Modelling flow over bedform fields in tidal environments
Final Report Abstract
In rivers, estuaries, tidal inlets and shallow seas, the movement of water above the sandy bed often cerates some large underwater dunes. These dunes interact with the flow in a complex manner. This project aimed at modelling at high-resolution tidal flows above three-dimensional (3D) dune fields. The modelling system Delft3D was used to develop a 3D numerical model which, using the nonhydrostatic module, can accurately simulate the movement of water above dunes. The model was calibrated and validated against laboratory data of unidirectional flow above idealised 3D dunes. It was then used to simulate water velocity and turbulence above a dune field from the Rio Parana (Argentina). The model simulations showed that the presence and size of the flow separation zone and turbulent wake depend on the presence and properties of a steep face (defined here as the portion of the lee side with angles >15°) and not on those of the crest. When present, the flow separation and wake lengths are, for the tested settings, respectively, around 5 and 13 times the steep face height. A steep face orientation of 25° or more compared to the flow increases cross‐stream flow and suppresses flow reversal over the steep face. To understand and predict the interaction between flow and bedform, the steep face rather than the crest position should be identified and analysed. In the second part of this project, the distribution and morphology of bedforms in the Weser Estuary, Germany, were investigated, with a strong focus on characterising the detailed bedform morphology, especially the presence and position of a steep face. Bedforms were identified in bathymetric data of monthly multibeam echosounder surveys along the navigation channel during the years 2009 to 2013. Their size and shape were characterized. Bedforms were present along most of the channel, except at the position of the estuarine turbidity maximum, due to the presence of fine sediment, and in the Outer Weser, where dredging is carried out. Average bedform length varied between 20 and 60 m and bedform height between 0.3 and 1.6 m. Bedform asymmetry varied spatially and temporally along the estuary. In times of high river discharge, bedforms were generally more ebbasymmetric than in times of low discharge. The bedforms were predominantly two-dimensional lowangle dunes with their steepest slope situated near the bedform crest. A significant proportion of bedforms possessed a steep face (portion of the lee side steeper than 15°). This implies that they are likely to create flow separation and a turbulent wake, with a strong potential to induce high bedform roughness. However, important variations in steep face area density were noted, both spatially along the estuary and temporally as a function of the tidal phase (ebb or flood) and the seasonal variation in river discharge. The results have wide implications in terms of understanding and modelling hydrodynamics and sediment transport in estuaries.
Publications
- (2019). Three-Dimensional Flow Above River Bedforms: Insights From Numerical Modeling of a Natural Dune Field (Río Paraná, Argentina). Journal of Geophysical Research: Earth Surface 124, 2241-2264
Lefebvre, A.
(See online at https://doi.org/10.1029/2018jf004928) - (2021). Morphology of estuarine bedforms, Weser Estuary, Germany. Earth Surface Processes and Landforms
Lefebvre, A., Herrling, G., Becker, M., Zorndt, A., Krämer, K., Winter, C.
(See online at https://doi.org/10.1002/esp.5243) - (2021). Special issue: Marine and river bedforms dynamics, MARID VI. Earth Surface Processes and Landforms 46, 1646– 1651
Lefebvre, A., Winter, C.
(See online at https://doi.org/10.1002/esp.5091) - (2021). The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary. Earth Surface Processes
Herrling, G., Becker, M., Lefebvre, A., Zorndt, A., Kramer, K., Winter, C.
(See online at https://doi.org/10.1002/esp.5170)
