基于HEC-RAS模型的汉江上游旬阳西段超长尺度古水文演化重建

通过对汉江上游河谷开展广泛细致的野外考察,在汉江上游旬阳西段全新世黄土-土壤剖面发现四层典型的古洪水滞流沉积物。对于系统采集的样品,进行粒度成分、磁化率分析,从沉积学角度证明了它们是典型的古洪水滞流沉积物。这些古洪水滞流沉积层夹在全新世风成黄土-土壤地层序列中,其每一层古洪水滞流沉积物记录了一期古洪水事件。根据OSL测年数据,并结合考古年代学和典型剖面地层对比,确定了这四期古洪水事件分别发生在8 500~8 400 a B.P.、4 200~4 000 a B.P.、3 200~2 800 a B.P.和1 800~1 700 a B.P.。利用古洪水SWD尖灭点高程法,恢复了这四期古洪水洪峰水位,介于233.0~239.2 m之间;进而基于HEC-RAS模型重建了四期古洪水洪峰流量,介于26 500~46 800 m3/s之间。将古洪水研究成果加入后,得到了远超过实测洪水和历史洪水重现期的稀遇洪水的水文信息,延长了汉江上游安康-旬阳段流域洪水的数据序列至万年尺度,使洪峰流量-频率曲线的稀遇洪水部分有了点据控制,百年和千年一遇的洪水洪峰流量的计算由外延变为内插,提高了设计洪水的精度。并且通过古洪水水文计算得到,该河段万年一遇洪水洪峰流量为46 900 m~3/s,千年一遇洪水洪峰流量为37 800 m3/s,百年一遇洪水洪峰流量为28 900 m~3/s。这对于汉江上游水利工程、防洪工程和城镇建设的洪水设计提供了十分重要基础数据。

RECONSTRUCTION OF LONG-TERM PALEO-HYDROLOGICAL EVOLUTION IN THE WEST XUNYANG REACH ON THE UPPER HANJIANG RIVER BASED ON HEC-RAS MODEL

Palaeoflood hydrological studies were carried out in the upper reaches of the Hanjiang River valley.Four bedsets ofpalaeoflood slackwater deposit (SWD) was found in the eolian loess-soil profile at the LJT site at the riverbank near the Luojiatan Village in the Xunyang County.After detailed field observations,a series of sedimentary samples were taken from the profile.Grain-size distribution and magnetic susceptibility were analyzed in the laboratory.The results showed that these SWD consist of sandy silt,originated from the suspended sediment load of the palaeo-floodwater.Each bedset of the SWD represented one group ofpalaeoflood event.According to the OSL dating in combination with the archaeological dating and pedo-stratigraphic correlations,these palaeoflood events occurred during 8 500-8 400 a B.P.(SWD1),4 200-4 000 a B.P.(SWD2),3 200-2 800 a B.P.(SWD3),and 1 800-1 700 a B.P.(SWD4),respectively.The palaeoflood peak discharges were calculated by using hydrological models.They were estimated to be between 26 500 and 46 800 m3/s.Improvement of the frequency curve estimation was made by the use of gauged flood data from 1935 to 2010;historical floods of 1583,1867,and 1921 at the Ankang hydrological station;and Holocene palaeoflood discharges.These results have the archives of flood data extended to a time-scale of 10 000 years in the Ankang-Xunyang reach in the upper reaches of the Hanjiang River.Thus,the data generated from palaeoflood hydrology allowed us to refine the flood frequency-peak discharge curves and potentially increase its accuracy.From the frequency plot,we conclude that the flood discharge was 46 900 m3/s,with a return period often thousand years;37 800 m3/s with a return period of one thousand years;and 28 900 m3/s with a return period of one hundred years,respectively.These results are of great significance for mitigating flood disasters and hydraulic engineering.These results are also very important for water conservancy construction and flood hazard mitigation.