[M9] Lecture and exercise Wave modeling - Lecture Introduction to Delft3D FM part 2 [50:00]

Summary

This video explores the Del 3D wave model used for simulating wave and current interactions in coastal environments. It highlights the challenges of modeling variable tidal and wave conditions, and the importance of reducing wave conditions to manageable levels due to computational limitations. The video covers the setup of wave and current grids, boundary conditions, and the interactions between waves and flow, such as vertical mixing, longshore currents, and rip currents. It also provides insights into the Del 3D user interface, model calibration, and the significance of wave-current interactions in coastal modeling.

Takeaways

• 😀 The Del 3D model setup is designed to simulate wave-current interactions in coastal regions with tidal and wave conditions, ensuring computational efficiency.
• 😀 Running simulations for every tide and wave condition is too expensive, so the model simplifies the wave climate into manageable conditions.
• 😀 Wave schematization reduces complex wave conditions (e.g., varying heights, periods, directions) to a smaller set of representative wave conditions for simulation.
• 😀 The model setup includes defining boundary conditions, wave grid resolution, and spectral space to accurately simulate wave propagation and currents.
• 😀 Sensitivity studies are crucial to ensure model convergence, assess boundary effects, and validate physical parameters like breaker models and bottom friction.
• 😀 High-resolution grids are necessary in the nearshore areas to accurately capture wave-breaking, dissipation, and wave-driven currents.
• 😀 The Del 3D interface allows users to select grid settings, define spectral spaces, and input boundary conditions to set up wave and flow models.
• 😀 Wave-current interactions are modeled using complex physics to account for vertical mixing, mass flux, and momentum flux, especially in the surf zone.
• 😀 Waves induce current profiles, including longshore and cross-shore currents, which are important for understanding coastal dynamics and potential hazards like rip currents.
• 😀 Proper boundary conditions, including constant wave conditions at depth contours, help simplify the computational model while maintaining accuracy for morphological simulations.
• 😀 The flow model and wave model communicate via a communication file, with wave forces and orbital velocities passing to the flow model, which produces water level and current data.

Q & A

What is the primary challenge when running Del 3D wave models for morphological studies?

The main challenge is handling highly variable wave and tidal conditions while maintaining computational efficiency. Running models for every tide and wave condition would be too expensive, so the wave climate must be simplified into representative conditions.

How are tidal cycles represented in Del 3D models?

In Del 3D models, tidal cycles are typically schematized into a simplified cycle that is representative of the spring-neap cycle. The water level fluctuates above and below the mean tide level, with the goal of capturing the key tidal dynamics without requiring excessive computational resources.

What is the purpose of wave schematization in the context of Del 3D modeling?

Wave schematization is used to simplify the complex wave climate by grouping waves into classes based on height, direction, and frequency. This allows for a manageable number of wave conditions to be used in simulations, ensuring computational efficiency while still accurately representing the wave environment.

What is the key aspect to consider when setting up nested grids in Del 3D models?

The key aspect is ensuring that the resolution is higher in areas where detailed wave-current interactions are critical, such as in the nearshore region. Nested grids allow for finer resolution in areas of interest while maintaining a coarser resolution for the rest of the domain.

Why is it important to reduce the number of wave simulations in Del 3D models?

Reducing the number of wave simulations is crucial to avoid computational overload. By simplifying the wave conditions to a manageable number, modelers can perform simulations over tidal cycles and various wave conditions without excessive computational cost, which is vital for large-scale studies.

How are wave boundary conditions typically defined in Del 3D models?

Wave boundary conditions are typically defined using parametric values such as significant wave height, peak period, and direction. These conditions are often kept constant along the boundary for morphological studies, although more complex, time-varying spectra may be used for specific cases.

What role do bottom friction and wave breaking play in Del 3D modeling?

Bottom friction and wave breaking are crucial for accurately representing wave dynamics in shallow waters. The model often uses the Buchan and Johnson model for depth-induced wave breaking, and bottom friction is typically activated using a John Swap model to simulate energy dissipation in nearshore environments.

How does the wave-current interaction affect the flow model?

Wave-current interactions enhance vertical mixing near the surface and bottom, influencing the current profile, especially in the cross-shore direction. These interactions can also generate longshore currents and rip currents, which are critical to understanding sediment transport and coastal dynamics.

What is the significance of the 'com file' in Del 3D simulations?

The 'com file' serves as the communication file that passes data between the wave and flow models. It ensures that variables like wave forces, orbital velocities, water levels, and currents are exchanged between the two modules, enabling an accurate representation of wave-current interactions.

How can structures, like thin dams, be represented in Del 3D wave models?

Structures such as thin dams need to be represented as obstacles in both the flow and wave models. A polygon is created to define the structure's location, ensuring that it appears in both models at the same position to avoid inconsistencies in simulation results.