Alain Crave

Landscape response to climate change: Insights from experimental modeling and implications for tectonic versus climatic uplift of topography

We present the results of an experimental investigation of the concurrent action of tectonic uplift and climate variation on relief evolution. We designed an experimental apparatus that allows the study of erosion of laboratory-scale topographies that evolve under given uplift and rainfall rates. For constant uplift and rainfall rates, the experimental topography evolves toward a statistical steady state defined by a mean elevation constant with time. Starting from such a steady state and keeping the input uplift rate constant, a subsequent change in the rainfall rate yields a change in the mean elevation of the landscape to a new equilibrium elevation. An increase in precipitation yields a lower mean steady-state elevation, whereas for a decrease in precipitation the surface is uplifted. We define this phenomenon as a climatically induced surface uplift, as opposed to a tectonically induced surface uplift. The climatically and tectonically induced surface uplifts correspond to different dynamics of denudation so that it is theoretically possible to differentiate between the climatic or tectonic causes of surface uplift from records of output sediment fluxes.

A stochastic precipiton model for simulating erosion/sedimentation dynamics

Abstract We present a stochastic modelling of erosion–sedimentation processes, based on cellular automata, which mimics the natural variability of climatic events with deterministic transport processes. The numerical procedure calculates the runoff water discharge as a function of the return period of elementary walking elements. This procedure generates water flux distributions at each point of the system, that depends on the local drainage area, and on the walker rainfall, and that are statistically analogous to natural river discharge distributions. It happens that a wide range of non-linear transport processes can be directly simulated in a intuitive way, relating the water flux and the local slope to the sediment transport activity. The present version of the code encompasses the main characteristics of erosion–sedimentation processes, from the simple hillslope diffusive law to the more complex non-linear fluvial transport. To illustrate the versatility of the model to reproduce complex natural dynamics, we calculate several geomorphological instabilities which are crucial in relief dynamics.

Upscaling local-scale transport processes in large-scale relief dynamics

The objective of this paper is to discuss the form and parameters of the macroscopic continent-scale erosion law. The law cornes from a spatial integration of local transport processes on the resulting topography. It thus depends on the relief/drainage organization that results from the development of geomorphic instabilities such as differentiai incisions. Assuming local transport processes to depend on local slope and water discharge, we have calculated topographie evolution to derive the characteristic time scales of erosion dynamics. W e especially focused on the case of a declining plateau which presents two main phases : a first phase when the drainage pattern establishes, and a declining phase when topography decreases almost exponentially in the absence of tectonic input. The latter phase is characterized by a time scale which depends on the system size, on the organization of the drainage network, and on the parameters of the transport process. We show that the macroscopic erosion law has the characteristics of an abnormal diffusion whose basic time-length exponent a is determined by the parameters of the fluvial process. This result sheds new light on the observed negative correlation between current denudation rates and drainage areas in world-wide fluvial watersheds.

Influence of piedmont sedimentation on erosion dynamics of an uplifting landscape: An experimental approach

Models of relief development generally assume that eroded products are evacuated far from the landscape, whereas in nature they are often deposited at the foot of mountain belts, within continental environments. Because piedmont aggradation can modify the base level for erosion, we investigate the influence of piedmont sedimentation on the dynamics of an upstream relief. We developed an experimental study of relief dynamics using laboratory-scale models submitted to uplift under runoff-driven erosion. We compare the dynamics of topographies surrounded, or not, by a depositional belt made of eroded products coming from upstream. Piedmont aggradation acts on the dynamics of the upstream relief by modifying the relative uplift rate (applied uplift rate minus aggradation rate) that denudation tends to balance. Relief denudes at a lower rate than the applied uplift rate, so the mean elevation of the uplifting topography rises. When the time scale of aggradation is higher than the time scale of relief development, the topography cannot reach a steady state between denudation and the applied uplift rate as long as aggradation occurs. However, in this case denudation balances a continuously varying relative uplift rate during a dynamic equilibrium phase of the topography.

Estimating uplift rate and erodibility from the area-slope relationship: Examples from Brittany (France) and numerical modelling

Abstract We used the local slope/drainage area relationship to derive the basic erosion and tectonic parameters from a topography. Assuming a dynamic equilibrium between uplift and erosion, this relationship is expected to depend quite simply on the rock erodibility, and on the tectonic uplift. This relationship may then be used to quantify independently the effect of lithological variation on the erodibility, and the uplift rate. We tested the method on a computer simulated topography and showed that the uplift information can be precisely calculated from the topographic analysis alone. We then analysed the topography of Brittany (France), and obtained a good agreement with uplift data from comparative levelling studies and river incision analysis.

Analogue modelling of relief dynamics

Abstract Natural landscape analysis and numerical modelling point to a lack of physical data on relief dynamics. Experimental modelling is therefore an interesting approach for obtaining physical information on eroded systems with runoff transportation and topographic incision. The main technical challenge, in reproducing regional topography at the laboratory scale, is to obtain mm-scale incisions and a limitation of the smoothing action of diffusive transport processes. An experimental design using newly developed rain making apparatus and silica as a model material, satisfies the required conditions, and allows simulation of geomorphic instabilities. An example of “plateau instability” modelling is presented to illustrate the suitability of this experimental procedure.

