Hydrodynamic transition of tsunami waves to overland flows

Abstract

The hydrodynamic transition of tsunami waves from offshore propagation to overland inundation remains poorly resolved owing to limited high-resolution measurements of velocity and flow depth within the nearshore region. This study presents large-scale laboratory experiments investigating the hydrodynamic processes governing tsunami waveform evolution from offshore propagation through the nearshore and into overland flow. Froude-scaled tsunami waves were generated using pneumatic tsunami generators in two flume geometries at HR Wallingford: a long run-up slope representing an idealised infinite beach and a short sump geometry representing abrupt topographic transitions such as overtopped seawalls or inland depressions. Results suggest the presence of a distinct hydrodynamic transition during tsunami inundation, characterised by nonlinear offshore waveform evolution, rapid attenuation of momentum flux within the shallow nearshore region, and systematic changes in the phase relationship between flow depth and velocity. Tsunami-length waves exhibit evidence of nonlinear amplitude growth during offshore propagation despite nominally constant-depth conditions, indicating that offshore scalar elevation records alone may not fully represent the incident conditions governing subsequent inundation. In the shallow nearshore region, momentum flux decreases rapidly, suggesting a friction-dominated transition prior to shoreline impingement. Offshore, peak velocity generally precedes maximum wave amplitude, whereas the timing of these maxima progressively converges toward the shoreline. Under the tested long-wave conditions, measured near-bed shoreline velocities remained generally subcritical, suggesting that simplified critical-flow assumptions adopted in some engineering loading approaches (e.g., ASCE 7-22 load cases 2 and 3) may not fully represent all inundation regimes. Abrupt onshore topographic transitions further modify inundation behaviour through localised energy losses, suggesting that simplified gradually varied flow formulations may under-represent loading in such regions. Collectively, these findings suggest that the nearshore region acts as a friction-dominated hydrodynamic transition zone that fundamentally modifies tsunami inundation processes. Explicitly resolving this transition may improve definition of shoreline hydrodynamic loading conditions and reduce uncertainty in tsunami load estimation and coastal infrastructure design.