How do gravel-bed rivers braid?

Abstract

Sedimentary processes and bed forms leading to the onset of braiding were observed in small-scale hydraulic models of gavel-bed streams. The laboratory streams had a variety of combinations of (constant) discharge and slope but identical bed-material particle-size distrihutions. From initially straight channels, braiding occurred by four different processes: deposition and accurnuIation ol'a central bar, chute cutoff of point bars, conversion of single transverse unit bars to mid-channel braid bars, and dissection of multiple bars. In these experiments the chute cutoff mechanism was the most common, but the predominant braiding mechanism depends upon sediment mobility (excess bed shear stress) and the bed-form regime. At very low excess bed shear stress the central bar process dominates, but at higher excess bed shear stress slip-face unit bars are more common, bed scour at confluences is more pronounced, and propogation of alternate convergence (scour) and divergence (deposition) is more likely; thus chute cutoffs and bar conversion dominate. The multiple bar mechanism is restricted to channels with very high widthldepth ratio. All of these processes, along with avulsion, are significant for maintenance of an established braided channel.The direct physical sedimentary cause of primary braiding is essentially the same in all these processes: local aggradation (often by stalling of bed-load sheets) and loss of competence in a lateral flow expansion. The chute cutoff process occurs in a morphologically distinctive setting and may be aided by other factors, but it is usually triggered by the local thalweg shoaling that is the fundamental physical mechanism causing the onset of braiding by the other processes. Local short-term pulses in bed-load supply are often the trigger for the initiation and maintenance of braiding, regardless of the exact braiding process.Les processus sedimentaires et les formes de litage qui sont associCs au dCveloppement d'un rCseau anastornos6 ont Ct C observks par modClisation hydraulique de fdets d'eau sur lit de gavier, une khelle rCduite. Les filets d'eau utilisds en laboratoire combinaient diverses conditions de debit (constant) et de pente, cependant les distributions granulomCtriques du mathiel du lit de sCdiment Ctaient toujours les memes. Quatre processus ont modifiC les chenaux rectilignes initiaux et ont contribuC 2 crCer un rkseau anastornos6 : dCpBt et accumulation d'une barre centrale, recoupement transversal des lobes de mkandre, transformation de barres transversales individuelles en barres anastomosCes B chenaux multiples, et dissection des barres multiples. Le processus le plus souvent observC dans cette expkrimentation fut le recoupement transversal des lobes mkandriques, toutefois le mCcanisme dominant dans la formation du rCseau anastornos6 dCpend de la mobilitk du sediment (force supCrieure au seuil de cisaillement du lit) et du rCgime des formes de litage. Le processus de formation des barres centrales pridomine en prksence d'une trks faible force exctdante au seuil ...

Network response to disturbances in large sand-bed braided rivers

“…Hence, a single disturbance in a large braiding river could affect an entire reach, beyond the extent of individual engineering projects. The bar and branch dynamics of braided rivers have been studied by means of flume experiments (e.g., Fujita, 1989;Ashmore, 1991a), numerical modeling (e.g., Nicholas, 2013;Schuurman et al, 2013;Yang et al, 2014), and field observations (e.g., Bristow, 1987;Klaassen and Masselink, 1992;Klaassen et al, 1993;Ashworth et al, 2000;Best et al, 2003). However, in these studies, any artificial constraints and disturbances such as non-erodible (flume) walls, engineering works (Fig.…”

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.…”

A genetic classification of floodplains

“…Knighton, 1984;Ashmore, 1991), the cause of the other multi channelled river state, anastomosis, has never been adequately explained. It is proposed here that anastomosis may be the product of an abundant supply of water over low-gradient unconfined and relatively cohesive (or well-vegetated) floodplains.…”