Capital Hill PUD, Tentative (14-020) <br />August 22nd, 2017 <br />*Discharge Site 5 was not analyzed because the total drainage across Capital Drive onto the adjacent <br />neighbors to the west will be reduced by directing upland areas to the western gutter line south on <br />Capital Drive instead. <br />It is worth noting that the basin area that will be released to the Discharge Sites 1, 2, 3, & 4 under <br />the post developed condition will be larger than the existing conditions. The public and private <br />roadway improvements will capture more upland areas and redirect some of the volume <br />previously draining onto the neighbors to the west and sheet flowing into the park land to the <br />east. For that reason, multiple underground storage tanks will be necessary, as previously <br />discussed, to detain the water and reduce the peak discharge rates to match pre-developed levels. <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 Site 4 <br />South Capital <br />Greenway <br />North Capital <br />South Capital <br />Discharge Location <br />Drive (East <br />between Lot 13 & <br />Drive, ditch east <br />Drive (West <br />Summary <br />Gutter Line) <br />14, and greenway <br />of gravel road. <br />Gutter Line) <br />east of lot 14. <br />Private Roadway <br />New Pavement <br />draining toward <br />New Pavement <br />New Pavement <br />Basin Summary <br />from Private & <br />the low point <br />from Private & <br />from Public <br />Public Roadway <br />west of the <br />Public Roadway <br />Roadway and <br />and upland areas. <br />and upland areas. <br />upland areas. <br />geenway. <br />Impervious Area <br />10,945 <br />18,470 <br />13,960 <br />6,045 <br />(SF) <br />Pervious Area (SF) <br />12,580 <br />58,810 <br />34,350 <br />33,815 <br />Total Area (SF) <br />23,525 <br />77,280 <br />48,310 <br />39,860 <br />Peak WQ Discharge <br />0.06 <br />0.08 <br />0.06 <br />0.04 <br />Rate (cfs) <br />Peak 10-YR <br />Discharge Rate with <br />0.27 <br />0.16 (x 2)* <br />0.19 <br />0.32 <br />detention (cfs) <br />"The combined flow rate out of the detention vault releasing runoff into Discharge Site 2 is 0.32 <br />cfs, however, the flow is split into two level spreaders, each releasing 0.16 cfs per 38' level <br />spreader, which matches the existing peak release rate per 38' section into Hendrick's Park. <br />In comparing the two tables, it can be seen that the new development meets or reduces peak runoff <br />rates during the 10-year storm when compared to the pre-developed condition. <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. The combination of <br />underground storage facilities is modeled in Appendix C and detailed in the plans on Sheet C2. <br />Branch Engineering, Inc. <br />6 <br />