Under what conditions do climatic variations lead to incision of alluvial fans? Insights from coupled catchment-fan system in the lab
Authors: Bonnet Stéphane , Zavala-Ortiz Valeria , Carretier Sébastien 
Published: March 2021

&lt;p&gt;Although a transition from aggradation to incision is observed in many natural alluvial fans and is usually related to variations in climate, the condition driving such change remain unclear. We address this problem here by considering laboratory experiments of coupled catchment-fans systems forced by oscillatory precipitation. In the 8 experiments considered here, eroded materials from an uplifting mountain may deposit on a plateau, erosion being driven by the surface runoff of water from an artificial rainfall device. These experiments, 700 to 900 minutes-long, were performed with the same uplift rate but with different sequences of variations of the rainfall rate (10 to 40 minutes-long) between two extreme values. The topography was digitized every 10 minutes thanks to a high-resolution laser sheet.&lt;/p&gt;&lt;p&gt;We observe that the sediment (Qs) and water (Qw) fluxes at the outlet of mountain catchments continuously vary with time, not only because of precipitation oscillations but also because of the dynamic state of the landscape. For a given precipitation rate, variations of Qs and Qw depend on whether the catchments enlarge or shrink, with additional variations of Qs depending on the equilibrium degree (steady-state or not). Depending on these conditions, we document a large variety of trends of Qs vs Qw in rivers at catchment outlets, Qs increasing or decreasing with Qw, or being independent of Qw. For a given catchment, oscillations in precipitation rate then drive alternations between two individual Qs vs Qw trends, the slope of these trends being indicative of the sediment concentration in the mini-rivers at the outlet of catchments that feed alluvial fans.&lt;/p&gt;&lt;p&gt;Our experiments indicate that incision of alluvial fans is controlled by two necessary conditions. First, it occurs when rainfall increases and goes with a decrease of the Qs/Qw ratio, i.e. with a decrease of concentration at the outlet of the catchment. Second, this control is modulated by the slope of the fan, incision only occurring for fans above a threshold slope. Then, the decrease in sediment concentration required to initiate the incision is weak for steeper fans, those that developed under a mean dry climate. A larger drop in concentration is necessary for gentle fans, those that develop under a mean wetter climate.&lt;/p&gt;&lt;p&gt;Several studies already demonstrated how a decrease of Qs or an increase of Qw drives incision. We show here that these two parameters are also coupled and covariate following the dynamical state of catchments. We also demonstrate that the decrease of the Qs/Qw ratio required for initiating the incision of a fan is lower for steeper fans, that is for fans that develop under more arid condition.&lt;/p&gt;&lt;p&gt;We conclude that a given amplitude of climate change won&amp;#8217;t have the same imprint on landscapes, erosion and transport depending on the mean prevailing climate.&lt;/p&gt;