Inundation risk assessment of underground spaces in the downtown of Fukuoka City, Japan

Two flood events, which were due to short-time heavy rainstorms, hit the downtown of Fukuoka City in Japan in 1999 and 2003. The Mikasa River in this city overflowed its banks, and then the overflowing water moved down the roads to the railroad station, called 'Hakata-eki'. This area is a dense downtown with many office buildings and underground spaces utilized for restaurants, parking garages, shopping malls and a subway station. The flooding water inundated the basements of many buildings and the other underground spaces. The purpose of the present study is to develop an inundation risk assessment method for underground spaces. Firstly, two types of underground spaces near the 'Hakata-eki' railroad station can be distinguished; one is a small-scale underground space utilized for restaurants and parking garages, and the other is a large-scale underground space for a subway station and shopping malls. Secondly, the floodwater behaviour within the two types of underground spaces is simulated. Finally, on the basis of the simulation, a method for risk assessment of inundation of underground spaces due to a flood event is proposed.