The Delta21 tidal lake: towards a dynamic equilibrium

Abstract

The goal of this research is to understand the impact of the Delta21 construction on the hydrology and morphology of the current situation. Such understanding helps reveal how the system moves toward a dynamic equilibrium, which is crucial for developing a stable tidal lake. The study employs numerical modeling using Delft3D to simulate the hydrodynamics and sediment transport within the system. The effects of the inlet width and hydraulic forces on sediment transport and bed evolution are investigated to identify how these factors control erosion, sedimentation and long-term morphological changes.

The model focuses on three main hydraulic forces: tidal motion, river discharge, and operational flows from pumps and turbines. To simplify the system, wind and wave effects are excluded, as the Delta21 construction largely protects the tidal lake from incoming waves. A representative tide is imposed, neglecting spring–neap tidal variability, and river discharge is held constant at 1000 m³/s. Pumps and turbines are modeled using maximum discharges rather than realistic fluctuating operations. The sediment transport is limited to sand with a median grain size of 160 µm, with only a single hydrodynamic layer and one active sediment layer. These simplifications allow the study to capture general morphodynamic trends while recognizing that small-scale and those involving finer sediments or varying tides and river flows, are not resolved.

Results show that inlet dimensions critically control flow velocities and sediment dynamics. Narrow inlets accelerate flow, causing substantial erosion, while wider inlets reduce velocities and promote deposition. Because the inlet is a fixed hard structure, the width cannot adjust naturally; the only way for the system to increase the inlet cross-section is by eroding its bed. The equilibrium depth emerged as a key concept: it represents the depth at which erosion stops. Once this depth is reached, cumulative sediment transport patterns indicate that the system may begin importing sediment from outside, suggesting a potential shift toward flood-dominant behavior. In the simulations, only the 2000 m inlet without river discharge approached equilibrium and showed signs of flood-dominant sediment transport. The 1500 m inlet eroded toward equilibrium but had not yet shifted to flood dominance, while all scenarios with river discharge remained ebb-dominant, exporting sediment.

Future developments, such as sea level rise and increasing extreme river discharges, influence morphological stability. Sea level rise drives long-term sediment loss, whereas extreme discharges induce short-term sediment redistribution inside the tidal lake. These findings highlight the importance of designing inlet dimensions to maintain flow velocities near the critical threshold for sediment transport and of understanding equilibrium depth dynamics to guide the system toward long-term morphological stability.

In conclusion, the research demonstrates that inlet geometry and hydraulic forcing strongly influence the morphodynamic evolution of the tidal lake. Simplified numerical models, informed by natural system analogies, can effectively support the design of engineered tidal lakes and provide insight into the hydrodynamic and morphodynamic processes that control inlet evolution, sediment transport, and overall system stability.