Lunchtalk by Thomas van Veelen

Title: Process-based modelling of bank-breaking mechanisms of tidal sandbanks

To improve our understanding of long-term plan view dynamics of tidal sandbanks, we have developed a nonlinear morphodynamic model. Current hypotheses describe how an isolated bank breaks into multiple banks, but they lack process-based support. We aim to provide this with the inclusion of nonlinear interactions between tidal flow, topography, Coriolis and bottom friction in the hydrodynamic formulation and depth-dependent wind wave stirring and slope effects in the sediment transport formulation. The pseudo-spectral solution procedure is characterised by an expansion of the hydrodynamics in the ratio of bank height and water depth and separate treatment of the morphodynamics.

The model is applied for two sample sandbanks: one breaks and one does not. We distinguish four stages in the breaking mechanism: (a) bank with straight outline; (b) bank ends rotate and the middle part erodes; (c) the crests at the ends grow and elongate, while the middle part continues to erode: the bank breaks in two or more parts; (d) the middle part of the separate banks rotates clockwise, which leaves a characteristic S-shape. The non-breaking bank grows, elongates and evolves into an S-shape as well. Bank breaking occurs under the condition that banks are sufficiently long and are not initially oriented in their preferred angle of deposition. The proposed mechanism is consistent with hypotheses by Huthnance (1982b) and Smith (1988). The characteristic final S-shape shows resemblance to field data.