A simulation study on the vertical structural evolution of seabed sediment erodibility

This study focuses on the simulation of vertical structure evolution in seabed erodibility under wave–current interactions in estuarine areas. Accurate representation of sediment erodibility in numerical models is essential for predicting sediment transport and morphological changes. A "consolidation–erosion" sediment model is employed, which comprehensively incorporates sediment erosion, resuspension, settling, and consolidation processes. The model simulates the evolution of bed erodibility under various hydrodynamic conditions, including still water, spring–neap tidal variations, storm surges, and their combined effects. The primary formation mechanisms of easily erodible surface sediments are examined based on the simulation results. The influence of periodic shear stress variations and historical shear stresses on the vertical seabed structure is analyzed. Additionally, the evolution of sediment erodibility in vertical profiles is examined from the perspectives of internal consolidation time and external shear stress cycles.

The model reveals that under periodic shear stress, a fresh sediment layer with lower critical erosion shear stress forms on the seabed surface, making it more prone to erosion. This fresh layer, primarily composed of easily erodible sediments, develops through cyclic erosion and deposition processes driven by tidal and storm events. The thickness of this layer is chiefly governed by the magnitude of the maximum shear stress experienced during these events. Notably, sediments in this fresh layer exhibit lower critical erosion shear stress than those in the underlying, more consolidated layers, resulting in heightened erodibility.

The vertical structure of the seabed displays distinct evolution patterns depending on the interaction between the consolidation time of seabed sediments (internal time, T_I) and the periodic variation of external shear stress (external time, T_O). Specifically, T_O is longer than or equal to T_I, the seabed may regain stability before the next stress peak, thus influencing the extent of sediment erosion. In such cases, if the subsequent cycle's peak shear stress is comparable to the previous one, newly deposited sediments, though partially consolidated, may be re-eroded and resuspended into the water column. Conversely, if the next peak shear stress is smaller, the newly consolidated sediments will undergo partial erosion, resulting in a reduced maximum erosion depth. If the subsequent peak shear stress is larger, not only will the newly consolidated sediments be fully eroded, but sediments below the prior maximum erosion depth will also be partially eroded, increasing the maximum erosion depth.

When T_O is shorter than T_I, newly deposited sediments may not fully consolidate, leading to greater erodibility and deeper erosion during subsequent stress peaks. Under these conditions, even if the next peak shear stress is smaller than the maximum critical erosion shear stress of the newly deposited layer, the maximum erosion depth may still increase. However, due to insufficient consolidation, seabed erodibility remains high, resulting in a deeper erosion depth compared to scenarios where T_O is greater than or equal to T_I. If the next peak shear stress is larger, the newly deposited sediments will be fully eroded, and erosion will extend into the older, more consolidated layers.

Furthermore, the study finds that long-period shear stress leads to thicker sediment deposition with better consolidation and lower erodibility. In contrast, short-period shear stress results in thinner, less consolidated sediments that are more easily eroded. This difference arises from the longer deposition phases associated with long-period shear stress, allowing greater sediment accumulation and consolidation. Despite the thicker deposition, the simulated erosion and consolidation processes show that sediments at the same depth under long-period stress are more resistant to erosion due to extended consolidation time.

The historical shear stress also plays a significant role in determining the seabed's vertical structure evolution. When the historical peak shear stress is lower than the current peak, the current stress controls the maximum erosion depth. However, if the historical peak shear stress is higher, the vertical evolution within the erosion depth is influenced by both historical and current stresses.

In summary, this research provides key insights into the vertical structure evolution of seabed erodibility under complex hydrodynamic conditions. The findings highlight how the interaction between internal consolidation time and external shear stress cycles critically shapes seabed erodibility and structure. These results offer theoretical guidance for parameter selection in sediment–water numerical models, vital for accurately predicting sediment transport and morphological changes in estuarine and coastal systems. By improving our understanding of seabed erodibility, this study contributes to improved management and mitigation of sediment-related hazards, such as coastal erosion and sedimentation in navigation channels.