Variability of the particle size of suspended sediment during highly concentrated flood events in a small mountainous catchment

Purpose There is a growing interest in the characterization of the particle size of sediment due to its impact on particle dynamics, especially for connectivity purpose. This study determined the particle size distribution of suspended sediment in a mountainous catchment, with the aim to evaluate the variability of particle size during floods, the main controlling factors, and if indirect information from hillslopes was useful for the interpretation of particle size measured at the catchment outlet. This work involved the development of a measurement protocol. Material and methods Samples were collected automatically from streamwater during flood events using an ISCO 3700 sampler. Five events were analyzed for their particle size distributions using a Malvern Mastersizer 2000. Because the samples were too concentrated, two different protocols were tested to address the errors made during the subsampling step: using a pipette and a home-made device with successive dilution phases. Results and discussion High errors occurred when using a pipette to extract particles within a stirred sample. The maximum errors were reduced from 1,600 to 30 % using the device described within this study. Particles were found to be aggregated at various levels regardless of the discharge they were sampled at. Their size was found to be either variable or stable at the event scale, and statistical analyses revealed that discharge was the factor that best correlated with particle size. The results obtained in this study are in agreement with the few other studies in comparable environments. Some hypothesis are put forward and discussed to explain the positive relationship between particle size and discharge. Input from hillslopes seems to have a measureable effect in this headwater catchment. Conclusions While the need for in situ measurements has long been stressed in lowland rivers, estuaries, and coastal environments, it was shown that the use of an accurate dilution protocol could provide some physically interpretable measurements on the particle size distributions of suspended sediment transported in a mountainous catchment. It also appears that hillslope information has to be considered when studying particle size measured at the catchment outlet.