The combined effect of an increase in typhoon intensity and sea level rise could pose significant challenges to coastal defences around Tokyo Bay. In the present chapter the authors determined the storm surge that could result from an increase in typhoon intensity around Tokyo Bay around the turn of the twenty-first century. This was then combined with a variety of sea level rise scenarios to obtain potential water levels for a 1 in 100 year design storm in the year 2100. The results show that the various settlements around Tokyo Bay, and particularly those along the low-lying areas in the Koto delta in Tokyo, are at considerable risk of storm surges and sea level rise in the future. If these defences are breached the potential direct economic consequences would be significant, potentially in excess of 100 trillion yen (around 0.92 trillion USD or 0.72 trillion euros, or 20% of the current GDP of Japan), with the indirect costs likely to be even greater. As a result it is likely that sea defences will have to be strengthened around Tokyo Bay in the future, which could cost in the order of 370 bn yen to defend against a 1 in 100 year storm by the year 2100. Alternatively, a storm surge barrier could be built, which would cost in the range of 700-800 bn yen, though such a structure could raise the protection level and cope with a 1 in 500 year storm, amongst other benefits.
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