Hydrodynamic response induced by free-surface flow over rippled seabed morphology in coastal regions

In many coastal and estuarial areas, large-scale continuous submarine sandbars have been measured in the past few decades. Besides, strong tidal, nearshore, or runoff currents also exist in these regions. Due to the co-existence of free-surface water waves, ambient flow, and rippled seabed morphology, the hydrodynamic characteristics, especially the interaction effects among these elements, are very complex. Previously, the interactions above were usually studied separately in coastal hydrodynamics, i.e., Bragg resonance between water waves and rippled bottoms, as well as the wave-current interaction.

If we consider wave-current-bottom interactions from the perspective of wave hydrodynamics, the rippled seabed morphology could alter the wave-current interaction behaviors and even induce intensive resonant interactions among free surface waves, ambient currents, and rippled topography. The corresponding free-surface wave field and the flow field under the free surface will be affected if these intense resonances are triggered.

As one specific phenomenon, the upstream-propagating waves were observed in flume experiments for steady free-surface flow over rippled bottoms. The study for this specific wave component provides new insights into wave-current-bottom resonant interaction. We have performed a series of flume experiments, in which the different flow depths and flow velocities were adjusted above the rippled topography. Under a specific range of flow conditions, the new free-surface wave components are induced to propagate upstream continuously.

This specific resonance-induced hydrodynamic phenomenon could also be induced in estuarial areas, potentially. With the existence of rippled seabed morphology and tidal/runoff currents, this kind of wave stimulation might affect the wave field if it is triggered. However, the generation of this new wave component on the free surface not only depends on the resonant condition but also relies on some specific critical conditions (i.e., the critical flow velocity of wave energy stagnation for upstream-propagating waves).

In this study, based on the parameters of continuous submarine sandbars, flow velocity, and water depth conditions in typical estuarial and coastal areas, the potential conditions and parameter range for triggering the new free-surface wave components are calculated and evaluated for various resonant combinations for steady flow over rippled bottoms. The associated critical flow conditions underlying the resonant conditions are also discussed, along with the temporal evolution and spatial distribution behaviors of the resonant free-surface wave components involved or induced.