The concept of travelling bedload and its consequences for bedload computation in mountain streams

Abstract

In bedload transport modelling, it is usually presumed that transported material is fed by the bed itself. This may not be true in some mountain streams where the bed can be very coarse and immobile for the majority of common floods, whereas a finer material, supplied by bed‐external sources, is efficiently transported during floods, with marginal morphological activities. This transport mode was introduced in an earlier paper as ‘travelling bedload’. It could be considered an extension of the washload concept of suspension, applied to bedload transport in high‐energy, heavily armoured streams. Since this fine material is poorly represented in the bed surface, standard surface‐based approaches are likely to strongly underestimate the true transport in such streams. This paper proposes a simple method to account for travelling bedload in bedload transport estimations. The method is tested on published datasets and on a typical Alpine stream, the Roize (Voreppe, France). The results, particularly on active streams that experience greater transport than expected from the grain sizes of their bed material, reinforce the necessity of accounting for the ‘travelling bedload concept’ in bedload computation. The method relevance is discussed regarding varying flood magnitudes, geomorphic responses and eventual anthropic origin of the ‘travelling bedload’ phenomena. To conclude, this paper considers how to compute bedload transport for a wide range of situations, ranging from sediment‐starved cases to the general mobile bed alluvial case, including the intermediate situation of external source supply on armoured bed. Copyright © 2017 John Wiley & Sons, Ltd.

Particle transport in gravel‐bed rivers: Revisiting passive tracer data

“…The structured bedload may define the scale of roughness elements in SP channels and influence flow resistance, while the travelling bedload may relate to the supply-limited and more mobile sediment introduced sporadically into the channel. Very recently, Piton et al (2016) and Piton and Recking (2017) built on this concept of travelling bedload, which they explicitly state as an extension of the washing load concept to bedload transport. According to these authors, in steep SP systems, relatively fine sediment supplied by external sources may be efficiently transported during floods, with marginal morphological activity and without the breaking up of coarse armoured surfaces (Egashira and Ashida, 1991;Piton and Recking, 2017).…”

Breaking from the average: Why large grains matter in gravel‐bed streams

“…We believe that the stability of a bed is related to the fraction of the bed area covered by immobile grains, and that these grains create a framework over which bed material sediment transport may occur without significantly altering the channel morphology, similar to what has been observed in steep step‐pool channels (e.g., Phillips, ; Lenzi et al. ; Piton and Recking, ). Both immobile size fractions and partially mobile size fractions contribute to stability – a nuance that is lost when we simply focus our attention on a single grain size.…”

Steep Bedload‐Laden Flows: Near Critical?

“…This correlation, still described in many papers notably in Grant () or Schneider et al (), but here accurately measured on a large data set, is the consequence of coupled hydraulic and geomorphic features of high gradient channels. Boulders and cobbles, supplied by colluvial processes and debris flows, greatly increase channel roughness (Piton & Recking, ). Autogenic arrangements in cascade and step pool morphologies increase energy dissipation even more since such bedforms are closely coupled and self‐adjusted, with energy dissipating hydraulic processes such as hydraulic jumps and tumbling flows (Zimmermann et al, ).…”