Capital Hill PUD, Tentative (14-020) <br />March 3, 2017 <br />In order to calculate the pre-developed rates that need to be met in the post developed condition, the existing <br />basin areas to those discharge locations needed to be determined. Using the existing surface in the software <br />program AutoCAD-Civil3d and the new road and lot layout, the basin areas were determined and the following <br />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 />Greenway between Lot <br />Discharge Location <br />South Capital Drive <br />14 & 15, and greenway <br />North Capital Drive <br />Summary <br />east of lot 15. <br />' <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 developed <br />condition will be significantly larger than the existing condition as the private roadway will capture all of the <br />private roadway as well as the upland lots (28-32). For that reason, multiple underground storage tanks will be <br />necessary to detain the water and soften the peak discharge rate being released to the level spreaders. <br />The areas and peak flow rates under the post developed condition are displayed in the following 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 />14 & 15, and greenway <br />North Capital Drive <br />Summary <br />east of lot 15. <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 />8320 <br />18470 <br />2630 <br />Pervious Area (SF) <br />8425 <br />27050 <br />3525 <br />Total Area (SF) <br />16745 <br />45520 <br />6155 <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 />0.11 <br />(cfs) <br />*The Discharge rate is released to two level spreaders, so the total discharge rate is 0.32 cfs. <br />Branch Engineering, Inc. <br />3 <br />