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9/30/2013 <br /> RATIONAL METHOD <br /> for Conveyance Facility Sizing <br /> (Based on City of Everett Stormwater Management Manual Section 5.3) <br /> Project: Boeing Gate E-70 Parking Lot <br /> Description: Rational method for 12"existing and proposed conveyance pipe sizing <br /> Design Storm: 25 yr <br /> Q=CIA <br /> Where: Q=peak flow(cfs) I=peak rainfall intensity(inches/hour) <br /> C=estimated composite runoff coefficient A=drainage subbasin area(acres) <br /> Composite Runoff Coefficient <br /> Cc=(C1*A1+C2*A2...)/At <br /> Where: Cc=composite runoff coefficient A#=area of land cover(acres) <br /> C#=runoff coefficient for Area# At=total area(acres) <br /> Area <br /> C# Description Area(sf) (acres) C A"C <br /> 1 Onsite/New Impervious Surface 99,743 2.29 0.90 2.06 <br /> 2 Onsite/New Pervious Surface 0.00 0.25 0.00 <br /> Totals: 2.29 2.06 <br /> Cc= 0.90 (total C#*A#)/(total area) <br /> Time of Concentration <br /> Travel <br /> Upper Lower Slope Time <br /> Seg.# Description of Flow Path Segment Length(ft) kr Elev Elev (ft/ft) (minutes) <br /> 1 Paved Area(sheet flow)and shallow gutter flow 775 20 533.59 528.6 0.006 8.05 <br /> Totals: 775 8.0 <br /> Unity Peak Intensity Factor <br /> ir=m*Tc^-n <br /> where: Tc=time of concentration(minutes) <br /> m and n=coefficients from Table 5.2 j <br /> Tc= 8.05 minutes(from table above or 6.3 minimum or 100 max) L • <br /> m= 7.83 (from Table 5.2) <br /> n= 0.582 (from Table 5.2) <br /> ir= 2.33 <br /> Peak Runoff Rate <br /> Q=C*ir*A <br /> C= 0.90 Cc(unitless)from above <br /> ir= 2.33 Ir(inches/hour)from above <br /> A= 2.29 total area(acres)from above <br /> Q= .79 cfs Ci <br /> Pipe Capacity Calculations(Manning's Equation) <br /> Full Flow(d/D=0.90) <br /> Pipe <br /> Wetted Per. Hyd. Manning's Slope Velocity Capacity Req'd Flow <br /> Description ID(inches) Area(sf) (ft) Radius(ft) n (ft/ft) (ft/s) (cfs) (cfs) <br /> 12"for new parking <br /> lot and e isting <br /> outfall under road 12 0.7853982 3.141592654 0.25 0 013 0.02 6.43 5.052 .79 Capacity OK <br /> 4 E`--r V <br /> Central Tributary Area_Pipe Sizing_Rational Method_20130926.xls <br /> Calculations DCG,Inc. Page 1 of 1 <br />