2300 MERRILL CREEK PKY OPUS NW 2018-01-01 MF Import

5.0 ANALYS[S AND DESIGN OF REQUIRFD RWOFF CONTROL [3NIPs � The 3.33-acre project site is located within one drainage basin, which is known as Narbeci:Creek, which is the drainage basin that the site drains lo under existing conditions. Convevnnce• For purposes of conveyance and collection of stormwatcr nmoff generated from the development area, the Rational Mcthod was uscd based on the 2�-year design storm using a precipitation value of 2.6 inches. Please see the convcyance calculations within the Appcndix oC this report. Detention Facilitics: For the purposes oC detention calculation, the precipitauon values as dictated by the Ciry of Everett were uulized for the sitc crcating hydrographs for thc 100-ycar (3.25 inches), l0-year, (2.25 inches), and 2-year (1.55 inches) design storms as well as the pre-developed condition storms. For the pre-developed condition, [he si[c was assumcd to havc becn forested with a runo(f Curve Number oF 76. For the developed condition, a pervious runoff Curve Numbcr of 86 was utilized Cor the landscape area. For the impervious areas, a runoff Curve Number of 93 was utilized. The methodo(ogy utilized to size the detention system Cor the site was such that one-half oE[lte 2-year�,re-developed stortn even[ was the release r.tte during a 2-year developecI condition storm. In addition, thc (0-ycar prc-dcvcloped pcak runoff ratc was the relr.ase rate during a f 0-year post-developed condition storm. Finally, the 100-year pre-developcd E�eak rate of runoff �vas the release rate during a 1(�-year/24-hour developed condition storm event. In addition to matching thc release rates, a sa(ery factor based on the impervious amount of surface percentage is utilizcd for tliis project site, �vhich totuls approximately 42 perccnt safety factor basecl on a � 80 percent impervious covcragc for thc projcct site. A single de�ention pond is proposed to handle the detention mquirements for.he site. W:�tcr Oualitc Fncilitics: For the purposes of water quality, a wet pond is proposed located below the live storage in u weddctcntion pond sized to srorc, as dead storuge, thc cntire 6-monttd24-hour design storm volume from thc full buildout condition. 1 12270.003.da