Numerical study on the effects of floodplain vegetation on river planform style

Abstract

The effects of floodplain vegetation on river planform have been investigated for a medium-sized river using a 2D morphodynamic model with submodels for flow resistance and plant colonization. The flow resistance was divided into a resistance exerted by the soil and a resistance exerted by the plants. In this way it was possible to reproduce both the decrease in bed shear stress, reducing the sediment transport capacity of the flow within the plants, and the increase in hydraulic resistance, reducing the flow velocities. Colonization by plants was obtained by instantaneously assigning vegetation to the areas that became dry at low water stages. This colonization presents a step forward in the modelling of bank accretion. Bank erosion was related to bed degradation at adjacent wet cells. Bank advance and retreat were reproduced as drying and wetting of the computational cells at the channel margins. The model was applied to a hypothetical case with the same characteristics as the Allier River (France). The river was allowed to develop its own geometry starting from a straight, uniform, channel. Different vegetation densities produced different planforms. With bare floodplains, the river always developed a braided planform, even if the discharge was constant and below bankfull. With the highest vegetation density (grass) the flow concentrated in a single channel and formed incipient meanders. Lower vegetation density (pioneer vegetation) led to a transitional planform, with a low degree of braiding and distinguishable incipient meanders. The results comply with flume experiments and field observations reported in the literature.

Network response to disturbances in large sand-bed braided rivers

“…Furthermore, the opposite process of bank erosion, which is bar-floodplain conversion by, for example, vegetation encroachment, is not considered in Delft3D. The necessity of this bar-floodplain conversion for channel migration was demonstrated by Schuurman et al (2016), and the large effect of riparian vegetation on braiding river morphology has previously been demonstrated by, for example, Murray and Paola (2003) and Crosato and Saleh (2010). This missing mechanism must be included to fully understand the contribution of floodplain-channel interaction on the morphodynamics in braiding rivers.…”

“…Many river modeling studies have applied constant discharge, assuming a morphologically dominant or representative discharge exists that gives similar yearly morphodynamics to the "real" hydrograph (e.g., Nicholas, 2010;Schuurman et al, 2013). However, other studies have shown that discharge variation has a large effect on river morphology (e.g., Kiss and Sipos, 2007;Crosato and Saleh, 2010) due, among other things, to vegetation colonization on exposed bar sections (Gordon and Meentemeyer, 2006;Tealdi et al, 2011). Also, Egozi and Ashmore (2009) demonstrated that braiding intensity increased with increasing discharge, although this was temporary and braiding intensity decreased after the channel adapted to the new discharge.…”

Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

“…1) is successfully used in other studies with real vegetation: such as the simulation of the flow field over a river floodplain with grass, reed and softwood [48,49], and marsh vegetation [50]. Following the approach of Baptist et al [1], the representative Chézy value at every location with vegetation is described by (Eq.…”

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.…”