Hydraulics, morphology, and energy dissipation in an alpine step-pool channel

To investigate the relationship between hydraulics and channel morphology in step-pool channels, we combined three-dimensional velocity measurements with an acoustic Doppler velocimeter and topographic surveys in a steep step-pool channel, the Rio Cordon, Italy. Measurements were organized around step, pool, and tread units and occurred within a range of 36%-57% of bankfull discharges. As flow moved from steps to their downstream pools in our study reach, an average of approximately two thirds of the total energy was dissipated, as measured by relative head loss through step-pool sequences. Much of this head loss was achieved by elevation (potential energy) loss rather than velocity reductions. Although an overall, expected pattern of flow acceleration toward step crests and deceleration in pools was present, pool velocities were high, especially where upstream step crests were irregular and where residual pool depths were low. Many steps were porous or "leaky," with irregular cross-channel bed and water surface topography, producing high-velocity jets and less flow resistance than channel-spanning dammed steps. Longitudinal variations in hydraulics are thus often overshadowed by lateral variations arising from morphologic complexities. Velocity and turbulence characteristics in the Rio Cordon show marked differences from data we have collected in a more stable and wood-rich channel in the Colorado Rockies, in which "ponded" steps are more prevalent and pools are slower and more turbulent. Comparison of these channels illustrates that step-pool structure and hydraulics are strongly influenced by flow regime, sediment supply, lithology, time since the last step-forming flood, and availability of in-stream wood.