The effect of imbrication on the porosity value of non-uniform gravel bed: a case study from the Buëch River, France

Abstract Existing porosity predictors for fluvial sediments are mainly derived from laboratory-generated, randomly packed sediment samples. However, such predictors could not adequately describe beds with non-random grain arrangements that occur widely in fluvial deposits. In this work, the effect of grain imbrication on non-uniform gravel-bed porosity has been quantified using fluvial sediment samples showing imbrication and no imbrication collected from the river Buëch, France. The in-situ porosity of the undisturbed samples was directly measured on-site, while the ex-situ porosity was measured by randomly packing the particles of a sample in a cylindrical container in the laboratory. The in-situ porosity and the ex-situ porosity of the same sample were compared. Apart from the porosity measurement, a relatively new and simple workflow was applied to quantify the degree of bed imbrication based on the X-Ray Computed Tomography images of frozen sediment samples. For samples showing no imbrication, the in-situ and the ex-situ porosity showed similar values, indicating that sediment samples randomly packed in the laboratory (with shaking) are well representative of the fluvial sediment with random grain orientation formed under natural conditions. For samples showing imbrication, the in-situ porosity values were about 30% lower than their ex-situ porosity values, indicating denser packing structure due to imbrication. This increase in structural compactness is believed to stem from the ordered arrangement of sediment particles, thereby reducing the formation of large pores. Graphical Abstract ( a ) Comparison of the n in-situ and n ex-situ of samples showing imbrication and no imbrication, ( b ) Dip direction distribution, ( c ) Imbricated particles