On the spatial variation of resistance to flow in upland channel networks

Detailed field measurements of channel properties and flow characteristics collectedin the Ashley and Cropp catchments (New Zealand) are used to investigate the spatialvariation of resistance to flow across upland channel networks. The application of theDarcy-Weisbach equation and semilogarithmic flow resistance relationships reveals thatmean flow velocities calculated from local measurements of bed material particle size,hydraulic depth, and channel bed slope may be inaccurate. The Manning-Gauckler-Strickler equation with resistance coefficient independent of bed material particle size isfound to be relatively more reliable but not sufficiently general to reproduce the spatialvariation of resistance to flow across a complex channel network. A new methodology isdeveloped by combining a hydraulic equation of the Manning-Gauckler-Strickler type, aflow discharge-upstream drainage area relationship, and geomorphological fluvialrelationships for mean flow velocity, Gauckler-Strickler resistance coefficient, hydraulicdepth, and friction slope. This methodology is found to improve the reproduction of thespatial variation of mean flow velocity across the Ashley catchment and appears of generalapplicability for the parameterization of resistance to flow in distributed catchmentmodels.