On the use of packing models for the prediction of fluvial sediment porosity

Abstract. Obtaining accurate porosity information of fluvial sediment deposits is helpful and desirable for many tasks of river engineers. Besides direct measurements of single samples and empirical formulas specialized for specific cases, packing models promise efficient predictions due to their theoretical and extensible foundation. The objective of this work is thus to investigate the usability of three such models in order to obtain a suitable porosity prediction method for the challenging case of fluvial sediment packings. There, the complexity originates from wide continuous size distributions, from silt to gravel, and different grain shapes. We use data obtained from extensive numerical packing simulations to determine the required model parameters and to verify the models' accuracy for moderate size ratios. This study reveals systematic deficits in one of the models which can be attributed to the absence of a built-in mixture packing model. By combining these findings with data from laboratory measurements and extending the model to include cohesive effects, we exemplify for the Rhine River in Germany that reasonable porosity predictions can be obtained with the Compressible Packing Model. Through an additional comparison with data from French rivers, guidelines for a successful prediction in cases with limited prior knowledge of the model parameters are developed. Future model enhancements, of the packing models directly as well as by incorporating more effects that are known to influence porosity, are expected to improve the predictive performance.