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<br />~~ <br />4 <br />2 ~" <br />HYDRAULIC CALCULATIONS <br />Permanent Ditches <br />~~~1, ~~ <br />li" <br />`~~_ r <br />1~v U <br />Peak flows resulting from the 10-year, 24-hour storm event were used to design <br />all permanent diversion ditches. Manning's Equation for open channel flow was <br />used to size the ditches as follows: <br />O = (1.486/n)AR'~'S'~= ~~ <br />Where: O =peak discharge from 10-year, 24-hour event (cfs) <br />n = Manning's roughness coefficient (0.035) <br />A =cross sectional area of channel flow (ft~ <br />R =hydraulic radius (ft) <br />S =channel slope (ft/ft) <br />The Manning's roughness coefficient of 0.035 represents an earthen channel ~~IL <br />with some weeds and stones (Ref: Barfield et. al., 1981, page 159). <br />For channel sections with flow velocities exceeding 5.D feet per second, rock or= <br />riprap will be provided. The following equation is used to determine riprap size. <br />,~~ <br />D70 = 12(118OS21(R/P))°~' <br />Where: D„ = Riprap mean particle size (inches) <br />O =Peak discharge from 10-year, 24-hour event (cfs) <br />~ ~ ~ S =Channel slope (ft/ft) <br />~ R =Hydraulic radius (ft) ,. a ~ <br />V P =Wetted perimeter (ft) ALP ''~.`~,~ <br />~ J~ 2 <br />rti~~~J '~~~lti~ <br />~•f The diameter of the largest stone size should be 1.5 times the D„ size. The <br />C~,/ minimum thickness of the riprap laayer will be 1.5 times the maximum stone size, <br />//'S~~wnyJ S~~ CIG.~~ ~ :~r, r~'i ~. 7.f ,'F/SO ~i:/ <br />but not less than six inches. , b <br />zY ~.;0 S. ~ .. ~ ai <br />~~3 . <br />~` ~ 7 <br />~~i,k _ . ~~ .i <br />'~ <br />`~ ~ <br />IlJ <br />~ '!,) <br />13 <br />