Response of the Eddystone Lighthouse to storm-wave impacts

Abstract

Structural response to wave action is difficult to predict due to complex wave–structure interactions, particularly for structures subject to impulsive loads from breaking waves such as rock lighthouses. This study examines the response of the exposed Eddystone Lighthouse, located some 21 km offshore from Plymouth, UK, using field measurements from geophones, video cameras and an offshore wave buoy. Accelerations and displacements are derived from geophone velocity measurements recorded just below the lantern during 3085 response events in the exceptional UK winter storms of 2013–2014. These are complemented by video records of breaking waves with estimated heights up to 10–15 m. Response characteristics are shown to vary systematically with the frequency content, and four primary response types are described: strongly-impulsive, relatively-impulsive, slightly-impulsive, and non-impulsive. Video observations indicate that the frequency content depends on how waves approach the lighthouse. The largest accelerations, up to 0.1g, are associated with plunging waves breaking on or immediately in front of the structure. However, maximum velocities and accelerations are not associated with maximum displacements (∼0.1 mm). Deflection amplitude is largely controlled by depth-limited conditions at the toe of the lighthouse, probably by way of their influence on breaking wave height, impact extent, force-line elevation, aeration and impact duration.