-hours. Table 1 describes the Atlas 14 rainfall depth and resulting rates and volumes of runoff anticipated. Table 1: Summary of anticipated subwatershed conditions draining to the proposed rain garden. Rainfall Runoff Runoff Rainfall Depth Rate Volume Event (in) (cfs) (cu ft) 1" Event 1.0 0.3 655 1-Year 2.46 0.6 1440 2-Year 2.86 0.9 2035 5-Year 3.59 1.4 3250 10-Year 4.26 2.0 4490 The rain garden was also analyzed for water quality. The results can be used as a planning tool to make decisions based on this rain garden size and similar watershed conditions. In other words, if additional rain gardens were planned in this neighborhood, estimated pollutant load reductions can be prorated based on these results. Table 2 below summarizes the anticipated total suspended solids (TSS) and total phosphorus (TP) reductions. Table 2: Summary of pollutant loading and removal. Pollutant Loading Description Quantity Unit Post development annual runoff volume 0.70 acre-ft Annual runoff volume removed by BMPs: 0.26 acre-ft Percent annual runoff volume removed: 38 Post development annual particulate P load: 0.32 Ibs Annual particulate P removed by BMPs: 0.12 Ibs Post development annual dissolved P load: 0.26 Ibs Annual dissolved P removed by BMPs: 0.1 Ibs 38 % Percent annual total phosphorus removed: Post development annual TSS load: 104 Annual TSS removed by BM Ps: 39 Percent annual TSS removed: 38 Ibs Ibs o