Sediment pulses in mountain rivers: 1. Experiments

Abstract

Sediment often enters rivers in discrete pulses associated with landslides and debris flows. This is particularly so in the case of mountain streams. The topographic disturbance created on the bed of a stream by a single pulse must be gradually eliminated if the river is to maintain its morphological integrity. Two mechanisms for elimination have been identified: translation and dispersion. According to the first of these, the topographic high translates downstream. According to the second of these, it gradually diffuses away. In any given river both mechanisms may operate. This paper is devoted to a description of three controlled experiments on sediment pulses designed to model conditions in mountain streams. Each of the experiments began from the same mobile‐bed equilibrium with a set rate and grain size distribution of sediment feed. In one experiment the median size of the pulse material was nearly identical to that of the feed sediment. In the other two the pulse material differed in grain size distribution from the feed sediment, being coarser in one case and finer in the other case. In all cases the mode of pulse deformation was found to be predominantly dispersive, a result that constitutes the main conclusion of this paper. The pulses resulted in a notable but transient elevation of sediment transport rate immediately downstream. When the pulse was coarser than the ambient sediment, the bed downstream remained armored, and a migrating delta formed in the backwater upstream. When the pulse was finer than the ambient sediment, translation was observed in addition to dispersion. The presence of the finer material notably elevated the transport of ambient coarse material on the bed downstream. In part 2 [Cui et al., 2003], the experimental results are used to test a numerical model.

The role of landslides in mountain range evolution

“…by debris flows (Benda et al, 2003), landslides add noise to ideally concave river longitudinal profiles (Eq. (1)), thus affecting their use for isolating specific external forcing on profile development (Sklar and Dietrich, 1998;Schlunegger, 2002;Cui et al, 2003;Korup et al, 2004). Catastrophic long-runout rock or debris avalanches may completely obliterate valley floors by scouring and subsequently covering them with tens of metres of debris for lengths >10 km, thus interrupting and slowing down fluvial bedrock incision (e.g.…”

“…Therefore, changes in base level at the basin outlet, e.g. may not be communicated to all parts of the basin until the landslide dam is removed and any accumulated sediment is dispersed downstream (Cui et al, 2003).…”

Formation and evacuation of a large gravel-bar deposited during a major flood in a Mediterranean ephemeral stream, Nahal Me'arot, NW Israel

“…Depositions of fine sediment and consequent vertical accretion in natural pools were described by Lisle and Hilton (1999), and following artificial disturbances in low-gradient reaches by Madej and Ozaki (1996) and Wohl and Cenderelli (2000). Bedload waves, sediment pulsation and sediment slugs were described both in laboratory flumes (Iseya and Ikeda, 1987;Lisle et al, 1997;Cui et al, 2003) and in mountainous gravel-bed rivers following extreme precipitation (Maita, 1991;Marutani et al, 1999;Lisle et al, 2001;Sutherland et al, 2002;Kasai et al, 2004), and also in hyperarid desert ephemerals (Lekach and Schick, 1983).…”

Influence of consecutive sediment replenishment on channel bed morphology

“…The channel bed is fixed, thus accounting for bed armouring, and composed of gravels with a mean grain size diameter of 11.5 mm (see also Battisacco et al 2015). Overall the experimental flume properties, such as slope, bed grain size and Froude number are representative for an alpine gravel channel (Parker et al 2002, Hersberger 2002& Cui et al 2003) and the applied scale of 1:10 can be considered as adequate for bed morphology modelling of mountain rivers (Weichert 2006in Heller 2011.…”