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I <br /> 1 7/12/2013 <br /> RATIONAL METHOD <br /> I for Conveyance Facility Sizing <br /> (Based on 1998 King County Surface Water Design Manual Section 3.2.1) <br /> Project: Boeing E-6 Parking Lot Expansion <br /> IDescription: Rational method for conveyance pipe sizing <br /> Design Storm: 100 yr <br /> I 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 /> I 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 /> I C# <br /> Area <br /> Description Area(sf) (acres) C A*C <br /> 1 Onsite/New Impervious Surface 31,703 0.73 0.90 0.66 <br /> 2 Onsite/New Pervious Surface 0.00 0.25 0.00 <br /> Totals: 0.73 0.66 <br /> I Cc= 0.90 (total C#*A#)/(total area) <br /> Time of Concentration <br /> Travel <br /> Upper Lower Slope Time <br /> I Seg.# Description of Flow Path Segment Length(ft) kr Elev Elev (ft/ft) (minutes) <br /> 1 Paved Area(sheet flow)and shallow gutter flow 250 20 555 540 0.060 0.85 <br /> Totals: 250 0.9 <br /> I 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.E <br /> I 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 /> ir= 0.82 <br /> IPeak Rainfall Intensity <br /> Ir=Pr*ir <br /> where: Ir=peak rainfall intensity(inches/hour) <br /> I Pr=total 24-hour precipitation for design return period(inches/24 hours) <br /> ir=unit peak rainfall intensity factor <br /> Pr= 3.25 precipitation(inches) <br /> ir= 0.82 unit peak intensity factor(from above) <br /> I Ir= 2.66 inches/hour <br /> Peak Runoff Rate <br /> Q=C*Ir*A <br /> l <br /> C= 0.90 Cc(unitless)from above <br /> Ir= 2.66 Ir(inches/hour)from above <br /> A= 0.73 total area(acres)from above <br /> I Q= 1.743 cfs <br /> Pipe Capacity Calculations(Manning's Equation) <br /> Full Flow(d/D=0.90) <br /> Pipe <br /> I <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 entire <br /> site 12 0.7853982 3.141592654 0.25 0.011 0.02 7.60 5.971 1.743 Capacity OK <br /> I <br /> I Pipe Sizing_Rational Method_20120711.xls <br /> Calculations DCG,Inc. Page 1 of 1 <br />