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<br />3. CROSS SECTION SUBDIVISION <br /> <br />The determination of total conveyance and the velocity coefficient <br />for a cross section requires that flow be subdivided into units for which <br />the velocity is uniformly distributed. The approach used in HEC-2 is <br />to subdivide flow in the overbank areas using the input cross stations <br />(X-coordinates) as the basis for subdivision. Conveyance is calculated <br />within each subdivision by the equation (based on English units): <br /> <br />k = <br /> <br />1.486 a r2/3 <br />n <br /> <br />(4) <br /> <br />where: <br /> <br />k = conveyance for subdivision <br /> <br />n = Manning's 'n' for subdivision <br /> <br />a = flow area for subdivision <br /> <br />r = hydraulic radius for subdivision (area divided by wetted <br />perimeter) <br /> <br />Flow in the main channel is not subdivided in normal applications. The <br />total conveyance for the cross section is obtained by summing the incremental <br />conveyances. <br /> <br />The velocity coefficient, a, is obtained with the following <br />equation: <br /> <br />a = <br /> <br />()2 3 2 3 2 3 2 <br />At [(Klob) /(Alob) + (Kch) /(Ach) + (Krob' /(Arob) ] <br />(K )3 <br />t <br /> <br />(5) <br /> <br />where: <br />At = total flow area of cross section <br />Alob' ACh' Arob = flow areas of left overbank, main channel and <br /> right overbank, respectively <br />Kt = total conveyance of cross section <br />Klob' KCh' Krob = conveyances of left overbank, main channel and <br /> right overbank, respectively <br /> <br />5 <br />