Floods Happened In Yishu River Basin And Their Climatic Backgrounds For1000Years

Flood is one of the severest natural disasters in China. Its historical records in Chinese abundant history of literature are unraveled in terms of its spectrum of time, the details of contents and the wide scopes involved. Yishu River Basin flows through the southeastern Shandong area, which is located in China’s north-south climatic transition zone, and has an uneven distribution of rainfall during the year which varies greatly among different years. Flood is the primary natural disaster in the history of this region.In this paper, the collection, collation and analysis of historical documents of historical flood records Shandong Yishu Basin for1000years reveals natural law of the time and space of the Basin Flood Occurrence and characteristics; and on the watershed historical climate data collected analysis, preliminary study of the watershed in the relationship between the historical period flood disasters and climate change, especially with the features of the climate of the relationship; in addition, the combination of historical changes status of the basin of the river, to explore the impact of the floods. Conclusions are as follows:(1) The period of1000years is divided into two phases, the first phase of the Northern Song Dynasty (AD960) to the second stage of the Yuan Dynasty (AD1367); during the Ming Dynasty (1368AD) to AD1950. In the Song and Yuan Dynasties, Shandong Yishu River Basin flood occurred overall less than normal, which has the two periods floods Year concentrated:1001-1050,1301-1350;nearly600years Yishu River Flood divided into three phases:Phase,1368-1530, a total of163years, the floods occurred9times occur less frequently; second stage,1531-1760, a total of230years, the occurrence of floods60times, an average of once every3.8years, accounting for58.8%of the number of floods; third stage,1761-1950, a total of190years, floods occurred33times, an average of once every5.7years, but the frequency of occurrence of changes in the magnitude. The occurrence of floods in the basin there are differences between regions, counties and cities with a higher frequency of floods, are distributed in the basin plain area, mountain counties lower the frequency of the floods occurred. This basin floods occurred in the past600years the division level:mild water logging of45 times, accounting for44.1%of the total number of floods; moderate water logging occurred37times, accounting for36.2%of the total; big floods in16times, accounting for the total number of15.6%; the catastrophic floods least only four times, accounting for4.1%of the total.(2) According to the instrumental precipitation data from1931to1985of this region, the analysis of the flood season total amount of annual precipitation and July precipitation found:the study area flood season total annual precipitation correlation is quite good the full performance of the precipitation in the study area, the impact of the floods; study area flood season total July rainfall correlation is better, the regional flood event with precipitation concentrated. Climate dry and wet conditions in the study area flood events an important factor influencing the frequency, humid climate of the period, the region’s history of floods more time; periods of dry climate, while the flood event frequency less Song and Yuan Dynasties, corresponding to the medieval Warm Period, the entire region is dry, the floods occurred less frequency. The study area nearly600years in the climate characteristic period of the Little Ice Age of the Ming and Qing, the climate there is a clear Climatic Change. Fluctuations of precipitation frequency fluctuations is remarkably consistent with the basin floods occur, changes in precipitation can be said is an important factor causing floods. Both the positive and negative correlations between temperature changes and floods in the history of this region exist, but it is certain that the fluctuation of the temperature is closely correlated with the occurrence of floods.(3) The changes of the basin of the river also are the primary reason leading to the frequent floods. In the year1128, Huang River changed its channel and flowed to the south, and started a700years’occupancy of Huai River’s channel into the East Sea, thus completely destroying the original water system of the downstream of Yishu River, plus the poor drainage, small outlets of the floods, in the end exacerbating the floods disaster happening in this region.