Quantitative Analysis of Grain Orientation as a Proxy for Historical Flow Dynamics

The spatial arrangement of grains in gravel-bed rivers significantly influences sediment transport, flow resistance, and ecological conditions. Inferring historical flow information from spatial grain arrangements has garnered considerable interest among researchers. This study presents a robust and feasible method for such inference, focusing on quantitative approaches to estimate grain arrangement through inclination analysis. Two parameters based on inclination analysis are proposed for estimating grain arrangement. Quantitative relationships between these parameters and grain arrangements are established using artificially generated grain surfaces with predefined grain features. Additionally, the degree of imbrication, represented by the standard deviation of inclination curves, is examined using these generated surfaces. At the macroscopic level, the irregular inclination curve of a riverbed arises from the spatial variability of local hydrodynamic processes, leading to different local grain arrangements. Leveraging this feature at the microscopic level, the spatial distribution of grain orientation, or the grain azimuth field, is obtained using the proposed quantitative relationship. To explore the relationship between this grain azimuth field and flow velocity directions, a three-dimensional turbulent model based on Detached Eddy Simulation is deployed to simulate the detailed flow field near a gravel bed surface. Comparisons between the grain azimuth field and the flow velocity field highlight similarities, affirming that historical flow direction can be inferred from grain arrangement information. This study contributes to advancing the understanding of the intricate connections between grain arrangements and historical flow dynamics in gravel-bed rivers.