Ship-induced edge waves: Reflected and trapped depression waves at waterway edges

With the increasing size of modern vessels, destructive long-period ship-induced waves are being reported more frequently, causing damage to adjacent waterways, promoting erosion, and posing risks to human safety and natural ecosystems. Particularly in shallow areas near embankments, unexpectedly large waves have been observed. Recent insights into the long-period components of ship wakes suggest that ship-generated edge waves, trapped along waterway margins, are a key mechanism that must be considered to fully understand ship-wave impacts. Edge waves can be viewed as long-period waves that reflect off the shoreline and subsequently re-reflect from an offshore depth transition. This repeated reflection can generate long sequences of waves traveling parallel to the coast. This work investigates whether the above outlined reflection mechanism applies to ship-induced edge waves. Results from a parametric study with the depth-averaged numerical tools FUNWAVE and REEF3D::SFLOW indicate that reflection of the inverse wave at the super-sub-critical depth-Froude boundary (caustic) governs the near-bank trailing wave system. These reflections of the ship-generated drawdown in shallow water determine both the wave heights and wave lengths along channel margins. The main findings show that edge waves' lengths depend solely on ship speed and local bathymetry and are independent of the vessel geometry itself. Moreover, under idealized conditions, near-bank wave heights may increase by up to a factor of four relative to the offshore wave height due to repeated reflections in shallow-water regions, underscoring the need to account for edge waves when predicting loads on waterways.