Performance evaluation of pedestrian bridge as vertical evacuation site during the 2011 tsunami in Japan

Abstract

  We evaluated the performance of pedestrian bridges as vertical evacuation sites during the 2011 Tohoku tsunami in six prefectures in Japan. Evaluation was performed by considering the vulnerability and tsunami hazard characteristics as parameters that influence the damage probability. Vulnerability is represented by the exposure, indicated by the bridge’s position from the shoreline relative to the maximum inundation distance. In contrast, tsunami hazard was identified from the observed flow depths relative to the bridge’s height. We found that pedestrian bridges that were positioned inside an area less than 0.2 times that of the maximum inundation distance (which is 6 km), or where the tsunami flow depth exceeded that of the bridge’s height by more than 1.5 times, have a high probability of being damaged by a tsunami. It is necessary to consider such limitations when determining the placement criteria of pedestrian bridges as evacuation shelters for the above areas. Starting with the concept of sudden expansion phenomenon, a series of numerical exercises was conducted to parameterize hydraulic conditions that affect the sudden drop in water level due to the building configuration at an intersection. We searched the areas where pedestrian bridges could still be used for evacuation with the existing height. Preliminary results indicate that if the sudden expansion ratio (D) > 1.5, and the ratio between the widths of road to the width of residential blocks (β) < 0.1, sudden expansion occurs with hydraulic gradient dependent on the Froude number (Fr). In this condition, the bridges can be used for evacuation as long as they are installed on the side of the road parallel to the coastline, or on the landward side of the road at an intersection. These results provide the opportunity to apply more distributed evacuation shelters to relatively flat and populated areas.