Capital Hill PUD, Tentative (14-020) <br />June 19, 2017 <br />series of level spreaders in the eastern greenway west of the Ribbon Trail. With all the <br />underground facilities (catch basins, storage tanks and mechanical treatment devices), this <br />system could almost be considered a treatment train. The runoff downstream of these <br />mechanical devices is then released to level spreaders where the drainage will be filtered <br />through the landscape as it runs across the ground before reaching the eastern property <br />line and the Greenway. No additional filtration or infiltration can be applied to this basin <br />area due to the lack of space available, the steep slopes and the areas nearby prone to <br />landslides. <br />(c) The third and last discharge location will be the northern basin that collects runoff west <br />of the second (northern) high point. The drainage will be collected in a catch basin and <br />released via bubbler into Capital Drive and discharged onto the pervious pavement, again <br />matching the pre-developed drainage conditions. The drainage area is a bit too small to <br />justify the expense of a mechanical treatment; therefore, this basin will trigger the higher <br />LID fees. There will still be some degree of filtration however, when runoff is released <br />through the bubbler onto the pervious pavement. No additional filtration can be applied <br />to this basin area due to the steep slopes and lack of space available. <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. <br />Table 1: Existing Basin Data: <br />Discharge Site 1 (a) <br />Discharge Site 2 (b) <br />Discharge Site 3 (c) <br />Discharge Location <br />Greenway between Lot <br />South Capital Drive <br />14 & 15, and greenway <br />North Capital Drive <br />Summary <br />east of lot 15. <br />Existing Driveway & <br />38' wide section up to <br />Existing Driveway & <br />Basin Summary <br />Landscape (SW) <br />top of Hill (^'380' long) <br />Landscape (NW) <br />Impervious Area (SF) <br />5620 <br />0 <br />1390 <br />Pervious Area (SF) <br />9345 <br />14900 <br />4220 <br />Total Area (SF) <br />14965 <br />14900 <br />5610 <br />Peak 10-YR Discharge <br />0.19 <br />0.16 <br />0.10 <br />Rate (cfs) <br />It is worth noting that the basin area that will be released to the Discharge Site 2 under the post <br />developed condition will be significantly larger than the existing condition as the private roadway <br />Branch Engineering, Inc. <br />3 <br />