Hydraulic geometry of stream reaches

Variation in river hydraulic geometry throughout stream networks is central to the problems of catchment management because of its influence on flow and sediment routing, physical habitats and channel-floodplain interactions. In its simplest form, at-a-station hydraulic geometry is described by power functions relating the surface width, water depth and mean velocity at a channel cross-section to discharge. This paper extends hydraulic geometry analysis to represent the mean and variation of hydraulic characteristics along a river reach as power functions of discharge. Using 54 surveys of 17 river reaches, mostly in south east Australia, it is shown that five reach hydraulic variables are sufficient to characterise the gross hydraulic conditions along a river reach: the means and coefficients of variation of surface width and hydraulic depth plus the coefficient of variation of cross-sectional velocity. Exponents for the mean width and mean depth hydraulic geometry relations are sensitive to measurement and cross-section location and may best be treated as constant for regional studies. Coefficients for the mean width and mean depth hydraulic geometry relations are correlated positively with mean discharge and negatively with reach gradient, respectively. Further analysis of variations in reach hydraulic geometry parameters, particularly the power function coefficients in relation to river type, flow parameters, sediment load, bank material and bank vegetation is warranted. Reach hydraulic geometry relations show considerable potential for improving analyses of fluvial processes and physical habitats across catchments.