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Tvoe ot Cover <br />TABLE 2-33 <br />Grand Cova Coelfieients <br />Forcst with heavy ground cover <br />Minimum tillage cultivation <br />Short pasturc grass or lawn • <br />Nearly barc ground <br />rasud waterway or small roadside ditch <br />� <br />Gutter flow 0.23 feet deep <br />0.30 feetdeep <br />0.75 feetdeep <br />�und crner eoeifieient) <br />1S0 <br />280 <br />420 <br />600 <br />900 <br />� f <br />I�300 <br />2�400 <br />3,lOG <br />• Concrete CNP <br />n=0.012 n=0.024 <br />Storm Sewers (Concrcte) 12 inch diameter 3,000 1,300 <br />l8 inch diameter 3,900 1,930 <br />24 inch dlameter §�700 2�350 <br />Open Channel Flow (n = 0.040) <br />2-3.k Rainfall tntmsitr <br />A. Rainfall Intensity Charts <br />1 foot deep <br />2 feet deep <br />4 feet deep <br />Narrow Medium Wide <br />WD=1 WD=2 WD=9 <br />1,100 1,500 2,000 <br />I,S00 2,300 3,I00 <br />2,E00 3,700 5,000 <br />When the time of concentration and storm frequency are known, the rainfall intensity is obtained from the rainfalt <br />intensity charts at the end of Ltiis chapter (Appendix 2-1). Each chart shows the intensity of Me rainfall for 100-, <br />50-, 25-, 10-, and 3-year storm frequencies for a duratian of 5 to 100 m[nutes It is the practice of the WSDOT <br />Headquarters Hydreulics Section to use a minimum time of concentration of tive minutes. It ahould be nomd that the <br />rainfall tntensity at any g[ven time 'u the averege from zero minutes ro the time considered and is not the <br />instantaneous rainfall. <br />� �, 6. Rainiall Intensity Equation <br />In addition to the raintall intensity charts, the designer also has Me option of dei�rmining rainlal: data by use of a <br />mathematica! equation. This equation has the form: � <br />��r','� <br />' .-r'a <br />where: m and n= dimensionless ccefficients <br />T = the rainfall duration (minutes) <br />i= the average rainfall intensity (inches per hourl <br />MRI = mean returrence interval <br />Z _ 7 May 19E9 <br />+ �. <br />