Tidal Effects on Modal Parameters of a Pile-Supported Concrete Quay Using Operational Modal Analysis

This paper investigates the influence of tidal water level fluctuation on the modal parameters of a pile-supported concrete ferry quay on the Norwegian coast. Ambient vibration data collected continuously over a 24-hour tidal cycle (range: 2.093\,m), excluding ferry docking periods, from 16 accelerometers are analysed using covariance-driven stochastic subspace identification applied to 10-minute windows. Five dominant vibration modes are identified in the 0.5--10~Hz range. The fundamental mode (lateral sway) at approximately 2.74~Hz shows a significant negative correlation with water level ($r = -0.76$, $p < 0.001$), with a sensitivity of $-0.074$~Hz/m, consistent with hydrodynamic added mass effects on the submerged piles. Damping of the first mode increases with water level ($r = 0.52$, $p < 0.001$), indicating that fluid-structure interaction contributes to energy dissipation. Modes 2--5 (second lateral, coupled lateral-longitudinal, and higher-order bending modes) show no statistically significant correlation with water level. The added mass ratio estimated from the frequency shift increases approximately linearly from 2\% at low tide to 15--19\% at high tide. The observed tidal variability of up to 5.5\% of the mean frequency has direct implications for vibration-based structural health monitoring of marine structures.