Effects of vegetation on flow and sediment transport: comparative analyses and validation of predicting models

Abstract

The presence of vegetation modifies flow and sediment transport in alluvial channels and hence the morphological evolution of river systems. Plants increase the local roughness, modify flow patterns and provide additional drag, decreasing the bed-shear stress and enhancing local sediment deposition. For this, it is important to take into account the presence of vegetation in morphodynamic modelling. Models describing the effects of vegetation on water flow and sediment transport already exist, but comparative analyses and validations on extensive datasets are still lacking. In order to provide practical information for modelling purposes, we analysed the performance of a large number of models on flow resistance, vegetation drag, vertical velocity profiles and bed-shear stresses in vegetated channels. Their assessments and applicability ranges are derived by comparing their predictions with measured values from a large dataset for different types of submerged and emergent vegetation gathered from the literature. The work includes assessing the performance of the sediment transport capacity formulae of Engelund and Hansen and van Rijn in the case of vegetated beds, as well as the value of the drag coefficient to be used for different types of vegetation and hydraulic conditions. The results provide a unique comparative overview of existing models for the assessment of the effects of vegetation on morphodynamics, highlighting their performances and applicability ranges.

The influence of a vegetated bar on channel-bend flow dynamics

“…where Cd,v is vegetation drag coefficient, Ac is projected vertical frontal area of vegetation (Nepf, 1999;Vargas-Luna et al, 2015, 2016, and Uc is the approach velocity. For Uc we substituted cross-sectional mean velocity, Um (after Jalonen et al, 2013).…”

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

“…Vargas-Luna et al [26] performed a comparative analysis and validation of fourteen models which describe the resistance effect caused by aquatic vegetation. Here we built further upon the approach proposed by Baptist et al [1] (Eq.…”

“…Here we built further upon the approach proposed by Baptist et al [1] (Eq. 1), which was identified by Vargas-Luna et al [26] as one of the model approaches that performs best to simulate both submerged and emerged vegetation, real and artificial vegetation, and rigid and flexible vegetation. It is important that the vegetation height in the formula corresponds to the deflected vegetation height in the field [47]; therefore in this study we consider reconfiguration as a varying deflected vegetation height in function of stream velocity.…”

“…Several approaches, based on this Manning equation, are derived to estimate the hydraulic resistance caused by submerged vegetation and have been implemented into hydrodynamic models [e.g. 1,10,11,24,25], see for Vargas-Luna et al [26] a comparative analysis. These equations divide the vertical velocity profile into two layers: one layer within the vegetation canopy and one layer above the vegetation canopy.…”

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)

“…Vargas-Luna et al (2015) analysed the performance of a large number of models on flow resistance, vegetation drag bed-shear stresses in vegetated channels to provide information for modelling purposes. The summary of measurements using real vegetation presented in this paper indicated a very small number of works referring directly to field measurements (Nikora et al, 2008;Ree and Crow, 1977).…”