While the Delft3D-Geotool already does a good job at simulating the deposition of sediments in fluvial dominated deltas, the removal (erosion) of sediment, which is also very important for understanding the sediment distribution patterns in the preserved deposits, is less well represented. On long timescales, the cumulative alongshore transport of sediment at the coastline can play a significant role in the depositional patterns. We investigated the potential of simulating this process using a new, computationally efficient, coastline evolution model: ShorelineS.
In Delft3D-GeoTool we can already include a rising sea level, but without fully accounting for the necessary coastal erosion processes the tool will continue to overestimates delatic deposition. This could be viewed as an acceleration technique, increasing the speed with which deposits are preserved. But then it would be important to explicitly consider the impact of this imbalance between deposited and preserved sediment on depositional patterns.
In this study we investigated the feasibility of using a fast, computationally efficient, coastline evolution model. ShorelineS, and iteratively run this model together with a Delft3D model. To get some constraint on hydrodynamic parameters, and sediment preservation, we chose a fairly well-studied case from the recent geological past: the Old Rhine Delta. This delta represents an eroding delta, embedded in a relatively straight and simple coastline. For this delta, many of the hydrodynamic control conditions have already been approximated (de Haas et al. 2019).
Output
A number of technical challenges were overcome to enable the iterative coupling of Delft3D and ShorelineS. In addition a number of bugs were identified and addressed in the ShorelineS code. Bug fixes were also incorporated into the main branch of the ShorelineS code, code which is being further developed in a new TKI project aimed specifically at the development of ShorelineS for engineering timescale models (TKI ShorelineS research and development).
Integration of ShorelineS and Delft3D did not prove fruitful for the longer timescales of the Delft3D-GeoTool type models. Here the bathymetry used in the ShorelineS and Delft3D iterations started to diverge (see figure below). While solutions are conceivable which could deal with this divergence, these would typically lead to much slower overall runtime of the Delft3D-GeoTool models as they require more frequent iteration. Different solutions to long term wave transport will need to be considered.
