 The reservoir 's flood storage capacity is 22 cubic kilometres ( 18 @,@ 000 @,@ 000 acre · ft ) . This capacity will reduce the frequency of major downstream flooding from once every ten years to once every 100 years . The dam is expected to minimize the effect of even a " super " flood . In 1954 the river flooded 193 @,@ 000 km2 ( 74 @,@ 518 sq mi ) , killing 33 @,@ 169 people and forcing 18 @,@ 884 @,@ 000 people to move . The flood covered Wuhan , a city of eight million people , for over three months , and the <unk> Railway was out of service for more than 100 days . The 1954 flood carried 50 cubic kilometres ( 12 cu mi ) of water . The dam could only divert the water above <unk> , leaving 30 to 40 km3 ( 7 @.@ 2 to 9 @.@ 6 cu mi ) to be diverted . Also the dam cannot protect against some of the large tributaries downstream , including the Xiang , <unk> , <unk> , Lishui , <unk> , and the Gan . 
