1001 SHUKSAN WAY 2016-01-01 MF Import

i � � 5. Analysis and Design of Required Runoff Control (Treatment and Stream Bank Erosion Control) BMPs (Hydraulic Report) I Ezisting Conditions ( The original topography of the site was sloping at about 3 percent from south to north, so the runoff from stormwater flowed into Glenwood Crcek. The existing tupography of the site is ( changing daily because the carrent use is a gravel pit. Portions of the site aze being mined while I other portions aze being reclaimed. The existing slopes vary from about 1 percent to vertical walls. Stormwater runoff from the existing gravel pit either infiltrates into the ground or flows { into an onsite pond. There aze no off-site tributary azeas contributing runoff to this project basin. I '['he soil as classified by the Soil Survey of Snohomish County Area by the Soil Conservation I 5ervice datecf luly 1983 is Alderwood gravelly sandy loam, Alderwood Urban L.and Complex, and Toku1 gravelly Io�m. This soil is classified as a Hydrologic Soil Group C. � To be consetvative in the detention pond calculations,the existing conditions for all azeas within the project were assumed to be undisturbed old growth forest with a Curve Number(Cf�of 76. The existing conditions time of concentration is 2.23 hours. The CSR Associated Aerial I Topographic Survey by the Nies Mapping Group, lnc.,dated November 1998, was used to determ'vie the flow rate through the FSA project site. See Figure 1. IDeveloped Conditions Ipgp will be improving the 37.4 acre subbasin of the former gravel pit into a product disvibution warehouse as previously mentioned. Stormwater runoff will be d'vected into a storm sewer � system which will convey the water to a detention pond with two open water cells and a wedand I treatment cell with an oudet control structure. 'i'he control structare, a type 2 catch basin, with thtee orifices will discharge the controlled runoff into an on-site 36-inch diameter storm sewer which will flow uito a proposed 36-inch diameter storm drain system in a proposed road. For the description of Ihe flow beyond this point, see the one quarter mile downstream inventory section in this report. ITo be conservative in the detention pond calculations for the developed conditi�ns, we assumed that 90 percent of the total area is to be impervious. The calculations for stormwater detention { i�iclude the build out for Phases 2 and 3. We used a Curve Number(CM of 98 for the impetvious � area and 86 for the pervious azea. The developed conditions time of concenvation is 0.20 hours ` for the impervious azea and 0.11 hours for the pervious azea. The developed conditions for , Phase 1 are shown on Figure 4. , See the attached reduced copies oE the Storm Drainage Plan,Detention Pond Sections and � Details, and Storm Drainage Details and Notes, Figums 4, 5, and 6,respectively. I � ,�� Fwd Scrvices uf America S�ormw��cr Site Plan a�d Starmwater Ddention Pond I Hydraulic Repc�: 4 ( p) v