Effects of vegetation on channel morphodynamics: results and insights from laboratory experiments

Abstract

A series of laboratory experiments demonstrates that riparian vegetation can cause a braided channel to self-organize to, and maintain, a dynamic, single-thread channel. The initial condition for the experiments was steady-state braiding in noncohesive sand under uniform discharge. From here, an experiment consisted of repeated cycles alternating a short duration high fl ow with a long duration low fl ow, and uniform dispersal of alfalfa seeds over the bed at the end of each high fl ow. Plants established on freshly deposited bars and areas of braidplain that were unoccupied during low fl ow. The presence of the plants had the effect of progressively focusing the high fl ow so that a single dominant channel developed. The single-thread channel self-adjusted to carry the high fl ow. Vegetation also slowed the rate of bank erosion. Matching of deposition along the point bar with erosion along the outer bend enabled the channel to develop sinuosity and migrate laterally while suppressing channel splitting and the creation of new channel width. The experimental channels spontaneously reproduced many of the mechanisms by which natural meandering channels migrate and maintain a single dominant channel, in particular bend growth and channel cutoff. In contrast with the braided system, where channel switching is a nearly continuous process, vegetation maintained a coherent channel until wholesale diversion of fl ow via cutoff and/or avulsion occurred, by which point the previous channel tended to be highly unfavorable for fl ow. Thus vegetation discouraged the coexistence of multiple channels. Varying discharge was key to allowing expression of feedbacks between the plants and the fl ow and promoting the transition from braiding to a single-thread channel that was then dynamically maintained.

Geometry of meandering and braided gravel-bed threads from the Bayanbulak Grassland, Tianshan, P. R. China

“…More specifically, field observations suggest that a heavier sediment load tends to increase the aspect ratio of a thread, other things being equal (Smith and Smith, 1984;Tal and Paola, 2010;Métivier and Barrier, 2012). This proposition remains speculative though, and needs to be thoroughly tested against dedicated field measurements, which we believe should include both braided and meandering threads.…”

Analysis of in situ water velocity distributions in the lowland river floodplain covered by grassland and reed marsh habitats - a case study of the bypass channel of Warta River (Western Poland)

“…Klaassen and Van der Zwaard, 1974). These include theoretical principles of the mathematical modelling of the phenomenon (Baptist et al, 2007;Horritt, 2006;Klopstra et al, 1997;Wu et al, 2001;Yang and Choi, 2010), numerical modelling with models of varying complexity (Crosato and Saleh, 2011;Perona et al, 2009;Velasco et al, 2008), flume experiments to verify models of vegetation roughness and in situ floodplain roughness estimations (Meijer and van Velzen, 1999;Murphy et al, 2007;Straatsma, 2009;Tal and Paola, 2010). Some studies focused on the accumulation of both thick and fine debris (Mazur et al, 2016) carried along with the flood wave.…”

Riparian proper functioning condition assessment to improve watershed management for water quality

“…Meanders create a low velocity zone adjacent to point bars that form through deposition of bedload and later suspended sediment. Vegetation growing on deposited coarse bedload material increases stability and roughness, decreases flow velocities, increases deposition of finer suspended sediment (figures 2e and 3b to 3g) (Robertson and Augspurger 1999;Steiger and Gurnell 2003;Rood et al 2003;Robertson 2006;Polzin and Rood 2006), and encourages meander development (Erskine et al 2009(Erskine et al , 2012Rominger et al 2010;Tal and Paola 2010), which decreases stream gradient and power (Bagnold 1966). Deposition of fine alluvial soil facilitates revegetation while it removes water pollution.…”