泾河泾阳段一级阶地前缘古洪水沉积与洪水变化

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

根据泾河泾阳段一级阶地前缘古洪水沉积剖面(JYc)中285个样品的粒度成分和元素含量分析,结合野外观察、剖面顶部样品光释光测年结果和历史文献记录,研究了泾河泾阳段公元960—1662年间持续约702年的JYc剖面沉积所指示的古洪水变化。结果表明,泾河泾阳段持续约702年的JYc剖面沉积物以粗粉砂、极细砂为主,各组分在整个剖面上变化明显,分辨率高,能够清晰地指示洪水以及降水量的变化。JYc剖面分为41个沉积阶段,指示了41次大小不同的洪水和41个降水较多的年份。JYc剖面中某些元素和化合物含量能够指示洪水变化。剖面中洪水沉积物的粒度与元素、化合物指示的洪水动力、洪水位、降雨量大小具有很好的一致性。泾河泾阳段洪水代表的是黄土高原泾河流域的洪水,特大洪水与大洪水来自整个泾河流域或泾河流域大部分地区,较小的洪水来自泾河流域小部分地区。泾河流域发生洪水的原因有3种:一是年降水量明显增加造成;二是年降水量没有增加,而是由短时暴雨造成;三是年降水量没有增加,而是由于降水集中、持续时间长造成的洪水。泾河泾阳段JYc剖面的洪水沉积主要是由中度涝灾、大涝灾、特大涝灾形成的,轻度涝灾一般不会造成洪水灾害。

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. Cata- strophic 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 annu- al 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.