Capital Hill PUD, Tentative (14-020) <br />August 22nd, 2017 <br />mechanical treatment or perhaps lined planters, rain gardens, sand filters, or swales. For <br />flow control, some of the mechanisms above might work, but the use of underground <br />storage tanks or oversized pipes may serve better. <br />For the lots bound by the new roadway improvements, upon treatment and flow control, <br />drainage disposal will discharge onto the new roadways. For the lots draining away from <br />the roadway to the east, disposal (after treatment and detention) would be into level <br />spreaders directed toward the existing park land to the east or north. <br />3.2 STORMWATER ANALYSIS <br />All stormwater calculations were determined using HydroCAD 10.0 modeling software. This <br />program utilizes the Santa Barbara Unit Hydrograph hydraulic modeling method. All results of the <br />analysis are included in Appendix C. 24-Hour Rainfall depths are based on City of Eugene <br />Stormwater Management Manual, Appendix C. <br />In order to calculate the pre-developed rates that need to be met in the post developed condition, <br />the existing basin areas to those discharge locations needed to be determined. Using the existing <br />surface in the software program AutoCAD-Civil3d and the new road and lot layout, the basin areas <br />were determined and the following summarizes the areas and peak discharge rates for the existing <br />conditions. <br />Table 1: Existing Basin Data: <br />Discharge <br />Discharge <br />Discharge <br />Discharge <br />Discharge <br />Site 1 <br />Site 2 <br />Site 3 <br />Site 4 <br />Site 5* <br />Greenway <br />North <br />Sheet flow <br />Discharge <br />South <br />between Lot <br />Capital <br />South <br />west off <br />Location <br />Capital Drive <br />14 & 15, and <br />Drive ditch <br />Capital Drive <br />Capital Drive <br />Summary <br />(East Gutter <br />greenway <br />east of <br />(West <br />onto adjacent <br />Line) <br />east of lot <br />Gutter Line) <br />neighboring <br />15. <br />gravelroad. <br />properties <br />Existing <br />38' wide <br />Existing <br />Existing <br />Existing <br />Basin <br />Driveway & <br />section up <br />Driveway & <br />Pavement <br />Pavement <br />Summary <br />Landscape <br />to top of Hill <br />Landscape <br />and upland <br />and upland <br />(SW) <br />(^'380') <br />(NW) <br />areas <br />areas <br />Impervious <br />6050 <br />0 <br />2095 <br />0 <br />N/A <br />Area (SF) <br />Pervious <br />13510 <br />14750 <br />10625 <br />30,885 <br />N/A <br />Area (SF) <br />Total Area <br />19560 <br />14750 <br />12720 <br />30,885 <br />N/A <br />(SF) <br />Peak 10-YR <br />Discharge <br />0.30 <br />0.16 <br />0.19 <br />0.38 <br />N/A <br />Rate (cfs) <br />Branch Engineering, Inc. <br />5 <br />