Presenter: Jae-Soon Jeong (Hiroshima University)

Description:
The Itsukushima Shrine located in Hiroshima Bay is preserved as one of the World Heritage Sites for its high cultural value in Japan. The shrine was built offshore, 30 cm above the highest tide to prevent it from being submerged. However, from 1991 to 2012, the shrine has been submerged 6 times during September due to abnormal high tides enhanced by internal surge at high tide time. The surge was caused by a remote typhoon passing through an area 400 km east of Hiroshima Bay. To study the precise mechanism of this surge, we applied the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) in this bay. The SCHISM utilizes unstructured grids and its numerical stability was enhanced by adopting a semi-implicit time stepping scheme. Ota River, the source of freshwater in Hiroshima Bay, has narrow tributaries, so it demands a more high-resolution stable model for its modelling. To simulate the internal surge that occurred on September 23, 2013, atmospheric external forcings are adopted from meso-scale model results from the Japan Meteorological Agency. Modelling results with SCHISM reproduced the internal surge in the bay caused by constant northerly winds due to the remote typhoon passage. A surface mixed layer in the stratified sea water in summer was transported towards south by the northerly winds. So, the mixed layer became thicker in the south and thinner in the north, which made the stratification below unstable. After the typhoon disappeared, the unstable stratification adjusted to its equilibrium state. During the adjustment, internal waves propagated toward the south, and came back to the northern part of the bay after 5-6 days. If this wave and high tide were overlapped, internal surge will likely occur. The SCHISM successfully applied to the internal surge event in Hiroshima Bay. It is also shown that such internal surge can be occurred in a semi-enclosed bay under certain circumstances, and it can be predicted for the safe management.