Spatial Variations in River-Bed Porosity - Stratification and Imbrication

• Applicants: Professor Dr.-Ing. Holger Schüttrumpf; Dr.-Ing. Stefan Wolf-Dietrich Vollmer, since 5/2020
• Subject Area: Geotechnics, Hydraulic Engineering ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 283927452

River beds are composed of sediment particles and water-filled pores. The pore content (or porosity) determines the suitability of the river bed as a habitat for aquatic organisms, the stability of navigation channels, the siltation rate of drinking-water reservoirs and the exploitable volume of oil and groundwater in ancient river deposits.Despite its economic and societal relevance, until recently, little was known about natural variations in porosity in fluvial systems. In a joint DFG Project (2016-2019) we addressed this knowledge gap. First, we developed innovative techniques to measure porosity. These techniques, with hitherto unprecedented accuracy, were then used to quantify the magnitude of spatial variations in river bed porosity at micro-, meso- and macroscale. Furthermore, we determined the effect of grain size and grain shape on porosity, and developed a new physics-based method to predict spatial variations in porosity. So far, this (ongoing) study resulted in 4 peer-reviewed journal papers and one PhD thesis (more following soon). The high precision of these new techniques revealed an interesting and unexpected phenomenon: porosity values measured in the field are fundamentally different from porosity values measured in the laboratory on the same sediment sample. This shows, that – other than often assumed - the porosity of fluvial deposits is strongly affected by the way in which sediment grains are positioned in sediment deposits, especially – in geological terms – by stratification and imbrication effects. Unravelling these effects is essential to understand and predict spatial porosity variations in sand-gravel bed rivers. The objective of the research proposed here is to determine the effect of stratification and imbrication on porosity, focusing on fluvial sand-gravel mixtures. The following questions are to be answered: How do stratification and imbrication affect porosity? How big is this influence? How can this effect be accounted for in porosity predictions? What is the relevance of this effect for a river engineer’s daily practice?To answer these questions, first a mathematical parametrization of the degree of stratification and imbrication will be developed based on theoretical principles from material sciences and geology. Laboratory experiments and numerical simulations then will provide a mathematical relation between the stratification and imbrication parameters and porosity. This relation will be validated against extensive field data to be collected in France and used to extend the porosity predictor from our current DFG project. For laboratory experiments, numerical simulations and field measurements use will be made of advanced, innovative techniques, specifically photogrammetry, magnetic resonance imaging and physics-based packing models. Finally, the benefits to river management of the study proposed here will be exemplified in a case-study of sediment management in the German Niederrhein.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Dr. Roy Frings, until 5/2020