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Hydrodynamic models are often used to obtain insights into potential dike breaches, because dike breaches can have severe consequences. However, their high computational cost makes them unsuitable for real-time flood forecasting. Machine learning models are a promising alternative, as they offer reasonable accuracy at a significant reduction in computation time. In this study, we explore the effectiveness of a Long Short-Term Memory (LSTM) neural network in fast flood modelling for a dike breach in the Netherlands, using training data from a 1D–2D hydrodynamic model. The LSTM uses the outflow hydrograph of the dike breach as input and produces water depths on all grid cells in the hinterland for all time steps as output. The results show that the LSTM accurately reflects the behaviour of overland flow: from fast rising and high water depths near the breach to slowly rising and lower water depths further away. The water depth prediction is very accurate (MAE = 0.045 m, RMSE = 0.13 m), and the inundation extent closely matches that of the hydrodynamic model throughout the flood event (Critical Success Index = 94%). We conclude that machine learning techniques are suitable for fast modelling of the complex dynamics of dike breach floods. [ABSTRACT FROM AUTHOR]

Estuaries worldwide are susceptible and adapting to climate change (CC) impacts from both the river and coastal boundaries. Furthermore, engineering efforts are undertaken to improve flood safety, to claim land for human use or for port operations, which change estuary morphology. This paper aims to gain an understanding of the combined effects of CC and human interventions on the estuarine‐wide morphological response by analyzing the sediment infilling of highly engineered estuaries. A schematized process‐based morphodynamic model is used (Delft3D‐FM, in 2DH), resembling a highly engineered estuary in the Rhine‐Meuse Delta, The Netherlands. Three types of changes were implemented, both in isolation and in combination: (a) local interventions (changing channel depth or wetland area), (b) upstream human interventions (changing fluvial sediment supply) and (c) extreme CC scenarios (with projections for the future forcings and bathymetry). Results show that a CC scenario can elicit both positive and negative changes in the estuary's sediment budget. The direction and magnitude of the change depend on the local intervention and can align with the effect of the local intervention, intensifying its impact. The combined effects can even reverse the sign of the sediment budget. This stresses the need of analyzing CC impacts in combination with human interventions. Additionally, a relationship was identified which quantifies how a change in peak flow velocity due to both local interventions and sea‐level rise affects the annual sediment budget. These findings can help determine how local interventions affect morphodynamics of engineered estuaries in present and future climates. Plain Language Summary: Estuaries are the regions where the rivers meet the sea. They are susceptible to climate change (CC) impacts to both the sea and river. Additionally, as these regions are among the most populated regions worldwide, they undergo constant human interventions to utilize the region's natural resources optimally. This study investigates the combined impacts of CC and human interventions on the development of estuaries by analyzing the amount of sediment accumulating or eroding from the estuary. The findings reveal that CC can intensify the influence of local interventions on the estuary's sedimentation. If the estuary has high sedimentation rates in the present, due to activities like channel deepening or wetland reclamation, sedimentation increases in the future. Conversely, if the estuary experiences little sedimentation, a decrease is observed in the future. These results highlight the importance of studying CC impacts in combination with engineering measures. As such, these findings contribute valuable insights for effective estuarine management and planning in the future. Key Points: Climate change can increase or decrease the annual sediment budget of estuaries, depending on the estuary's local interventionsThe estuary's sediment budget was found to decrease exponentially with the peak ebb flow velocityLocal interventions and relative sea‐level rise affect the annual sediment budget following the exponential relationship [ABSTRACT FROM AUTHOR]

Tide‐dominated sandy shelf seas, such as the Dutch North Sea, are covered by sand waves. Yet, basin‐scale hydrodynamic models do not include any sand wave information because their grid sizes are too coarse to resolve sand waves individually. We explore the possibility of parametrizing the effects of sand waves on the larger‐scale tidal flow by means of a form roughness. Specifically, our aim is to see to what extent the flow over a sand wave field can be reproduced by that over a flat seabed with an increased effective roughness (accounting for both grain and form roughness). To do so, we use two process‐based hydrodynamic models: a second order perturbation approach, and Delft3D. Both models demonstrate that the presence of sand waves causes amplitude decrease and phase shift of the tidal flow. We explore the dependencies of form roughness on different sand wave characteristics (wavelength, height and asymmetry). Shorter and higher sand waves cause a higher form roughness, while our analysis does not reveal any dependency on sand wave asymmetry. Notably, the consideration of a tidal flow, characterized by several tidal constituents, each represented by an amplitude and a phase, results in a more complex form roughness analysis than in a fluvial setting, where the flow is unidirectional and steady. We thus obtain an amplitude‐based form roughness and a phase‐based form roughness, each yielding a different value, yet displaying the same qualitative dependencies. Plain Language Summary: Tide‐dominated seas, such as the Dutch North Sea, are often covered by tidal sand waves, that is, seabed undulations characterized by wavelengths of hundreds of meters, heights of several meters and migration rates of several meters per year. The presence of sand waves causes an increase in the roughness experienced by the tidal flow. As a result, the tidal flow may for example, slow down or lower in amplitude. Yet, the effects of sand waves are generally not considered in larger‐scale hydrodynamic models. We explore the possibility of representing the effects of sand waves through an increase in bed roughness on a flat seabed, that is, a form roughness. To do so, we use two process‐based hydrodynamic models. Results show that higher and shorter sand waves result in a higher form roughness. Furthermore, the unsteady nature of the tidal flow gives rise to multiple criteria to determine form roughness. These criteria yield different values for form roughness, highlighting the complexities of considering form roughness in a tidal setting. Key Points: The characteristics of tidal flow (unsteady, multiple tidal constituents) make studying form roughness more complex than in a river settingThe presence of sand waves causes amplitude decrease and phase shift of the tidal flow through an increased effective roughnessHigher and shorter sand waves result in an increased form roughness [ABSTRACT FROM AUTHOR]

In bifurcating rivers, an intervention aimed at flood risk reduction may trigger a change in discharge distribution and thus influence water levels throughout the entire river system. This article aims at assessing the impact of interventions on system‐wide water levels, explicitly accounting for a range of discharges and model parameter uncertainty. An idealized 1D model with dimensions of the bifurcating Dutch Rhine River is used. The results show that an unwanted increase in water levels downstream of the intervention occurs due to an increased discharge if a single intervention is implemented in a distributary. This effect can be counteracted by implementing a second intervention that balances the hydraulic effect of the first intervention at the bifurcation. However, unwanted water level increases still occur at other discharges. Furthermore, while interventions may reduce local water‐level‐uncertainty, it appears that uncertainty in discharge distribution is not reduced. This implies that flooding probabilities cannot be reduced throughout the entire river system by the implementation of interventions in upstream reaches. Concluding, for intervention design in a bifurcating river, it is important to consider the entire river system and explicitly account for a range of discharge conditions to avoid unwanted water level increases throughout the river system. [ABSTRACT FROM AUTHOR]