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7/31/2013 <br /> RATIONAL METHOD <br /> for Conveyance Facility Sizing <br /> (Based on 1998 King County Surface Water Design Manual Section 3.2.1) <br /> Project: Boeing Gate E-70 Parking Lot <br /> Description: South Subbasin-Rational method for conveyance pipe sizing <br /> Design Storm: 100 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 12,035 0.28 0.90 0.25 <br /> 2 Onsite/New Pervious Surface 0.00 0.25 0.00 <br /> Totals: 0.28 0.25 <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 330 20 553.4 550 0.010 2.71 <br /> Totals: 330 2.7 <br /> Unity Peak Intensity Factor <br /> ir=ar*Tc^-br <br /> where: Tc=time of concentration(minutes) <br /> ar and br=coefficients from Table 3.2.1.B <br /> Tc= 6.30 minutes(from table above or 6.3 minimum or 100 max) <br /> ar= 2.61 (from Table 3.2.1.6) <br /> br= 0.63 (from Table 3.2.1.6) <br /> it= 0.82 <br /> Peak Rainfall Intensity <br /> Ir=Pr*ir <br /> where: Ir=peak rainfall intensity(inches/hour) <br /> Pr=total 24-hour precipitation for design return period(inches/24 hours) <br /> ir=unit peak rainfall intensity factor I c,"1- <br /> Pr <br /> Pr= 3.25 precipitation(inches) <br /> it= 0.82 unit peak intensity factor(from above) <br /> Ir= 2.66 inches/hour <br /> Peak Runoff Rate <br /> Q=C*Ir*A <br /> C= 0.90 Cc(unitless)from above <br /> Ir= 2.66 Ir(inches/hour)from above <br /> A= 0.28 total area(acres)from above <br /> Q= 0.662 cfs <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 /> 10"for south <br /> tributary area 10 0.5454154 2.617993878 0.20833333 0.011 0.02 6.73 3.672 0.662 Capacity OK <br /> South Tributary Area_Pipe Sizing_Rational Method_20120711.xIs <br /> Calculations DCG,Inc. Page 1 of 1 <br />