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SECTION 4 Water Quality Eualuatiou Step 1) Determine the number of drywells in the cluster. Example: Number ofDrywells in Cluster: 10 Step 2) A drainage area for each drywell cluster needing a surface infiltration/ rain garden CP was estimated based on the number of drywells in the cluster and assuming a 2.70 acre drainage area per drywell. Example: Drainage Area Per Drywell: 2.7 acres Total Drainage Area for Example Drywell Cluster: 10 x 2.7 27 acres Step 3) Given the number of drywell clusters and the level of uncertainty associated with the defined drainage areas for each drywell cluster, site - specific rain garden designs (as described above for the on- street, rain garden option) were not developed. Instead, using one of the ROW rain garden configurations (from one of the initial on- street rain garden options), the total drainage area associated with one "rain garden unit" was determined to be 1.12 acres. Step 4) Given the total drainage area for the drywell cluster, and the drainage area accommodated by one rain garden unit, the number of rain garden units required for treatment of the drywell cluster was estimated. Example: # of Rain Garden Units Req'd: 27 acres =1.12 acres 24 Rain Garden Units Step 5) The on- street rain gardens were sized to treat only the ROW area within the drainage area. This represented approximately 19% of the entire unit drainage area or 0.21 acres of the 1.12 acres. It was assumed that drainage from the remaining portion of the drainage area would be treated on -site. The rain gardens within one "rain garden unit" were designed to be 12" deep with 3:1 horizontal to vertical side slopes. The 5 -year, 24 hour design storm runoff volume from the ROW that would have to be managed by a rain garden was calculated to be approximately 2,120 cubic feet. Assuming a single, rectangular rain garden, the required surface area to manage the required volume of runoff from the 0.21 acres of ROW per rain garden unit is approximately 2,854 square feet. As a result, the rain garden total surface area required to treat runoff from the drywell cluster was estimated. Example: Rain Garden Surface Area Per Rain Garden Unit: 2, 854 ft2 Total Surface Area Req'd: 24 Rain Garden Units x 2,854 ft2 68, 496ft2 (or 1.6 acres) Step 6) Finally, assuming each rain garden unit has 2,854 square feet of rain garden, and that construction of one square foot of rain garden (with native soils) is approximately $8.00 (for non - engineered soils), the average cost of rain garden 0:\25695978 Eugene RR -SC Final Basin P1an\Master P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 4-11