Abstract:

The research of historical flood is playing a more and more important role in the risk assessment and prediction in recent years. The research on interactions between flood disasters and flood factors in history could provide analogies for better understanding of the underlying mechanism and impacts of the current and future floods. To investigate the decisive role of different floodfactors interactions on flood hazards, the five types of floodfactors in the Yangtze River Delta during 1644-1949 including continuous rainfall, typhoon, tidal disaster, upstream flood and heavy rainfall were yearly identified and evaluated based on historic documents. And the yearly total floodfactors values sequence was accordingly obtained. The results showed that: (1) The average value of the five floodfactors is 1.66. Among them, 88.56% of the years were shown as a singlefloodfactor years and twofloodfactors years, and only 11.44% of the years were three and fourfloodfactors years. There are no fivefloodfactors year and only 1 zerofloodfactor year occurred. Based on the total sequence of yearly floodfactors, the studied period was divided into three stages. Stage 1 (1644-1718) contained most of the threefloodfactors years and fourfloodfactors years, and the average of the floodfactors in this stage was higher than the mean average of the whole sequence as well as the other two stages; Stage 2 (1719-1864) showed relatively large fluctuation. In this stage, singlefloodfactor years and twofloodfactors years showed higher frequency of flood occurrence. The average of the floodfactors in this stage was the lowest. Heavy rainfall factor was the dominating factor in singlefloodfactors years; Stage 3 (1865-1949) consisted of mostly singlefloodfactor years and twofloodfactors years showed a stable sequence with small fluctuation. (2) The singlefloodfactor years, mostly showing heavy rainfall factor and then continuous rainfall factor, occurred most frequently in the whole period. The frequency of twofloodfactors years was secondly highest, and most of the years exhibited continuous rainfall factor and heavy rainfall factor, followed by continuous rainfall factor and typhoon factor. The combination of continuous rainfall typhoonheavy rainfall factors was most common in threefloodfactors years, followed by continuous rainfalltidal disasterheavy rainfall factor and then continuous rainfalltyphoontidal disaster factor. And the combination of continuous rainfalltyphoonupstream floodheavy rainfall factors interactions mostly appeared in fourfloodfactors years. There was no fivefloodfactors year. (3) Six extreme flood disaster and seven extreme typhoon disaster years in the Yangtze River Delta during 1644-1949 were identified by the disaster influence. Extreme flood disaster years, including 2 fourfloodfactors years, 3 threefloodfactors years and 1 twofloodfactors years, were mostly found in middle of Stage 1 and second half of Stage 2, all associated with continuous rainfall and heavy rainfall. On the other hand, most extreme typhoon disaster years occurred in the first half of Stage 2. Most of them were the results of concurring typhoon and tidal disasters. If occurrence time and intensity vary among the five flood factors, a higher number of flood factors does not necessarily mean that the year is an extreme flood disaster year or an extreme typhoon disaster year.