| At present,the increase of natural disasters caused by global climate change has become a major challenge for human survival and development.Among them,the flood disaster is one of the most serious natural disasters in the world today and has become the focus of attention of all countries in the world.China is one of the countries with the most serious flood disasters in the world.With the development of the economy,the loss caused by flood disasters to human beings is increasing day by day.The upper reaches of the Hanjiang River,which is located in the transition zone between southern and northern China,is a sensitive region of global climate change.In recent years,frequent floods in the upper reaches of the Hanjiang River not only caused serious economic losses,but also restricted the smooth implementation of the Middle Route Project of South-to-North Water Diversion.Therefore,grasping the flood data and laws in the upper reaches of the Hanjiang River,scientifically and accurately simulating the flood evolution process and adopting corresponding disaster prevention and reduction measures have become an urgent demand for socio-economic development in the basin.Due to the short time scale of hydrological observation data in China,the existing flood data in the upper reaches of the Hanjiang River can neither reflect the regularity of flood changes on a long-term scale,nor meet the needs of flood prevention,disaster mitigation and basin development in the upper reaches of the Hanjiang River.The research group has carried out a series of research work on the palaeoflood in the upper Hanjiang River,which has extended the flood data and provided basic data for flood control and disaster reduction in upper reaches of Hanjiang River.Among them,the collation of palaeoflood research results found that the Loess-Palaeo-soil sedimentary profile of the T1 terrace in the upper reaches of the Hanjiang River contains palaeoflood events in the Eastern Han Dynasty and Northern Song Dynasty;Historical literature research suggests that it was possible to record an palaeoflood event in the second year of Jianan in the Eastern Han Dynasty(197 AD)and in the seventh year of Taiping Xingguo in the Northern Song Dynasty(982 AD).However,in terms of the discharge calculation of palaeoflood events,most of them are single-section and single-river sections,and less comprehensive studies on multiple sedimentary section systems that record the same palaeoflood event are conducted.Therefore,this paper takes the palaeoflood research in the Eastern Han Dynasty and the Northern Song Dynasty as an example.Based on the HEC-RAS model,the water surface line of the palaeoflood was calculated using the steady flow module,and the simulation of the palaeoflood routing was studied using the unsteady flow module.In addition,through the analysis of the climatic background of palaeoflood events,the response relationship between palaeoflood events and climate change in the Eastern Han Dynasty and Northern Song Dynasty was discussed.This has important practical significance for flood prevention and disaster reduction and flood forecasting in the upper reaches of the Hanjiang River.The main conclusion of this paper is as follows:(1)The analysis of the rainstorm characteristics in the upper reaches of the Hanjiang River shows that the weather circulation patterns of the severe rainstorms in the upper reaches of the Hanjiang River mainly consist of three types:southwest airflow pattern,westerly trough and low whirlpool shear type.Among them,the low whirlpool shear type is the main circulation form that causes the storm flood in the upper reaches of the Hanjiang River.There are three major storm centers in the upper reaches of the Han River,namely the Micang Mountain,Daba Mountain and the Qinling Mountain.According to the analysis of the flood characteristics,the flood types are mainly the Shiquan above type flood,the Shiquan Ankang interval flood and the upstream type flood.The flood hydrograph have multi peak type,single type and fat peak type.Extreme floods are mostly caused by peak floods.(2)Based on appropriate terrain data and hydrological parameters,using the one-dimensional steady flow module of the HEC-RAS model to calculate the palaeoflood water surface profile of the Eastern Han Dynasty and Northern Song Dynasty.The simulated peak flood discharges in the Eastern Han Dynasty and Northern Song dynasties were 60,800 m3/s and 57,500 m3/s respectively,and the simulated water level error at each section in the Eastern Han Dynastyb and Northern Song Dynasty was-0.30 to 0.46 and-0.52 to 0.60 m,respectively,and the relative errors were-0.18%to 0.25%and-0.31%to 0.34%,respectively.In addition,the calculation of palaeoflood water level in the Eastern Han Dynasty and the Northern Song Dynasty were verified using the floods in August 1983 and floods in July 18,2010 respectively.Based on the same topographical data and hydrological parameters,the water level errors of 1983 and 2010 were between-0.37%-0.26%and-0.18%?0.32%respectively.The certainty coefficient(DC)of simulated water level sequence and actual measured sequence is 0.95 and 0.96,respectively.This proves that the simulation results based on the HEC-RAS model for the water surface profile of palaeoflood events in the Eastern Han Dynasty and Northern Song Dynasty are accurate,and the selected terrain data and hydrological parameters are reasonable.(3)Based on the one-dimensional unsteady flow module of the HEC-RAS model,the palaeoflood routing in the Eastern Han and Northern Song Dynasty was studied in the same reach.The palaeoflood routing in the Eastern Han Dynasty lasted about 3 hours,and the peak discharge decreased by 1.43%.The palaeoflood routing in the Northern Song Dynasty lasted about 1.15 hours,and the peak discharge decreased by less than 1%.In the Eastern Han Dynasty and the Northern Song Dynasty,the distribution of palaeoflood water depth was higher than that of the lower reaches,and the downstream flow velocity was greater than the upstream.In addition,based on the same terrain data and hydrological parameters,the MIKE 11 software was used to simulate the palaeoflood in the Eastern Han Dynasty and the Northern Song Dynasty.Comparing the simulation results with the HEC-RAS,it was found that the certainty coefficients(DC)of the water level and flow series of the MIKE 11 and HEC-RAS models in the Eastern Han Dynasty and Northern Song Dynasty were all greater than 0.95.In the Eastern and the Northern Song Dynasty,the peak time errors of palaeofloods at each profile were less than 1.21%.This shows that the HEC-RAS model is reliable and reasonable for palaeoflood routing in the Eastern Han Dynasty and Northern Song Dynasty.(4)By analyzing the climatic background of the palaeoflood events in the Eastern Han Dynasty and Northern Song Dynasty,we can see that climate fluctuations have changed during the Eastern Han Dynasty,especially in the late Eastern Han Dynasty(180-220 AD),and the climate was extremely variable;The Northern Song Dynasty was a turning point in China's climate change in the past 2000 years.The Northern Song Dynasty was a turning point in China's climate change in the past 2000 years,especially in the 980s,when the climate changed suddenly.The late Eastern Han Dynasty and the Northern Song Dynasty 980s were all frequent periods of extreme hydrological events.This proves that the palaeoflood events mostly occurred in Climate fluctuation and mutation periods,which is the instantaneous response of the river hydrological system to the abnormal and changeable climate at that time.(5)The current researches on palaeohydrology are mostly single-section and single-site discharge calculation.It is rare to record multiple profiles and comprehensive studies of palaeoflood on multiple sedimentary profiles.This paper selects the palaeoflood sedimentary profiles that are concentrated in the upper reaches of the Hanjiang River,and calculate the water surface profile based on one-dimensional steady flow module of the HEC-RAS model.In addition,the palaeoflood routing was simulated using one-dimensional unsteady flow modules.It is of great scientific value to understand the extreme flood routing law in the upper reaches of the Hanjiang River,and it is of great practical significance for the flood forecast and flood control in the upper reaches of the Hanjiang River. |
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