A Comparison of At-A-Station Hydraulic Geometry for Step-pool Channels

This study compares the at-a-station hydraulic geometry for several step-pool channels. While past studies have compared at-a-station hydraulic geometry for many different channel types, to my knowledge none have included sites with step-pool morphology. For this study, hydraulic geometries are compared using ternary plots of the b, f, and m exponents for the width, depth, and velocity power equations, respectively. Data from step-pool channels in several widely separated locations are used to compare exponent characteristics. Similar to most other channels types, depth and velocity exponents are generally much greater than the width exponent. In contrast to other channel types, the velocity exponent is usually greater than depth, indicating that velocity changes faster than depth as flow increases. No clear relationship is apparent between bed material caliber and the size or relative magnitudes of the exponents. It has been suggested that the wide variation in at-a-station hydraulic geometries observed in past studies might be reduced by only comparing channels that occur in similar geomorphic settings (such as step-pool channels). However, as shown here, the variability in exponent values for channel width, depth, and velocity is still large; indicating that channel response to flow changes in step-pool channels cannot be predicted simply by knowledge of gross morphologic form.