Large-amplitude internal waves near a wharf: Observations and implications for navigation

Large-amplitude, nonlinear internal waves are rarely considered in operational navigation, yet they can generate strong and rapidly varying currents in stratified coastal environments. This study investigates their potential impact on ship manoeuvrability near the Grande-Anse Terminal in the Saguenay Fjord (QC, Canada), motivated by a 2019 docking incident involving a cargo vessel under otherwise calm conditions. Field measurements were collected during fall 2023 and summer 2024 using ADCPs, CTDs, an echosounder, shore-based imaging, and a drone. The measurements reveal the frequent occurrence of large-amplitude internal wavetrains, observed almost daily and phase-locked with the semi-diurnal tidal cycle. These waves propagate across the fjord and interact with the wharf, generating rapid current fluctuations on time scales of one to two minutes. Wave amplitudes of up to 10 m and associated horizontal current variations approaching 1 m/s were measured, significantly larger than previously reported in this region. Such conditions can substantially alter the local flow field and potentially affect ship manoeuvrability during critical operations such as docking. These results demonstrate that internal waves constitute a previously underappreciated source of current variability in stratified coastal environments and highlight the need to account for their effects in pilot training, navigation guidance, and ship-handling simulators.