Suspended sediment dynamics during a period of consecutive explosive cyclogeneses in rural temperate catchments: A case study from small-sized catchments in NW Spain

Abstract

During the winter of 2013/2014, a sequence of explosive cyclogeneses occurred in NW Spain, causing severe flooding and substantial economic damage. Peak discharges were extremely high in many areas, with return periods in excess of 2–5 years. The heavy flow registered during these episodes provides a unique opportunity to evaluate the sedimentological effects resulting from high-magnitude flow events. Such events are relatively rare and the technical difficulties in sampling with sufficiently high frequency means that few observations on sediment transport can be gathered during the same. In this study, we present detailed hydrological and sedimentological observations across a period of consecutive explosive cyclogeneses from two small-sized catchments (i.e., the Corbeira stream and headwater of the Mero river) located in Galicia (NW Spain), in order to examine the factors controlling suspended sediment (SS) dynamics during high-magnitude flow events. The study sites are representative of the biophysical conditions (e.g. rainfall pattern, land use, vegetation cover and management practice) in NW Spain. The catchments of the Corbeira stream and headwater of the Mero river are similar with respect to climate, land use and management practice. The main differences are the distribution of land use and catchment size (Corbeira: 16 km2 and Mero: 65 km2).Large amounts of sediment were transferred in both catchments in response to flow events. A high variability in SS load during these was also observed in the two catchments, which is attributed to variation in rainfall characteristics, magnitude peak discharge, antecedent conditions controlling sediment availability, and suggests that sediment supply and transport vary widely over time. Higher concentration and variability of SS was observed in the Corbeira catchment. Differences in SS dynamics were also noted between the catchments that may be attributed to differences in the spatial distribution of land use, which influence the location of the erosion source and SS availability, and directly and indirectly affects connectivity between the sediment source area and the stream. In the Corbeira catchment, the SS load was closely linked to the hydrological response and rainfall, whereas in the Mero basin, preceding conditions, i.e. the sediment depletion during the previous events, played an important role. The results demonstrated that peak SS concentrations may coincide with, or lag behind, peak discharge, and factors other than discharge (mainly the location of the sediment source and its connectivity with the drainage network) have a major influence on SS concentrations.