Research On Weihe River Flood Changes In Tang Dynasty And The Past 100 Years In Northeast Xi'an

The floods often inundate the land and cause serious losses or damages of property and even threaten the safety of people’s lives.Therefore,the study of flood has been highly valued all the time.However,there are relatively less attentions regarding researches on the restoration of paleo-flood flow and water level of Wei River,which is the largest tributary of Yellow River.This paper extracts the paleo-flood information contained in the flood sediments of the Wei River to restore the ancient river flood discharges and water levels in modern times,which would have important scientific significance for extending the historical hydrological data of this river.Based on the detailed filed investigations,we collected 813 samples from SL1,LT1,LT2 three typical profiles of sediment on floodplain and the front edge of the first order terrace in Gaoling and Lintong.By analyzing the grain-size of samples and with the help of software analysis of results of the experiment,we studied the flood changes of Wei River in Xi’an in Tang dynasty and nearly 140 years period and then generated the curve of water level and discharge,and restored the ancient flood discharge and water level.The main findings of the research are as follows:(1)The floodplain sediment of LT1 and LT2 profiles located in Lin Tong occurred in nearly 140 years,which the section of thickness is large and coarse appeared in the bottom of the profiles but occasionally doped with gravel.The flood sediment of SL1 profile occurred in Tang dynasty in Gaoling,which the thickness is very large,and the inside grain are rough and often doped with few gravel.This is different from the traditional fine-grained sediments,and it indicates that the flood of Wei River at that time in the study area was significantly in large scale.According to the comparison of the grain sizes of the three sections,it is shown that the flood scale in the Tang Dynasty was larger than that in the flood season in the past 140 years and the flood power was strong.(2)The flood sediments in the Gaoling section of Wei River in the Tang Dynasty and the Wei River in Lintong section in recent 140 years have obvious layering,which recorded the information of the Wei River flood events at that time.The changes of grain-size parameters reflected strength of the hydrodynamic at flood event,which is,the rougher the general grain-size is,the stronger is the hydrodynamic of flood and vice versa.(3)The thickness of LTI,LT2 and SL1 profiles is 4m,4m,9.39m respectively.The LT1 and LT2 profiles formed by the flood events of the nearly 140 years,which could be divided into 18 flood sediments.This indicates that the flood events occurred totally 18 times.Meanwhile,the SL1 profile formed by the flood events in Tang Dynasty,which could be divided into 36 flood sediments.This indicates that the flood events occurred totally 36 times and each flood typically represents the period of one year.The floods were generally discontinuous,therefore,the 18 floods in Lintong formed in the past 140 years and the 36 floods in Gaoling formed in the past 290 years in Tang Dynasty.(4)Compared to the lower ten layers of LTI profile,the grain-size of the upper 8 layers is much finer,the sorting is in the middle,the skewness is more symmetric and the kurtosis is narrower,which indicates that the flood hydrodynamic of the upper 8 layers is much weaker than the following layers.After eliminating the effect of topographic increase caused by the sedimentary thickness,we can confirmed the order of flood scale of the 18 layers were:9th>10th>13th>11th>12th>4th>16th>3rd>15th>17th>5th>7th>14th>8th>1st>6th>2nd>18th.The order of LT2 profile is as same as the LT1 profile.(5)The flood changes in the SL1 section of the Tang Dynasty in Gaoling can be divided into 3 flood periods,the early flood scale is the smallest,the middle period flood is the largest and the late flood is in the middle.The depth of flood in the early river-bed is generally between 5～12m,and the depth of the medium river-bed flood is generally slightly more than 12m in the middle period,and the depth of the late river-bed is generally between 9～11m.The larger water depth of the river-bed indicates that Wei River was suitable for transportation channel at Tang Dynasty.(6)The flood scale in the early Tang Dynasty was similar to the flood in the last 140 years,but the scale of the middle and late Tang Dynasties was significantly larger than that of the last 140 years.In the middle and late Tang Dynasty,when the flood occurred,the flood depth of the river bed was mostly 11～12m.In the past 140 years,the depth of the riverbed flood was generally between 5-8m.The depth of the river bed when the flood occurred was about 5m larger than that of the latter when the flood occurred.(7)Based on the actual flood data of Lintong Hydrological Station in recent years the flood peak discharge range of the flood season of the Wei River in the past 140 years was restored from the value of 3683 m3/s to 9874 m3/s according to the stage-discharge relationship method.Comparison with measured data and previous research shows that our calculated peak discharge data is believed.Adding this research result to historical floods and measured flood series can effectively extend the series of flood hydrology data in the area.(8)The occurrence of flood events in the Wei River were mainly due to the increase in annual precipitation in the year,and were also in some cases caused by short-term heavy rain.Deforestation and reclamation,planting on steep slopes,etc.had destroyed the ecology of rivers,which has also contributed to the multiple occurrences of flood events.