Application of numerical modelling to establish the thresholds of delta formation

Around 40% of rivers globally currently have deltas, but the factors that facilitate or prevent delta formation are not well understood. Previous work has suggested that the critical factors governing their formation are (mean annual) fluvial sediment delivery rate, significant wave height, and tidal range (Caldwell et al., 2019). In light of ongoing climate-change driven changes to wave-generating weather, as well as changes to river sediment flux due to land-use change and river management, understanding how variability in these factors affects delta development is critical to developing sound coastal management strategies.

Here a comprehensive set of numerical simulations conducted using Delft3D is presented, with the aim of identifying the limits of the above factors beyond which a delta is prevented from forming. In order to retain a reasonable scope, analysis is restricted to variation of significant wave height and tidal range only, with sediment delivery rate held approximately constant. The resultant depositional landforms are then classified as either deltaic or non-deltaic, based primarily on the ultimate presence or absence of new unsubmerged regions of land. Depositional environments are further classified as river-, wave-, or tide-dominated in order to link delta presence to dominance regime, following the methodology of Nienhuis et al. (2020). Algorithms are also developed to facilitate classification of the resulting depositional features, and metrics are investigated that are time invariant, so as to formalise the process of classification.

Results indicate that increasing significant wave height and tidal range lead to a reduced rate of formation of new unsubmerged land, with higher values preventing the formation of such land altogether. At this point the depositional landform no longer meets the criteria for being defined as a delta.

References
- R. L. Caldwell, D. A. Edmonds, S. Baumgardner, C. Paola, S. Roy, and J. H. Nienhuis. A global
delta dataset and the environmental variables that predict delta formation on marine coastlines.
Earth Surface Dynamics, 7(3):773{787, 2019.
- J. H. Nienhuis, A. D. Ashton, D. A. Edmonds, A. Hoitink, A. J. Kettner, J. C. Rowland, and T. E.
Tornqvist. Global-scale human impact on delta morphology has led to net land area gain. Nature,
577(7791):514-518, 2020.