Benoît Crouzy

Experimental characterization of vegetation uprooting by flow

We investigate vegetation uprooting by flow for Avena sativa seedlings with stem-to-sediment size ratio close to unity and vanishing obstacle-induced scouring. By inducing parallel riverbed erosion within an experimental flume, we measure the time-to-uprooting in relation to root anchoring and flow drag forces. We link the erosion rate to the uprooting timescales for seedlings with varying mean root length. We show that the process of continuous erosion leading to uprooting resembles that of mechanical fatigue where system collapsing occurs after a given exposure time. By this analogy, we also highlight the nonlinear role of the residual root anchoring versus the flow drag acting on the canopy when uprooting occurs. As a generalization, we propose a framework to extend our results to time-dependent erosion rates, which typically occur for real river hydrographs. Finally, we discuss how the characteristic timescale of plant uprooting by flow erosion suggests that vegetation survival is conditioned by multiple erosion events and their interarrival time.

Non-proportional Repartition Rules Optimize Environmental Flows and Energy Production

In order to meet the growing demand for energy, the development of hydropower leads to an increase of the river exploitation by human activities. Thereby, water management has become a major issue in the energy transition. A better definition of the flow release rules is now required to improve the Minimal Flow Requirement (MFR) concept, which has long been used in spite of its environmental inconsistency. In this work, we present a class of non-proportional redistribution rules that broadens the spectrum of dynamic flow releases based on proportional redistribution for run-of-the-river power plants. We adapt the mathematical form of the Fermi-Dirac statistical distribution to engineer a novel class of redistribution functions. In particular, such functions are used to define the fraction of water allocated to the environment depending on the inflow at the intake. The theoretical background as well as the economic interpretation is presented, and the ability to generate variable flow releases carefully discussed. MFR, proportional and non-proportional distribution policies are then applied to a real case study and their respective economic and environmental efficiencies quantitatively compared. We show that non-proportional distribution policies allow for operating conditions actually close to the Pareto frontier, which improve both efficiencies with respect to those obtained from some traditional MFR and proportional policies.

Biomechanics of plant anchorage at early development stage.

We propose a minimal model for the response of seedlings to pullout constraints. Central to our approach is the idea of capturing not only average mechanical properties but also the stochastic component of the uprooting process. Our model accounts on one hand for the tensile elastic response of root fibres and on the other hand for the friction between root fibres and the soil matrix. We present for validation a dataset of 98 uprooting experiments using Avena sativa L. seedlings (common oat), growing in non-cohesive sediment under controlled conditions. We show that even if the architecture of the roots used in the experiments and, as a consequence, the components of our model are very basic, the uprooting curve (stress vs. strain) presents a complex response, with sudden jumps followed by partial elastic recovery. Depending on the maturity of the root system, we identify a crossover in the response of the seedling to the constraint. While for younger seedlings the anchorage rapidly fails after the peak force has been reached, more mature root systems recover from partial failures. Finally, we discuss the importance of the characteristics of the uprooting curve (maximal uprooting force and total uprooting work) regarding the ability of seedlings to withstand environmental constraints in terms of duration or intensity.

Resilience of riverbed vegetation to uprooting by flow

Riverine ecosystem biodiversity is largely maintained by ecogeomorphic processes including vegetation renewal via uprooting and recovery times to flow disturbances. Plant roots thus heavily contribute to engineering resilience to perturbation of such ecosystems. We show that vegetation uprooting by flow occurs as a fatigue-like mechanism, which statistically requires a given exposure time to imposed riverbed flow erosion rates before the plant collapses. We formulate a physically based stochastic model for the actual plant rooting depth and the time-to-uprooting, which allows us to define plant resilience to uprooting for generic time-dependent flow erosion dynamics. This theory shows that plant resilience to uprooting depends on the time-to-uprooting and that root mechanical anchoring acts as a process memory stored within the plant–soil system. The model is validated against measured data of time-to-uprooting of Avena sativa seedlings with various root lengths under different flow conditions. This allows for assessing the natural variance of the uprooting-by-flow process and to compute the prediction entropy, which quantifies the relative importance of the deterministic and the random components affecting the process.

Quantifying snowfall and avalanche release synchronization: a case study

We quantify the synchronization between snowfall and natural avalanches in relation to terrain properties at the detachment zone. We analyze field statistics of 549 avalanche events in terms of slope, aspect, timing, coordinate, and release area, identified by a georeferencing procedure applied on terrestrial photography. The information from the digital pictures, together with associated meteorological data, provides us with the input needed for model calibration, namely, the magnitude of snowfall, the snow compaction rate, and the timing of precipitation and of avalanche events. Synchronization between snowfall and avalanches is established for different slope categories. We obtain an average probability of release after a snow event of 30% and 16% for the high- and low-slope categories (average slope 44 degrees and 36 degrees, respectively). Using the notion of information entropy, we quantify the uncertainty in predicting avalanche occurrence from a snow event. The steeper slopes correspond to a larger entropy in avalanche prediction. Further, the presented method allows us to establish the return period of avalanches without requiring a long series of data. When considering events regardless of their release depth, the avalanches had a return period of 48 days (higher slopes) and 88 days (lower slopes). Finally, we determine the average daily detachment rate as a function of snow depth and the return period of avalanches as a function of the release depth.

Stability analysis of ecomorphodynamic equations: STABILITY ANALYSIS OF ECOMORPHODYNAMIC EQUATIONS

Although riparian vegetation is present in or along many water courses of the world, its active role resulting from the interaction with flow and sediment processes has only recently become an active field of research. Especially, the role of vegetation in the process of river pattern formation has been explored and demonstrated mostly experimentally and numerically until now. In the present work, we shed light on this subject by performing a linear stability analysis on a simple model for riverbed vegetation dynamics coupled with the set of classical river morphodynamic equations. The vegetation model only accounts for logistic growth, local positive feedback through seeding and resprouting, and mortality by means of uprooting through flow shear stress. Due to the simplicity of the model, we can transform the set of equations into an eigenvalue problem and assess the stability of the linearized equations when slightly perturbated away from a spatially homogeneous solution. If we couple vegetation dynamics with a 1D morphodynamic framework, we observe that instability towards long sediment waves is possible due to competitive interaction between vegetation growth and mortality. Moreover, the domain in the parameter space where perturbations are amplified was found to be simply connected. Subsequently, we proceed to the analysis of vegetation dynamics coupled with a 2D morphodynamic framework, which can be used to evaluate instability towards alternate and multiple bars. It is found that two kinds of instabilities, which are discriminated mainly by the Froude number, occur in a connected domain in the parameter space. At lower Froude number, instability is mainly governed by sediment dynamics and leads to the formation of alternate and multiple bars while at higher Froude number instability is driven by vegetation dynamics, which only allows for alternate bars.