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Preliminary Stormwater Treatment Flow Rate Design <br />Estimated Stormwater Volumes Generated Based on Landuse and Storm Events <br />Client Name Bayley <br />Client Contact Stewart Potter <br />Project Name Everett Outdoor Events Center <br />Property Profile Acres Sq. Ft Proposal Type Stormwater Treatment <br />Disturbed Area 5.00 217,800 (area of excavation)Issue Date 10/27/2025 <br />Proposal Version 1.0 <br />Soil Profile Curve #% of site Area Product <br />Impervious 0.98 40%87,120 85,378 Discharge Information <br />Open Dirt 0.6 60%130,680 78,408 Receiving Water(s) <br />217,800 163,786 Proximity to Receiving Water <br />TMDL Waterbody/Special Monitoring Required?TBD <br />Soil Type silt <br />Water Quality Objectives <br />Composite CN 0.75 (Indicate the results being requested) <br />(aka Run-off Curve #) <br />Design; Precipitation Rates Inches Ft cf Gallons Permitting <br />"Significant Rain Event"0.25 0.02 3,412 25,523 NPDES Permit Required?TBD <br />"Good Rain"1.00 0.08 13,649 102,093 SWPPP Prepared?TBD <br /> 2 Year - 24-hour Storm 1.98 0.17 27,025 202,144 Permission to discharge to SS TBD <br />x 10 Year - 24-hour Storm 2.95 0.25 40,264 301,174 Permitted to Discharge to Storm Drain?TBD <br />100 Year - 24-hour storm 5.00 0.42 68,244 510,465 <br />Click here for Isopluvial Map pdf <br />Detention Volume Qty Volume (gallons) <br />Detention Tank 2 36000 <br />Ponds 0 0 <br />Total Detention 36000 <br />Treatment System Sizing Gallons System Type Hrs/Day <br />On-Site Detention 36,000 Shift 12 300GPM capacity quoted <br /> 2 Year - 24-hour Storm 202,144 <br />Difference 166,144 <br />Treatment System Sizing Gallons System Type Hrs/Day <br />On-Site Detention 36,000 Shift 12 300GPM capacity quoted <br /> 10 Year - 24-hour Storm 301,174 <br />Difference 265,174 <br /> <br />Site Stormwater Volume Summary for Consumable Estimation <br />Month Inches Site Volume <br />Jan 5.63 574,784 <br />Feb 4.09 417,560 <br />Mar 3.95 403,267 <br />Apr 2.66 271,567 <br />May 1.79 182,747 <br />Jun 1.49 152,119 <br />Jul 0.71 72,486 <br />Aug 1.05 107,198 <br />Sept 1.82 185,809 <br />Oct 3.73 380,807 <br />Nov 6.09 621,747 <br />Dec 5.81 593,160 <br />Annual 38.8 3,963,251.2 <br />10yr-24 hr storm event <br />Western Regional Climate Center - Summary Data 2yr - 24hr storm event <br />Table shows extreme rain event volumes (blue) over last 70 years, lines represent <br />storm volumes used in sizing calculations <br />369 <br />Average Monthly Rainfall - Seattle <br />Estimated Monthly <br />Stormwater <br />Volume (gallons) <br />231 <br />Stormwater Only <br />System Size <br />System Size <br />Progressive Design-Build Contract for Everett Outdoor Event Center <br />Exhibit to Amendment No. 1 - Phase 1B Work and Fee Proposal