STUDY OF THE PALAEO-FLOOD SEDIMENT IN FRONT OF THE FIRST TERRACE AT THE JINGYANG REACH OF JING RIVER AND ITS IMPLICATION FOR FLOOD VARIATION

Based on the grain size and geochemical data from the 285 samples taken in front of the first terrace at the Jingyang reach of the Jing River, together with field investigation, top sample OSL data, and historical records, we identified the palaeo-flood sediment in the section of JYc. The variation of paleoflood in the period between 960 AD and 1662 AD lasted for about 702 years was carefully studied. The result shows that the sediments formed during the 702 years are mainly composed of coarse silt and very fine sand. The changes in grain sizes were sensitively related to the variation of flood and precipitation. According to the grain size data, the section JYc could be divided into 41 layers, indicating 41 floods on different scales and the years with high precipitation. Our results show that the concentrations of the chemical elements and compounds of the flood sediments are consistent with that indicated by grain sizes. Therefore, those data could be used as useful proxies to the flood power, flood level, and precipitation. Catastrophic floods and big floods came from the events covering the whole drainage basin of the Jing River or large parts of the basin. There are three causes to cause flooding in Jing River, i.e drastic increase in annual precipitation, episodic rainstorm without increase in annual precipitation and concentrative and long time rainfall without increase in annual precipitation. The flood sediments in the section of JYc were supposed to be resulted from the moderate floodiing disaster, big flooding disaster and catastrophic disaster.