Capital Hill PUD, Tentative (14-020) <br />June 19, 2017 <br />will capture all of the private roadway as well as the upland lots (28-32). For that reason, multiple <br />underground storage tanks will be necessary to detain the water and soften the peak discharge <br />rate being released to the level spreaders. <br />The areas and peak flow rates under the post developed condition are displayed in the following <br />table. <br />Table 2: Proposed Basin Data: <br />Discharge Site 1 <br />Discharge Site 2 <br />Discharge Site 3 <br />Discharge Location <br />Greenway between Lot <br />South Capital Drive <br />12 & 13, and greenway <br />North Capital Drive <br />Summary <br />east of lot 13. <br />Private Roadway <br />New private roadway <br />draining toward the low <br />New private roadway <br />Basin Summary <br />southwest of high point. <br />point west of the <br />northwest of high point. <br />greenway. <br />Impervious Area (SF) <br />8,320 <br />18,470 <br />3,225 <br />Pervious Area (SF) <br />81425 <br />27,050 <br />3,525 <br />Total Area (SF) <br />16,745 <br />45,520 <br />6,155 <br />Peak WQ Discharge <br />0.04 <br />0.08 <br />0.02 <br />Rate (cfs) <br />Peak 10-YR Discharge <br />Rate with detention <br />0.19 <br />0.16 (x 2) * <br />0.12** <br />(cfs) <br />"The Discharge rate is released to two level spreaders, so the total discharge rate is 0.32 cfs. <br />**No Detention Applied. <br />As can be seen from the table, the areas discharging to sites 1 and 3 are quite comparable. <br />Therefore, minimal detention to these discharge sites will be required. <br />Through an iterative process, it was determined that multiple underground storage tanks would <br />provide the most effective tool in reducing the peak discharge rates from the developed private <br />road to Discharge Site 2. These tables, the HydroCAD analysis and the proposed stormwater <br />design will be discussed in more detail in the following sections. <br />3.3 WATER QUALITY TREATMENT <br />As noted previously, this report is intended to address the stormwater created by the <br />improvements to the public roadway on Capital Drive, and the new private road circulating <br />through the site. The primary stormwater management tools will be underground storage and <br />mechanical treatment for the private roadway runoff due in part for the lack of available space for <br />filtration facilities. The pervious pavement for the public improvements on Capital Drive is not <br />analyzed in HydroCAD. According to the City of Eugene Stormwater Management Manual <br />permeable pavements are impervious area reduction techniques and it is therefore assumed that <br />this stormwater mitigation technique will match pre-existing conditions and provide treatment <br />and match the exiting flow & discharge patterns. <br />Branch Engineering, Inc. <br />4 <br />