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

Abstract

The growth and establishment of riparian vegetation on river bedforms is of hydrological as well as ecological importance as it helps in enhancing spatial heterogeneity and thus the biodiversity of river corridors. Yet, during floods, flow drag and scouring may reduce the rooting length of plants determining plant mortality via uprooting. In order for uprooting to occur, bed scouring must proceed until the rooting length reaches a critical value and drag forces exceed root residual anchorage. Therefore, the critical rooting length of a plant represents a crucial parameter to estimate the probability of plant removal due to flow erosion. However, difficulties in quantifying such length at the field scale have limited so far the performances of biomorphodynamic models for river bed evolution. In this work, we propose to assess the critical rooting length from controlled plant pullout experiments. To this aim, a free-body model of the forces acting on a flexible plant in a stream at different erosion stages is developed. At incipient uprooting, we conjecture that the root resistance at the critical rooting length equals that of a plant with equal rooting length when pulled out in static conditions. To illustrate our approach, we validate our model on three different data sets obtained from small-and real-scale plant uprooting experiments. A comparison between modeling and experimental observations reveals that the model provides valid results, despite its deterministic approach. The critical rooting lengths are finally used to assess the probability density function of the time to uprooting via a physically based stochastic model.

Biomechanical Properties and Resistance to Uprooting of Laboratory‐Scale Wood Logs

“…The force components acting on a submerged plant were evaluated following the approach proposed by Bau et al. (2019), whereby the drag force, F d , is given by the sum of normal F d,n and tangential F d,t force components, such that …”

“…The modulus of F d,t was calculated as where C f is the friction coefficient, A s is the total surface area of the stems, and A l is the total area of the leaves exposed to the flow. To calculate the projected and surface areas A n , A s , and A l , the trunk, stem, and leaf were approximated by simple geometric shapes: a rectangle, a cylinder, and rhombus, respectively (see Bau et al., 2019). C d and C f were each assigned a representative value of 1 (Järvelä, 2002).…”

“…In the present computation, we assume a worst-case scenario where the impact between the longitudinal cross section of the log and the flow is perpendicular. The force components acting on a submerged plant were evaluated following the approach proposed by Bau et al (2019), whereby the drag force, F d , is given by the sum of normal F d,n and tangential F d,t force components, such that…”

“…Hence, further investigations are needed to assess other species characteristics. A second limitation is that the scouring processes, accounted in the free-body model of Bau et al (2019), were not considered here. As a consequence, the type of uprooting simulated here is of Type I.…”

“…Herein, the net buoyancy force F n is neglected, following previous studies (Bau et al., 2019; Bywater‐Reyes et al., 2015; Calvani et al., 2019). From a graphical perspective, uprooting takes place when the drag surpasses the root maximum resistance curve.…”