Deltas are dynamic landforms that develop through the interaction of sediment supply, hydrodynamics, and accommodation. They support large human populations and important ecosystems, yet many deltas are increasingly threatened by subsidence and sea-level rise. One of the key processes contributing to subsidence is sediment compaction. Compaction is an inherent process in delta systems. However, the impact of syn-depositional compaction during active delta formation over millennial timescales remains poorly constrained. This dissertation investigates how syn-depositional compaction influences delta morphodynamics, sediment distribution, and levee breaching, using a process-based numerical modelling approach. These questions are addressed in Chapters 2, 3, and 4 of this dissertation.

To address the above-mentioned questions, we apply and progressively refine compaction formulations within the Delft3D 4 - FLOW code. This approach focuses on mechanical compaction because it contributes to the largest sediment volume reduction compared to biological and chemical compaction. Additionally, it mainly operates within the active part of the delta top. This type of compaction occurs in two phases, primary and secondary compaction, driven by overburden weight and simulated time. Both phases lead to pore fluid expulsion, resulting in sediment volume reduction and lowering of the bed surface (subsidence). By switching compaction on and off in model simulations, the effects of syn-depositional compaction on delta development are assessed. Quantitative metrics are developed to enable comparison between simulations, including changes in delta geometry, sediment mass distribution, accommodation generation, and sediment erodibility.

Modelling results show that syn-depositional compaction generates additional accommodation during delta development, which alters delta morphology. Morphological changes are more prominent in mud-rich deltas than in sand-rich deltas, which experience larger compaction-induced volume reduction for the same compaction rate scenario. In higher compaction rate scenarios, accommodation increases at the delta top, leading to more sedimentation and more evenly distributed sediment at the delta top. This results in a less significant area increase and a wider delta top with a smoother coastline. These morphological responses emerge from feedback between compaction-induced additional accommodation, sedimentation, and channel dynamics.

Additional accommodation generated by syn-depositional compaction also affects the distribution of sediment mass across delta depositional areas. Modelling results show that increased accommodation on the delta plain promotes sedimentation in this area, thereby reducing sediment delivery to the mouth bar and beyond. Further increases in accommodation lead to enhanced lateral sediment redistribution associated with channel relocation, with sedimentation mainly occurring in the mouth bar. Changes in sedimentation within a depositional area are accompanied by compensating changes elsewhere, indicating interdependencies within the delta-wide sediment budget influenced by syn-depositional compaction. These results demonstrate that compaction-induced accommodation redistributes sediment beyond the immediate subsidence area, affecting sedimentation across the entire delta system.

In addition to generating additional accommodation, syn-depositional compaction increases sediment resistance to resuspension. Levees act as key sediment conduits in delta systems, and the location and timing of levee breaching are commonly assessed using proxies that describe the influence of topography on hydraulic forcing acting on levee deposits. However, the role of sediment properties, particularly levee resistance to resuspension, remains poorly constrained. The modelling results show that commonly used proxies, such as superelevation and gradient advantage, are relevant in predicting when and where levee breaching is initiated, but they are insufficient to describe breach progression, which depends on the balance between flow-induced shear stress and sediment resistance to resuspension. Syn-depositional compaction modifies both bed elevation and sediment erodibility, thereby influencing whether breaches are sustained or abandoned.

Overall, this dissertation demonstrates that syn-depositional compaction is a fundamental process influencing simulated delta evolution over millennial timescales. While numerical models cannot capture all processes operating in natural deltas, they provide a controlled framework to explore process interactions that are difficult to observe directly in the field. The results show that syn-depositional compaction affects delta morphology, sediment distribution, and levee breaching, and therefore represents a critical mechanism that should be included in process-based delta modelling studies.

Syn-sedimentary compaction or consolidation is an important process in deltaic environments because it affects both the local morphodynamics and hydrodynamics as well as the delta-scale accommodation space. However, the impact of syn-depositional compaction on the sediment distribution and the interdependency between different delta areas related to the sediment budget are not fully understood. This paper simulates syn-depositional compaction using improved 1D grain-size compaction formulations, integrated into hydrodynamic and morphodynamic modelling software Delft3D. The updated code is used to model sedimentation in mud-rich deltas under various compaction rate scenarios, which represents the maximum compaction rate potential of sediment that experiences the highest overburden stress in the delta. The simulated deltas are analysed by first classifying their plan-view area development into depositional elements: distributary channel, underfilled channel, delta plain, mouth bar, delta front and pro delta depositional elements. Then, sedimentation by mass, accommodation space and depositional segment metrics are calculated using the interpreted depositional elements. The results for zero compaction rate scenarios (0 mm year−1) show that limited space-varying and temporal-varying accommodation is available to deposit sediment in the delta plain depositional element. Therefore, the sedimentation mainly occurs in the mouth bar depositional element. For low-mid compaction rate scenarios (0.01–1 mm year−1), the additional syn-depositional accommodation space in the delta plain depositional element increases sedimentation in this area, limiting sedimentation in the mouth bar depositional element. For high compaction rate scenarios (>1 mm year−1), a further increase in the accommodation space in the delta plain depositional element leads to lateral sedimentation attributed to channel relocation, where the sedimentation mainly occurs in the mouth bar depositional element. This study shows that, although considered a gradual process, syn-sedimentary compaction does impact long-term delta evolution by influencing the distribution of sedimentation in the delta.

In natural deltaic settings, mixed hydrodynamic forcings and sediment properties are known to influence the preserved delta deposits. One process that has not received much attention yet is syn-sedimentary compaction of clastic sediment on millennial-scale delta evolution. To study how compaction interacts with delta morphodynamics and preserved sediment, a modelling approach is proposed. A 1D grain-size dependent compaction model was implemented into Delft3D-FLOW, which provides an opportunity to understand the underexplored connection between grain sizes supplied to the deltas and sediment compaction. The compaction model allows deposited sediment to decrease in volume due to the accumulation of newly deposited sediments above or the elapsed time. Differences in morphological trends are presented for scenarios defined by the composition of sediment supply (mud rich and sand rich) and the maximum allowed compaction rate in the model (0–10 mm year⁻¹). The resultant deposits are classified into sub-environments: delta top, delta front and pro delta. The delta top geometry (e.g. area increase, rugosity and aspect ratio), sediment distribution alongshore and across sub-environments, and delta top accommodation (e.g. volume reduction and average water depth) are compared. The modelling results show that compaction of the underlying delta front and pro delta deposits increases the average water depth at the delta top, driving morphological variability observed in the mud-rich and sand-rich deltas. The morphological changes are more prominent in the mud-rich deltas, which experience larger compaction-induced volume reduction for the same scenario. Moreover, higher compaction rates further increase the delta top accommodation, resulting in more deposition and evenly distributed sediment at the delta top. This leads to a less significant area increase and a wider delta top with a smoother coastline. The presented modelling results bridge the knowledge gap on the influence of syn-sedimentary compaction on long-term delta morphodynamics and preserved sediment. These findings can be applied to unravel the controlling processes in ancient delta deposits and predict the evolution of modern systems under changing climates.