Influence of inclination and permeability of solitary woody riparian plants on local hydraulic and sedimentary processes

Abstract

The role of solitary woody riparian plants with respect to local erosion and deposition of sediments is investigated. A focus is laid on the characteristics ‘inclination’ and ‘permeability’ of the plant's projected frontal area. Therefore, two experimental studies using cylindrical obstacles were carried out in a laboratory flume, one aiming at inclination, the other at permeability. The first series revealed that the total amount of mobilized sediment around the cylinder on average decreased by 8–10% per 5° increasing inclination in streamwise direction. Locations of maximum scour depth simultaneously shifted downstream. A horseshoe vortex system, causing the frontal and lateral scouring, ceased to exist below inclinations of 25–30°. The second series revealed that with increasing permeability, frontal scour incision is delayed, and the eroded sediment volume is significantly reduced. With permeable obstacles, two system states were observed: first, frontal scouring with leeside deposition at higher flow velocities and, second, moderate leeside scouring at lower flow velocities. For up‐scaling and comparison, a field study focussing on fluvial obstacle marks at poplars and willows in secondary channels of the River Loire was additionally conducted. A modified analytical model enabled us to quantify the amount of deposited sediments leeside of the plants. Leeside sediment ridges are significantly stabilized and have a higher preservation potential when covered by pioneer vegetation. Under such conditions, they may indeed induce the development of stable islands. Eventually, ‘sediment ridge width’ turned out to be a suitable indicator for leeside deposited sediment volume, irrespective of spatial scale. Copyright © 2012 John Wiley & Sons, Ltd.

Bed morphological changes around a finite patch of vegetation

“…In addition, a depositional ridge was observed in this study along the patch's lateral edge, which was particularly clear under the experimental conditions in Case 2 (Figure ). This depositional ridge was consistent with field observations made during a flood (Rodrigues et al ., ; Euler et al ., ). Several field studies found that trees and shrubs promote leeside sediment deposition during floods, which encourages the colonization of vegetation (Gurnell and Petts, ; Euler et al ., ).…”

Fluvial islands: First stage of development from nonmigrating (forced) bars and woody-vegetation interactions

“…Digital elevation models (Fig. 6) show a relatively plane surface of the bar in 2007 with superimposed low-amplitude longitudinal obstacle marks (Nakayama et al, 2002;Rodrigues et al, 2007;Euler and Herget, 2011;Euler et al, 2014) caused by the presence of patches of young trees.…”

“…Thin layers of fine sediments (mainly silts) enriched in organic matter and penetrated by roots are found in the densely vegetated areas (C12 and C22). These thin layers are markers of emersion phases and fixing of sediments by pioneer trees or herbs growing on obstacle marks (Rodrigues et al, 2007;Euler et al, 2014).…”

“…More specifically, the presence of shrubs and trees often creates obstacle marks (horseshoe scour hole, sediment ridge) which constitute a first step in the metamorphosis process from a bar to an island (Nakayama et al, 2002;Rodrigues et al, 2007;Euler et al, 2014). As a consequence of merging of these obstacle marks and vertical accretions, the morphology of bars evolves from a relatively flat bed to a ridge-and-swale topography.…”

“…Once established on bars, the permeable obstacle constituted by the vegetation influences their morphological development by increasing flow resistance (McKenney et al, 1995;Fathi-Maghadam and Kouwen, 1997;Darby and Simon, 1999;Freeman et al, 2000;Antonarakis et al, 2010), decreasing flow velocity (Li and Shen, 1973;Wu et al, 1999;Copeland, 2000), and enhancing deposition and fixing of sediments (Graf, 1978;Fielding et al, 1997;Wende and Nanson, 1998;Kollmann et al, 1999;Gurnell et al, 2001;Nakayama et al, 2002;Francis et al, 2006;Euler and Herget, 2012;Euler et al, 2014;Manners et al, 2014). More specifically, the presence of shrubs and trees often creates obstacle marks (horseshoe scour hole, sediment ridge) which constitute a first step in the metamorphosis process from a bar to an island (Nakayama et al, 2002;Rodrigues et al, 2007;Euler et al, 2014).…”