Uncertainty in the evaluation of sediment yield from badland areas: Suspended sediment transport estimated in the Araguás catchment (central Spanish Pyrenees)

Badlands are important in terms of sediment yield, particularly in mountain areas having subhumid and humid climatic conditions. Various studies concerning erosion and hydrological processes have demonstrated that suspended sediment transport is probably the main process in sediment export from badland areas. In the Araguás catchment (central Pyrenees) there is a statistically significant positive linear relationship between maximum discharge and the maximum suspended sediment concentration (SSC). The high frequency of hyper-concentrated fluxes of SSC (> 500 g·l− 1) recorded at two gauging stations close to badland areas in the Pyrenees (Araguás) and the Alps (Draix) indicates that these fluxes are not uncommon, and suggests that they may transport relatively large suspended particles, especially during extreme floods. In a study involving sampling during two moderate floods (August 2006 and February 2007) in the Araguás catchment it was observed that the mean suspended sediment particle size was significantly greater during the highest SSC conditions. The results showed the great heterogeneity of particle sizes that can affect suspended sediment transport, which is usually estimated from concentration determined from turbidity values obtained using infrared devices and associated with the corresponding discharge value. Infrared turbidimeters have problems in detecting particles with a diameter (D) > 0.1 mm, although discharge can be evaluated with relative high accuracy.The combination of these factors suggests that the evaluation of sediment yield from badland areas using turbidity values involves significant uncertainty. If most suspended sediment is transported during moderate–high floods, which carry large quantities of suspended particles having D > 0.1 mm, then the sediment yield will be underestimated. The uncertainty can be calculated by determining the percentage and mean diameter of particles not detected, and the specific weight of the material. However, the uncertainty is not linear because of the exponential relationship between increasing diameter and the volume/mass, and consequently the error will increase with the growth in the suspended concentration. In this study the physical factors associated with uncertainty in the estimation of sediment yield were investigated, and quantitative estimates of the errors involved are provided.

► Suspended sediment transport has been identified as the main global mechanism of fluvial sediment transport.
► Field measurements and data collection in terms of suspended sediment are generally difficult tasks.
► The evaluation of sediment yield from badland areas using turbidity values involves significant uncertainty.
► A significant effect of SSC increase on variations in the suspended grain size was observed.
► Automatic samplers typically show an overestimation of turbidity methods.