Comparison of different methods to predict the mean flow velocity in step-pool channels

Steep mountain streams have irregular bed topography, where the mean flow velocity is heavily affected by the coarsest bed components and by their arrangement to form step pools, cascades, and rapids. According to literature findings the mean flow velocity is often related with water discharge, channel slope, and grain-size related variables through power relationships. Several approaches consider dimensionless hydraulic geometry terms to develop the analysis over a wide range of channel sizes and hydraulic conditions. The aim of this research is to test the performance of some literature formulas to directly compute the mean flow velocity (V) in step-pool sequences. The study area deals with two fish ladders located in the Vanoi torrent (Trento Province, Italy), which were built by mimicking the step-pool morphology. Three reaches were selected to cover different channel slopes (2.6-10%). Data collection entailed three main phases: (1) topographical surveys, (2) granulometric analysis, and (3) flow discharge measurements (salt dilution method). Geometric and hydraulic variables were measured for the following step-pool cross sections: step head, pool center, and intermediate position between pool end next step. Particular attention has been reserved to determine the effective mean flow velocity over the whole path of each step pool sequence. The performance of different literature equations to predict V has been verified. The relations have been shared in three groups: dimensional (V), dimensionless with respect to the grain size (V*) or to a combination of grain size and slope (V**). In general, the V group of equations has produced the highest errors between computed and measured values. The dimensionless V*, V** groups have shown the best performance. In particular the V* equations, which use unit discharge and channel slope, have provided the better fitting, and the lowest root mean square error. The results highlight the difficult to estimate flow velocity in step-pool sequences, and the attitude of this channel-bed morphology to be highly dissipative. The good performance of some dimensionless equations to predict V could also support the hydraulic designer in case the ‘morphological rebuilding’ of mountain creeks is opportune. Further analyses are required to better understand the flow behavior in streams where very rough bed forms and hydraulic drops are the primary sources of flow energy dissipation.