A Physical Model for the Uprooting of Flexible Vegetation on River Bars

Abstract

Sediment deposition and bank accretion are promoted by the establishment and growth of pioneer plant species, a direct consequence of plant survival during flood events. Similarly, the uprooting of riparian vegetation on river bars during floods can subsequently alter hydraulics, sediment dynamics, and bar evolution. In this work, we focus on the removal of flexible seedlings due to both hydraulic forces and bed erosion, specifically examining failure mechanisms associated with root pull‐out. We provide a conceptual model and a new physical equation for predicting the flow and bed erosion conditions that promote the uprooting of plants. The model was validated by means of flume experiments employing two species of vegetation (i.e., common oats and a willow native to Europe). Furthermore, the Ombrone Pistoiese River (Tuscany, IT) was used as a case study to validate the physical model with respect to observed vegetation removal during a flood event. The results illustrate the capability of our model to predict conditions for vegetation removal and suggest that sediment transport is a necessary ingredient even for very young seedlings.

Return period of vegetation uprooting by flow

“…Vegetation uprooting under flow and scour constraints (Type II) was investigated by Edmaier et al (2015) in laboratory experiments with Avena sativa and by Bywater-Reyes et al (2015) in field measurements. Calvani et al (2019a) used flume experiments with Avena sativa and Salix purpurea and field measurements to test and validate a model able to predict the critical bed erosion depth for which uprooting occurs. All these studies agree that the amount of bed erosion leading to plant uprooting by flow is smaller than the initial rooting depth, thus supporting the critical rooting depth model (Edmaier et al, 2011;Calvani et al, 2019a).…”

“…Calvani et al (2019a) used flume experiments with Avena sativa and Salix purpurea and field measurements to test and validate a model able to predict the critical bed erosion depth for which uprooting occurs. All these studies agree that the amount of bed erosion leading to plant uprooting by flow is smaller than the initial rooting depth, thus supporting the critical rooting depth model (Edmaier et al, 2011;Calvani et al, 2019a). Perona and Crouzy (2018) hypothesized that for low plant size vs sediment size ratio, the critical rooting depth would correspond to a critical erosion depth.…”

Biomorphological scaling laws from convectively accelerated streams

“…, ) is unlikely to occur in riverine habitats with already established vegetation, and a certain flood duration is required for morphological changes (i.e., bed erosion) to reduce root anchoring and promote plant uprooting (Perona and Crouzy, ; Calvani et al. , ).…”

“…For instance, plant species of Groups 2 and 4 (e.g., Tamarix and Eleagnus) are prone to uprooting (i.e., high N d ) and can be uprooted at a shorter t d temporal scale. Conversely, plants species of Groups 1 and 5 (e.g., Populus and Celtis) were more resistant to uprooting (i.e, low N d ) and require, for As a result, it turns out that instantaneous uprooting (Type I according to (Edmaier et al, 2011) is unlikely to occur in riverine habitats with already established vegetation, and a certain flood duration is required for morphological changes (i.e., bed erosion) to reduce root anchoring and promote plant uprooting (Perona and Crouzy, 2018;Calvani et al, 2019). Moreover, we could correlate the average growth rate N g with the return period of the flow magnitude Q d , which represents a reasonable timescale for plants to start colonizing, establish and grow on river bare bedforms.…”

Extracting the Critical Rooting Length in Plant Uprooting by Flow From Pullout Experiments

“…For plants with a low flexural rigidity (Yagci et al, ; Nepf, ), the drag force progressively bends the portion of the plant above the ground until it lies parallel to the channel bed (Figure b). This horizontal reconfiguration of the plant was also adopted by Calvani et al (). At incipient uprooting, all the forces have to balance the resistance exerted by plant roots: where F n is the net buoyancy force, F d,n is the drag force, F d,t is the friction action, and R represents the resistance exerted by the root system.…”