Storm surge barrier Tokyo Bay

Abstract

Tokyo Bay is an area exposed to storm surges generated by typhoons. Consequently lowlands along its coastline are subject to flood risk. Dike walls, levees and barriers have been developed by the Japanese to protect these coastal areas. In the coming decades it is expected that a rise in sea level will occur. Also, climate change is expected to induce an increase in intensity of typhoons and therefore larger storm surges. These two circumstances would cause a need for the coast to be protected from higher water levels. The question arises whether the coastal defences of Tokyo Bay will be able to resist these future conditions in a satisfactory manner. In case there is a need for taking measures, a storm surge barrier could be an alternative course of action instead of dikes. The aim of this project has been to determine whether a storm surge barrier is a good solution for the flood protection of Tokyo Bay in the future. Moreover, a proposal for a conceptual design has been developed. The design addresses the challenges that this barrier would bring, mainly the large depths and the high seismicity of the area. Previous studies have analyzed the cost of upgrading coastal defences, considering conditions for the year 2100 and a typhoon of approximated return period of 100 years. In this report, this cost is compared to the cost of building a storm surge barrier, for the same scenario. Different locations have been studied for the barrier and the optimal one has been found to be close to the bay mouth, near cape Futtsu. The report also analyzes the convenience of using certain design conditions (sea level rise, typhoon return period). As a result, it is advised to consider as a safety standard larger return periods than the ones the Japanese have used up till now (100-200 years). Besides the cost calculation, a multi-criteria analysis is performed in order to compare all the considered options. The result is that barriers do not surpass coastal defences upgrading, if a return period of 100 years is considered. However, in case larger design return periods are applied, as recommended here, the barrier is supposed to increase its advantages, compared to the upgrading of coastal defences. The result of these considerations is a proposal for a barrier close to the bay mouth and built for a return period of 500 years. Once the overall design conditions are set for the hypothetical barrier, a layout is defined in order to satisfy the contemplated requirements and functions. The main requirements are flood protection (control water levels inside the bay during typhoons), navigational (allow shipping) and environmental (allow sufficient water exchange to protect the ecosystem inside the bay). The proposed barrier that fulfills these requirements is composed of a dam, a movable barrier part and a permanent opening. Finally, a conceptual design is developed. Part of the barrier is a dam, which would cross a long and deep section of the bay. The aim is to present an innovative design that offers advantages compared to standard solutions (rubble mound section, use of geocontainers). This thesis proposes a sand-filled geotextile structure for the dam core. This design offers a good performance during the dam lifetime, at a price that makes it competitive. The geotextile structure is composed of vertical cells and is expected to offer interesting advantages, like cost savings compared to a quarry run standard solution. Also, the expected performance during its lifetime improves with respect to other geotextile technologies, such as geocontainers. The proposed design allows for compacting the filling and avoiding gaps in the core of the structure, which is expected to help the stability of the dam and reduce the damage in case an earthquake happens.