Scaling relationships of channel networks at large scales: Examples from two large-magnitude watersheds in Brittany, France

Abstract We present a statistical analysis on two watersheds in French Brittany whose drainage areas are about 10,000 and 2000 km 2 . The channel system was analysed from the digitised blue lines of the 1:100,000 map and from a 250-m DEM. Link lengths follow an exponential distribution, consistent with the Markovian model of channel branching proposed by Smart (1968) . The departure from the exponential distribution for small lengths, that has been extensively discussed before, results from a statistical effect due to the finite number of channels and junctions. The Strahler topology applied on channels defines a self-similar organisation whose similarity dimension is about 1.7, that is clearly smaller than the value of 2 expected for a random organisation. The similarity dimension is consistent with an independent measurement of the Horton ratios of stream numbers and lengths. The variables defined by an upstream integral (drainage area, mainstream length, upstream length) follow power-law distributions limited at large scales by a finite size effect, due to the finite area of the watersheds. A special emphasis is given to the exponent of the drainage area, a A , that has been previously discussed in the context of different aggregation models relevant to channel network growth. We show that a A is consistent with 4/3, a value that was obtained and analytically demonstrated from directed random walk aggregating models, inspired by the model of Scheidegger (1967) . The drainage density and mainstream length present no simple scaling with area, except at large areas where they tend to trivial values: constant density and square root of drainage area, respectively. These asymptotic limits necessarily imply that the space dimension of channel networks is 2, equal to the embedding space. The limits are reached for drainage areas larger than 100 km 2 . For smaller areas, the asymptotic limit represents either a lower bound (drainage density) or an upper bound (mainstream length) of the distributions. Because the fluctuations of the drainage density slowly converge to a finite limit, the system could be adequately described as a fat fractal, where the average drainage density is the sum of a constant plus a fluctuation decreasing as a power law with integrating area. A fat fractal hypothesis could explain why the similarity dimension is not equal to the fractal capacity dimension, as it is for thin fractals. The physical consequences are not yet really understood, but we draw an analogy with a directed aggregating system where the growth process involves both stochastic and deterministic growth. These models are known to be fat fractals, and the deterministic growth, which constitutes a fundamental ingredient of these models, could be attributed in river systems to the role of terrestrial gravity.

Macroscale dynamics of experimental landscapes

Abstract We review results from laboratory-scale modelling of erosion and relief dynamics under variable uplift and rainfall rates. Under constant values of these forcing parameters an experimental landscape typically evolves towards a steady-state between uplift and erosion, and we show how the geometry of the steady-state landscape adjusts to the rates of uplift and rainfall. The comparison between these laboratory-scale landscapes and the natural ones is not straightforward because contrary to analogue modelling in tectonics, natural conditions of relief evolution cannot be downscaled to the laboratory without any scale distortions. Laboratory-scale modelling in geomorphology is therefore only experimental, not analogue. Despite these limitations, experimental models may be used to provide physical tests for numerical models and they give insights into first-order behaviours and directions for future research.

The impact of extreme El Niño events on modern sediment transport along the western Peruvian Andes (1968–2012)

Climate change is considered as one of the main factors controlling sediment fluxes in mountain belts. However, the effect of El Niño, which represents the primary cause of inter-annual climate variability in the South Pacific, on river erosion and sediment transport in the Western Andes remains unclear. Using an unpublished dataset of Suspended Sediment Yield (SSY) in Peru (1968–2012), we show that the annual SSY increases by 3–60 times during Extreme El Niño Events (EENE) compared to normal years. During EENE, 82% to 97% of the annual SSY occurs from January to April. We explain this effect by a sharp increase in river water discharge due to high precipitation rates and transport capacity during EENE. Indeed, sediments accumulate in the mountain and piedmont areas during dry normal years, and are then rapidly mobilized during EENE years. The effect of EENE on SSY depends on the topography, as it is maximum for catchments located in the North of Peru (3–7°S), exhibiting a concave up hypsometric curve, and minimum for catchments in the South (7–18°S), with a concave down hypsometric curve. These findings highlight how the sediment transport of different topographies can respond in very different ways to large climate variability.

Reply to comment by Yanni Gunnell and Marc Calvet on “Origin of the highly elevated Pyrenean peneplain”

Gunnell and Calvet [2006] (hereinafter referred to as GC) challenge the recent model that we proposed for the origin of the highly elevated Pyrenean peneplain by contest- ing our morphometric analysis of this chain and the relation we made between the morphological evolution and the piedmont sedimentation. Their reasoning is as follows: (1) According to Calvet [1996] (on which their comment is largely based) the high-elevation, low-relief surfaces in the Eastern Pyrenees are remnants of a peneplain that devel- oped before the Pliocene from applanation near to sea level, and which was later uplifted by 2000 m during the Plio- Quaternary (in other words, GC belong to the ‘‘applanation’’ school, whereas we woul d belong to the ‘‘altiplanation’’ school); (2) high-elevation, low-relief surfaces do not exist in the Central Pyrenees; (3) therefore the relationships we made between the morphology of the Central Pyrenees and the pattern of the detrital sedimentation in the adjacent Ebro foreland basin is meaningless; (4) contrary to the initial interpretation of Calvet [1996], GC recognize that crustal thickening did not develop since the Pliocene in the Eastern Pyrenees, so they appeal to another geodynamical process such as extension or lithosphere delamination to explain the supposed uplift.