Bedload transport under different flow conditions in a human-disturbed catchment in the Central Spanish Pyrenees

This paper reports on bedload transport in a small catchment in the Flysh area of the Central Spanish Pyrenees. The Arnás experimental catchment, located in the headwaters of the Aragón River, is affected by natural vegetation regrowth, resulting from farmland abandonment. The main stream is totally covered with coarse and tabular-shaped particles (D50 = 100), a legacy of intense erosive activity decades ago. Discharge and suspended sediment transport were continuously measured at the outlet of the catchment. Bedload was retained in a trap located upstream of the flume. During most floods, this trap overfilled and the volume of the accumulated bed material was obtained from measurements made by means of a 3 m long profilometer. Twelve floods of different characteristics were studied, making it possible to estimate a critical discharge around 0.4 m3 s− 1. The occurrence of an “exceptional” flood event showed the important role of low-frequency floods in bedload transport. Results show that bedload reasonably depends on peak discharge and, to a lesser extent, on effective runoff. In the Arnás catchment, bedload is never more than 30% of the total sediment yield. This low value can be related to the characteristics of the bed particles and those of the channel slope, but also to a reduction in stream torrentiality, as a consequence of an increase in plant cover.