Assessing coastal wave energy and the geomorphic evolution of rocky coasts.

摘要

Coastal wave energy is an important control on the geomorphic appearance and evolution of a rocky coast. To understand the influence of wave energy on coastal geomorphology, this study: documented microseismic sea cliff shaking response to changes in wave climate, identified a potential rock-fatigue mechanism linking cyclical wave loading and sea cliff strain, investigated the role of nearshore sediment in dissipating coastal wave energy, and presented a numerical model suitable to explore the long-term planform evolution of a rocky coast bearing an alongshore lithologic contrast.; Microseismic measurements of wave-induced shaking sea cliffs reveal how variations in deep water wave height, period, direction, and tide level dictate the temporal pattern of geomorphically-useful wave energy to the coast. By comparing a model that computes wave transformation and the time series record of deep-water wave character and tide, I show that shoaling and refraction greatly modulate wave power delivered to the coast, but energy dissipated by bottom drag across the shelf is relatively small. Much energy is dissipated in the surf zone, accounting for the strong dependence of cliff shaking on the tide.; Because the nature of wave energy delivery is episodic and concentrated, I examined data from a nearshore wave gage and sea cliff seismometers to understand sea cliff response to nearshore wave field. High frequency energy of sea cliff ground motion (20Hz) reflects the natural frequency of the sea cliff's ring in response to direct impact by the broken wave bore, whereas the low frequency energy in the 0.1 to 0.05 Hz (10--20 s) band of the microseismic data was consistently in agreement with the peak frequency power in the nearshore wave gage data. Mean sea cliff displacements of 50 mum (horizontal) and 10 mum (vertical) defined a displacement ellipse that exhibits downward and seaward sea cliff motion. Video footage suggested that water rushes upon the wavecut platform flexing the sea cliff downward in response to the imposed load of the wave bore. Two seismometers record flexing sea cliff rock on the order of 0.5 to 4 mustrains during loading and unloading of water from broken wave runup. These data identify a potentially effective process of fatiguing sea cliff bedrock through cyclical loading by broken waves. (Abstract shortened by UMI.)