Laboratory modelling of gravel braided stream morphology

Abstract

Hydraulic modelling principles, together with a knowledge of channel pattern thresholds, allow the development of a small scale model of a gravel braided stream with flow characteristics and equivalent dimensions of a natural river. The forms and processes of natural gravel braided rivers are reproduced by imposing a constant flume discharge and slope, and maintaining approximate equilibrium with an adjustable sediment feed.Beginning from a straight trough, braiding is initiated by development of a series of alternating bars and scour pools which produce bends of increasing amplitude, leading finally to channel division. These lobate bars accrete downstream by deposition of bed material at their margins, often in the form of avalanche faces. Together with the scour pools with which they are necessarily closely associated, these bars are the fundamental elements of the channel pattern. Channel migration and division is a response to the development of bars, and these adjustments leave portions of the originally active bars in the form of exposed and eroded remnants. Complex flats built from these lobate forms show varying degrees of preservation of the original depositional units, but the model allows observation of the systematic construction of some flats. Sorting of sediment on active bars with avalanche faces shows a distinct fining downstream. This may be the result of the accretion of fining upwards avalanche faces along the bar margins rather than a 'winnowing out' of fine material.The processes and forms observed in the model appear to be very similar to those occurring in natural gravel braided streams during peak flows.

Bed disturbance via foraging fish increases bedload transport during subsequent high flows and is controlled by fish size and species

“…Scaling up experimental results in time and space to natural situations in rivers is a key challenge in freshwater ecology (e.g., Peckarsky et al, 1997, Peckarsky, 1998, in fluvial geomorphology (e.g., Ashmore, 1982;Paola et al, 2009), and in ecogeomorphological studies (e.g., Rice et al, 2010;Johnson et al, 2011, their Figure 7b). However, it is reasonable to anticipate that what was observed in the flume is, at some level, representative of foraging effects in the field; and there are several good reasons to hypothesise that benthic feeding is a prolific and effective bed disturbance mechanism in rivers.…”

A theoretical analysis of river bars stability under changing channel width

“…Repetto et al, 2002) have investigated the influence of spatial planform variability on bars formation showing how spatial changes in channel width, influencing river bars, may produce planform instability and a related tendency to braid. The role of width unsteadiness may become relevant especially in laterally unconfined channels with non-cohesive banks, as it has been observed in laboratory experiments on the initiation of braided and of "pseudo-meandering" streams (Ashmore, 1982(Ashmore, , 1991Bertoldi and Tubino, 2005;Visconti et al, 2010). Evidence of this dynamics has been provided, also, by field observation on an artificially re-shaped natural river consequentially to a series of flood events (Lewin, 1976); this highlights the mutual influence between planform and bar instability in streams where the evolution of bed and banks occur at comparable time scales.…”

“…Note that this assumption should be less crude than it may appear, because we are interested here more in a simple mathematical law describing the trends and order of magnitude of width unsteadiness rather than in a predictive formnula quantitatively valid for a specific case. To this aim we use the empirical formula proposed by Ashmore (1982) to predict the width at river equilibrium stage for anabranch channels in braided rivers:…”

Modeling river delta formation

“…Field studies have been carried out for the Mississippi River delta (7)(8)(9)(10), the Niger River delta (11)(12)(13), and the Brahmaputra River delta (14). Laboratory experiments have also been set up in the last decades for quantitative measurements (15)(16)(17)(18)(19). For instance, in the eXperimental EarthScape (XES) project, the formation of river deltas is studied on laboratory scale, and different measurements have been carried out (20)(21)(22).…”