The determination of total conveyance and the velocity coefficient for a cross section requires that flow be subdivided into units for which the velocity is uniformly distributed. The approach used in HECRAS is to subdivide flow in the overbank areas using the input cross section n-value break points (locations where n-values change) as the basis for subdivision (see figure below). Conveyance is calculated within each subdivision from the following form of Manning's equation (based on English units):

 (1)
 (2)
SymbolDescriptionUnits

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conveyance for subdivision

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Manning's roughness coefficient for subdivision

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flow area for subdivision

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hydraulic radius for subdivision (area / wetted perimeter)

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