1901 MERRILL CREEK PKY 2018-01-01 MF Import

_ .. , . r . . � � � - _ associated with the wetland treatrnent cell and must be vegetated. Detention volume(6ve storage) � is combined with the wetpond open water cell vo(ume(dead storage)to reduce the number of vaults necessary to control the stom►water. � Stotm water cunoff from the area proposed for develoQment naturally flows into two separate drainage.pasins, the Narbeck Creek Basin and tt,e Metrill-Ring Creek Basin, with areas proposed for development of approximately 1.5 acres and 6.6 acres respectively. Drainage from both of these � basins eventuslly flow into Possession Sound. Drainage areas for each basin after development are consistent with the area flowing into that basin prior to development. [ Aforementioned above,characteristics of both basins include ground cover that is forest land with undisturbed older second growth vegetation. No impervious areas currently exist on site. Soils az�e Alderwood with the hydrological soil group considered to be a type "C" soil. As such, tun-off [ would be expected to be moderate. � As can be scen by the attached calculations used to size the detention and water quality facitities for this site the required detention volume for Memll-Ring Creek Basin is 47350 cf with a total of 47840 cfprovided in the detention vault. The required volume for water quality in the Memll-Ring Creek Basin is 12862 cf with a total of 12900 provided. There will be 11040 cf provided in the � detenNon vault and 1969 cf provided in the wetland treatment cell. For the Narbeck Creek Basin 14500 cf of storage is required with 15200 cf provided. Note that there is no water quality feature � proposed for the Nazbeck Creek Basin as this basin will collect only mof drainage and will therefore not require run-off treatment. � Offsite Drainage Description: � Upon leaving the site drainage is directed towPrd either of two streams aforementioned above. Upon drainage leaving the site through Merrill Ring Creek, stormwater continues north for approximately 1.2 miles where it finally empties into Possession Bay in the Puget Sound. A typical � cross section of this stream is a trapewid with a bottom width of approximately 4 to 5 feet and 1:I side slopes. The channel has a gentle slope and is lined with small trees and thick under brush on the banks. At the time of investigation the stream was flowing at a depth of 8 inches and had nearly 3 feet of freeboazd. During inspection slight bank sloughing and erosion problems where noticed. ',. There was no evidence of inadequate capacity or overtopping. I [Jpon drainage leaving the site through Narbeck Creek,stonnwater continues for approximately 1.2 miles where it empties into Possession Bay in the Puget Sound. A typical cross section of this stream is a trapezoid with a bottom width of approximately 3 fect with itregular side slopes. The � sheam has a gentle slope and lined with small IIaes and thick under brush on the banks. At the time of the investigation the stream was flowing at a depth cf 4 to 6 inches. During inspection there was no evidence of capacity problems, overtopping, scouring,bank sloughing, or erosion problems. ( I : I