Coastal Structure Failures and Coastal Waves on the North Coast of Taiwan due to Typhoon Herb

Abstract

Super Typhoon Herb caused tremendous damages on the coastal defense structures and severe flooding near and at the National Taiwan Ocean University (NTOU) on the north coast of Taiwan. This article examined nearby structure failures, coastal waves, and water level conditions during this typhoon. It was found that the damaged sections of the coastal structures were related to coastal bathymetry, the severe seas, and great overtopping flows. A wave gauge, which was located 17 km east of NTOU and 30 km from the track of the typhoon center, recorded sea conditions at a mean depth of 11.5 m. The data showed that the recorded highest significant wave height during the storm was 7.8 m, which occurred during the passage of the typhoon center. The maximum wave height at that time was about 10.5 m, close to the breaking limit. It was also found that the time evolution of the wave conditions, including energy spectra, correlated well with the movement of the storm. Right after the typhoon center passed, the wave height quickly died down, much faster than the build-up rate of waves before the center passed. Water level analysis indicated that, historically, the highest storm surge at least in the past 40 years and high wave setups had increased the water level by more than 2 m in the shallow water. Together with high waves and over 5 hours of large overtopping flow rates (greater than 0.02 m3/s/m), superstructures of seawalls and scoured lee-side armor units of breakwaters were broken.