EROSAT - ERosion from geOchemistry and remote Sensing of lAndslides in the Tropics.
As a major agent of erosion and sediment supply to rivers, mass wasting are one of the most efficient physical processes shaping the Earth’s surface and is thus critical to landscape evolution. They promote the percolation of surface runoff in highly fragmented rock debris thereby creating favorable conditions for chemical weathering that is a major atmospheric carbon dioxide sink. EROSAT project aims to evaluate the role of slope instability in sediment and solute fluxes from watersheds, and to study their impact on the critical zone in tropical islands of Guadeloupe, where erosion and weathering rates are among the highest on Earth. By a combining an innovative multidisciplinary approach based on ObsEra data, geochemical analysis and multi-scale remote-sensing, EROSAT will lead to an analytical scaling between mass wasting and sediment/solute load in the river. Ultimately, EROSAT will deliver a quantitative modeling of the sediment and solute transport driven by mass wasting.
RainForeST – RAdiatIve traNsfert FOR assEssing Sediment Transport.
It is important first of all to have a good understanding of how sediment transport takes place in rivers. We know that fine particles represent an important fraction of the transported sediment mass (Syvitski & Saito, 2007). Suspended load is therefore an important factor in erosion. However, recent field measurements show that suspended fine particles interact with the bed (Misset et al., 2019). Based on this observation, it is therefore possible to develop a formalism of suspended transport that explicitly takes into account the exchange of fine particles between the bed and the water column. This approach makes use of three parameters which are : (1) a threshold water level above which the flow begins to erode the bed, (2) an erosion rate which characterises the intensity of sediment entrainment and (3) a characteristic sedimentation time.
However, if river flow and turbidity are measured with a 5-minute time step, measurements of suspended solids concentration are sporadic since they require field sampling of sufficient water during a flood event to collect sufficient material transported by the river. The sediment (turbidity) load of a river or water body is measured using a laser light emitter/receiver instrument. The manufacturer is responsible for providing the calibration curves of its instrument (turbidity versus received intensity) and the user has no control over this calibration, nor can the user directly relate the true concentration of suspended matter to the turbidity measurement.
Thus, until now, we have had to convert the turbidity signal into a suspended solids concentration from an empirical calibration curve that we have obtained from river samples (Fig. 1). However, such an approach does not take into account variations in the granulometry of the suspended matter since it integrates all the measurements made regardless of the collection period. Moreover, its non-linear nature implies a high uncertainty towards small values. We therefore propose to reproduce in the laboratory the measurement conditions in the field in order to establish a calibration based on physical considerations. This study could be completed by modelling the radiative transfer (Mobley, 2001).