An imagery-based monitoring system was developed for nearshore topography. The system was based on the wave breaking density observed using a land-based camera and calculated the bathymetry using a depth-controlled breaking wave model. The system was applied to the Fukude-Asaba Coast, Shizuoka Prefecture, where rapid topography change was expected around the outlet of pipeline-based sand bypassing. The validity of the system was verified through comparisons with bathymetry survey data. It is observed that the system can successfully capture the nearshore bathymetry change owing to a typhoon. It is also suggested that the accuracy is sensitive to the incident wave properties and therefore can be improved by introducing selective averaging in which only appropriate imageries were used for the depth estimation depending on the target depth zones.

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In this study, field surveys along the coasts of Wakayama Prefecture, Japan, were first conducted to investigate the coastal damage due to storm surges and storm-induced waves caused by the 2018 Typhoons Jebi and Trami. Special focus was placed on the characteristic behavior of nearshore waves through investigation of observed data, numerical simulations, and image analysis of video footage recorded on the coasts. The survey results indicated that inundation, wave overtopping, and drift debris caused by violent storm-induced waves were the dominant factors causing coastal damage. Results of numerical simulations showed that heights of storm-induced waves were predominantly greater than storm surge heights along the entire coast of Wakayama in both typhoons. However, computed gradual alongshore variations in wave and surge heights did not explain locally-concentrated inundation and run-up heights observed along the coasts. These results indicate that complex nearshore hydrodynamics induced by local nearshore bathymetry might have played a significant role in inducing such local wave characteristics and the associated coastal damage. Analysis of video footage recorded during Typhoon Jebi, for example, clearly showed evidence of amplified infragravity wave components, which could enhance inundation and wave run-up.

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