Effective discharge for sediment transport in a mountain river: computational approaches and geomorphic effectiveness

Dominant, effective, bankfull and channel-forming discharges are different concept-based flows, often applied as design parameters in river management and restoration. In order to achieve a better understanding of channel-forming conditions in high-gradient, boulder-bed streams, the long-term sediment loads data obtained from the Rio Cordon (Italian Alps) measuring station have been analysed. The effective discharge (Qe, calculated using both Wolman and Miller’s method and the so-called ‘mean’ approach) for bedload transport proves to be more appropriate than that determined for the suspended sediment load in describing the channel formation and maintenance for this type of channels. The analysis demonstrates that Qe is strongly influenced by the number of flow classes, the fraction of transported sediments and the methodology used in its computation. The result questions the appropriateness in considering Qe as an unique value, and also suggests the possible definition of two dominant discharge ranges for steep mountain rivers: (a) a relatively frequent flow range responsible for maintaining channel form; and (b) a more infrequent high flow range responsible for macro-scale channel shaping.