Capital Hill PUD, Tentative (14-020) <br />August 22nd, 2017 <br />Table 3: Detention Facilitites <br />Storage <br />10 -Year <br />10-year <br />Storage <br />Volume <br />Peak <br />Peak <br />Tank <br />Summary <br />(cubic- <br />Destination <br />Inflow <br />outflow <br />feet) <br />(cfs) <br />(cfs) <br />S1 <br />Utility Vault-776-Revised <br />130 <br />G1 - East Gutter <br />0.23 <br />0.19 <br />S2 <br />Utility Vault - 816 <br />871 <br />S6 & East Greenway <br />0.28 <br />0.09 <br />S3 <br />Utility Vault - 816 <br />871 <br />S6 & East Greenway <br />0.33 <br />0.12 <br />S4 <br />Utility Vault - 816 <br />871 <br />S6 & East Greenway <br />0.20 <br />0.05 <br />S5 <br />Utility Vault - 816 <br />871 <br />S6 & East Greenway <br />0.31 <br />0.12 <br />S6 <br />Utility Vault - 816 <br />871 <br />East Greenway <br />0.37 <br />0.32 <br />S9, 510, & Northern Ditch <br />S7 <br />Utility Vault-776 <br />163 <br />0.12 <br />0.07 <br />S8 <br />Utility Vault - 816 <br />871 <br />S9, 510, & Northern Ditch D <br />0.25 <br />0.07 <br />S9 <br />Utility Vault - 816 <br />871 <br />510, & Northern Ditch D1 <br />0.23 <br />0.13 <br />S10 <br />Two (2) 24" pipes, 185 Long <br />1162 <br />Northern Ditch D1 <br />0.38 <br />0.19 <br />6,000 Gal Tank, 18'L x 8'W x <br />S11 <br />936 <br />G2 - West Gutter <br />0.58 <br />0.30 <br />The peak discharge rate into Destination #1, the existing eastern curb on Capital Drive, is actually <br />reduced slightly under the proposed condition, in part due to the existing pavement draining <br />toward the eastern curb and the new detention tank proposed. <br />For the discharge to Location 2, it was determined through an iterative process that having various <br />underground storage tanks collecting runoff every few hundred was more efficient than collecting <br />all the runoff at one low point and then directing them to stepped/tiered underground storage <br />facilities between lots 13 and 14. The second option required far more storage and would be more <br />complicated to construct and install. The detention tank selected and used for the multiple storage <br />tank system design is the Oldcastle Vault-816. <br />By having all the storage facilities fall within the footprint of the roadway, or just outside the <br />roadway, the stormwater improvements are far less likely to encroach on the existing vegetation. <br />The existing condition draining to discharge location 2 was assumed to be a 38-foot-wide section <br />of sheet flow draining from the top of the hill, located about 380 feet away. The basin area of <br />roughly 14,900 square feet created a peak discharge rate during the 10-year design storm of 0.16 <br />cubic feet per second (cfs). With the current layout proposed for the private roadway, the basin <br />area directing runoff to the 38' level spreader increases dramatically (3x) up to 45,520 square feet. <br />It is for that reason that several tanks were selected, and multiple level spreaders are proposed. <br />By matching the peak discharge rate of 0.16 cfs / 38 linear feet of level spreader, the post <br />developed improvements match the pre-developed discharge rates and runoff conditions. <br />The peak discharge rate into Destination #3, the northern ditch, matches the existing rate by <br />utilizing a combination of detention facilities. Since the new basin area draining to this destination <br />is larger, a somewhat complex combination of facilities was required as well as the use of 24" <br />detention pipes that will need to be constructed into the bank of the hill to the east due to cover <br />Branch Engineering, Inc. <br />8 <br />