Laserfiche WebLink
�� _ � K)( s��.�— <br />TABLE 2-33 <br />C�01�IA COYK COCL(�CIMLS <br />Tvve ot Cover _ <br />Forest with heavy ground cover <br />Minimum tillage cultivation <br />Short pasturc grass or lawn � <br />Ncarly bare grm��d <br />Grassed waterway or smail roadside ditch <br />PavN �rc� <br />Gutter [low 0.23 feet deep <br />0.50 feetdeep <br />0.75 [eetdeep <br />�_(��pund cover coetficientl <br />ISO <br />280 <br />420 <br />600 <br />900 <br />1�200 <br />1,500 <br />2,400 <br />3,100 <br />Concrete <br />n = OA t 2) <br />Storm Sewers (Concretel l2 inch diameter 3,000 <br />IS inch diameter 3,900 <br />24 inch diameter 4,700 <br />Opcn Channel Flow (n = 0.040) <br />2-3.4 Raintall intensiri <br />.4. Naintall Intensi*y Charts <br />I foot �eep <br />2 feet deep <br />4 feet deep <br />CNP <br />(n = 0.024) <br />f�500 <br />1,950 <br />2�J50 <br />Narrow Medium Wide <br />WD=I WD=2 W%p=9 <br />I,100 f,500 2,000 <br />1�800 2,700 :,100 <br />2,SOC 3,700 5,000 <br />Wlmn the time of concentration and storm frequency are kr.own, the rainfall intensity Is obtained from the reinfall <br />intensity charts at the end ot this chapter (Appendix 2-1). Each chart shows the intensity of the rain[all tor 100-, <br />50-, 25-, 10-, and 5_year storm frequencies for a duration o[ 5 to 100 minutes. It is the practice of the WSDOT <br />Iieadquarters Hydraulics Section to use a minimum time of concentretion of live minutes. It �ould Ae noted that the <br />rainfa:: intensi:y at any given time is the average from zero minutes to the time considered and is not tlie <br />instantaneous raintall. <br />� � D. Rainfall Inten:ity Equation <br />In additien to the rainfall intensity cF.arSs� the de;igner aiso has the ontion of determining raintall data by use of a <br />mathematical equation. This equation has the forrn: <br />� _ m <br />T^ <br />. <br />where: m and n= dimensienless cor.fficients <br />7' = the rainfall duration (minutes) <br />f= the averege rainfall intensity (incha per hour) <br />M1121 - mean returrence interval <br />2-7 <br />May I°E9 <br />