Author(s): Sandor Baranya

Linked Author(s): Sándor Baranya

Keywords: Large river; Bed form; Computational modeling; Nested grid

Abstract: Quantification of bedload transport in large rivers is of primary importance for river engineers, as this kind of sediment motion directly contributes to morphological changes of the river bed. However, direct measurement of bedload transport is challenging or even unfeasible in large rivers, especially during floods. Indirect sediment measurement methods are under development to overcome these difficulties, using acoustic and imaging methods, moreover, sophisticated, 3D numerical simulation tools might also support the understanding of sediment dynamics. This research introduces a 3D, nested grid based computational method to simulate the migration of bed forms, such as dunes and ripples, which, in fact, represent the bedload transport in large sand and even in gravel bed rivers. Two case studies were chosen to test the capabilities of the developed numerical tool: i) a large sand bed river, Mississippi in the United States at Vicksburg, MS; ii) a gravel bed river, Danube in Hungary. The computational model is based on the Reynolds Averaged Navier-Stokes equations, completed with a two-equation turbulence model. The hydrodynamic solver is coupled with different sediment transport models, and the model computes the temporally and spatially varying nature of the local morphological changes. In order to describe the migration of bed forms, representing much finer space scale than the actual sizes of the investigated river reaches, a nested grid based approach was tested. This technique enables a locally refined discretization of the computational domain in the zones of interest, i.e. in the vicinity of the bed forms. The two case studies represent different natures of large river beds. The Mississippi River can be characterized with 2-3 m high and 20-40 m long sandy dunes, whereas the bed of Danube River in Hungary consists of gravel dominated sediment, with 0.2-0.3 high and 5-10 m long ripples. For model validation purposes, detailed, repeated multibeam scannings were performed at both sites, providing high accuracy bed elevation difference maps. These maps could directly be compared with the simulated ones. The validated simulation tool provides novel insights into both the locally complex hydrodynamic features and the related sediment movements, furthermore, it can be of great use to quantify bedload transport in flow conditions when measurements can hardly be performed.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221135

Year: 2022