Offshore breaking of impact tsunami: The Van Dorn effect revisited

We report on calculations of the shoaling and off‐shore breaking of typical wavetrains from sub‐km impactors into the deep ocean. We use the COULWAVE code to compute the propagation of waves through simple bathymetry profiles typical of the North American Pacific coast and the Gulf of Mexico. Numerical results are consistent with those predicted by nonlinear shoaling theory. Our primary result is that large long‐period waves of the type considered should indeed break far offshore, as suggested by W. G. Van Dorn for similar waves generated by underwater explosions. Typical breaking distances range from ≈3–17 km for the Pacific coast, and up to ∼200 km for the Gulf coast. The inclusion of bottom friction affects the results; for very gentle slopes like the Gulf coast, a modest amount of bottom friction supplies enough dissipation to suppress wave breaking.