Temporal distribution of suspended sediment transport in a Mediterranean basin: The Lower Tordera (NE SPAIN)

Abstract

Suspended sediment dynamics are still imperfectly understood, especially in areas of Mediterranean climate with strong seasonal contrasts, where few studies have been conducted. The aim of this study is to analyze the variability in the suspended sediment load at different temporal scales (within-flood variability, monthly-seasonal and annual) in the Tordera River (894 km 2), located in northeastern Spain. Suspended sediment samples were obtained during base flows ( Q ≤ 3.5 m 3 s − 1 ) and flood events ( Q > 3.5 m 3 s − 1 ). The analysis of the variability in the suspended sediment is structured in two consecutive steps: (i) the construction of statistical relations between suspended sediment concentrations ( C) and discharge ( Q) and (ii) the application of those relations to assess the suspended sediment yield. The results show that, in a single event, the sediment peak generally precedes the peak discharge, following a clockwise hysteretic loop. Furthermore, it was observed that the reduction in the suspended sediment concentrations in a sequence of events can be mainly associated with a phenomenon of sediment exhaustion. The total amount of suspended sediment transported during a single flood is thus related to its duration and time (during the year) as well as to its magnitude. At a seasonal scale, the export of suspended sediment takes place in two phases: (i) sediment preparation and, (ii) sediment transfer and exhaustion. During the latter phase, the exhaustion of the sediment load is primarily related to the sequence of flood events. At the annual scale, the results show high inter-annual variability according to the number of floods recorded yearly, since almost all the suspended load is transported during those events.