| The delta is an important area for human survival and development,and at the same time it is threatened by flood disasters.The combined action of extreme runoff,astronomical spring tides,sea level rise,and extreme wind and wave events may cause compound flood risks and aggravate regional flood disasters.For large estuaries with obvious runoff changes,the incident tidal waves in the estuary are significantly affected by the seasonal runoff changes,showing strong temporal and spatial characteristics.Under the influence of compound dynamics such as runoff,tide and sea level,the tidal characteristics of the estuary will change,and the extreme water level of the tidal reaches of the estuary will be raised to varying degrees,thus affecting the flood control and safe development of the region.Previous studies have paid more attention to the flood disasters in the river sections above the tidal zone boundary and the estuary section,and the research and understanding of the extreme water level and composite flood risk in the estuary section near the mouth are relatively weak.Taking the mouth of the Yangtze River as an example,the near-mouth section from Datong to Jiangyin is about 400kilometers long.There are important urban agglomerations and population distributions such as Wuhu,Nanjing,and Zhenjiang along the route.The study of extreme water level changes in this section has important application value.Based on the research case of the Yangtze River estuary and the continuous measured hourly tidal level data from multiple stations in a year,this paper analyzes the temporal and spatial variation characteristics of parameters such as tidal range,tidal amplitude and decay rate,and ebb and flow duration along the entire estuary.The causes of seasonal tidal changes are analyzed from the perspectives of typical runoff,differences along the route,and narrow river widths and beams.In addition,the tidal model of the estuary of the Yangtze River was set up and calibrated,and the extreme water level variation characteristics of the tidal reaches under extreme runoff,astronomical spring tide,sea level rise and other scenarios were simulated and analyzed,and multiple disaster-causing factors were simulated individually and in combination,and then estimated the influence law of extreme water level caused by each factor,and then discuss the flood control safety strategy under the combined situation.The main understandings are as follows:(1)Based on the analysis of continuous multi-station tidal level data in the Yangtze River estuary during the year,it is recognized that in addition to the typical differences in the flood and dry seasons,which have been paid more attention in the past,the tidal wave characteristics of the Yangtze River estuary also have significant inter-month changes.Taking 2019 as an example,the tidal average residual water level decreased significantly in the Datong-Jiangyin reach,with the largest gradient,and the residual water level gradient from Jiangyin to sea was smaller.The tidal range near the mouth is the largest in January-February and October-December,and the smallest in June-August,which is closely related to the annual change of runoff.The landward attenuation rate of the tidal range is the largest between the Zhenjiang-Jiangyin reach,and there is an obvious inter-monthly variation pattern.The Jiangyin-Weicunzha reach has the largest attenuation rate in July,at 0.0136 m/km,and the Xuliujing-Wusong reach is the largest in July.The decay rate of the river section in December was the slowest,only 0.00098 m/km;the average tidal range decay rate in dry season in the Sanjiangying-Weicunzha section was much lower than the adjacent upper and lower sections,only one third of that,reflecting the significant nonlinear characteristics of tidal range attenuation in the Zhenjiang-Jiangyin reach.In terms of tidal wave deformation,the duration of high tide water level gradually decreases along the course,and there is a decreasing inflection point near the Jiangyin section.Through the above quantitative analysis,the tidal wave propagation in the estuary of the Yangtze River has significant intra-annual changes and nonlinear characteristics along the journey.In the future,the refinement of shipping,agricultural management and safety risk prevention in the estuary area is the general trend.(2)Based on the comparative analysis and simulation study of the data of the catastrophic flood and the average water year,it is shown that the catastrophic flood will significantly increase the average water level and its gradient in the tidal reaches,and at the same time reduce the tidal range.The rate of ascent gradually slowed down.The water level gradient of the Zhenjiang-Jiangyin reach at the extreme flow rate of 84500 m~3/s is 2.17 times that of the average flow rate of 68000m~3/s.In the case of large runoff,the absolute attenuation of the entire estuary tidal range is low at both ends and high in the middle.The tidal range of the Zhenjiang-Jiangyin reach is the most significant,and the tidal range is 0.54-1.29 m smaller when the extreme discharge is 84500 m~3/s than at 68000 m~3/s.Compared with the flood peak flow of 60000 m~3/s in normal water years,the average spring tide water level of Zhenjiang Station is about 1.50 m higher in the case of Datong’s 100000m~3/s flood peak flow,but the maximum tidal range of the spring tide is reduced by about 0.46 m,resulting in a higher extreme maximum water level of about 1.5 m.1.26 m;the average spring tide water level at Nanjing is about 2.40 m higher,and the maximum spring tide tidal range is reduced by about 0.15 m,resulting in a higher extreme maximum water level of about 2.33 m;the average spring tide water level at Wuhu Station is about 2.84 m higher,and the maximum spring tide tidal range is reduced by about 2.84 m.0.06 m,resulting in an extreme maximum water level of about 2.81 m higher.(3)Simulate the tidal and extreme water level characteristics of the Yangtze Estuary under the influence of different sea level rise(0.2-2 m),and realize that the sea level rise leads to an increase in water depth and a relative decrease in resistance,which in turn leads to an increase in tidal range and tidal range.The degree shows nonlinear variation characteristics in the whole estuary.When the sea level rises by0-1 m,the tidal range of Jiangyin and Zhenjiang stations increases by 0-0.42 m and0-0.36 m,respectively;when the sea level rises by 1-2 m,the tidal range of the two stations increases by 0-0.43 m and 0-0.41 m.The tidal range of the Nanjing-Wuhu section has a relatively small and insignificant change with the sea level rise,but the sea level rise leads to the rise of the average water level and its backwater effect affects the extreme water level of the near-mouth section.The impact of sea level rise becomes smaller as the sea level rises in the mouth,and the ratio of extreme water level increase to sea level rise decreases from 95%in Jiangyin to 15%in Wuhu.That is to say,under the scenario of 1 m rise in sea level outside the mouth,the extreme high tide level in Wuhu may be raised by about 0.2 m.Considering the three factors of extreme runoff of 100000 m~3/s,astronomical spring tide and sea level rise of 2 m,the extreme water level of the Wuhu-Jiangyin reach may increase by 0.28-1.94 m.Through sensitivity analysis,it is recognized that when the sea level rise is less than 0.5 m,the extreme water level increment near the mouth is small;when the sea level rise exceeds 0.5 m,the extreme water level near the mouth may increase rapidly,increasing the regional composite flood risk.To sum up,the tidal wave characteristics and extreme water level of the Yangtze River Estuary will be affected by the extreme runoff,astronomical spring tide,and sea level rise,and show different extreme water level change trends and significant changes along the way.All of them will bring different degrees of security risks to the tidal reaches of the Yangtze River.The research results of this paper analyze and judge the safety risks of various regions along the route according to the actual intensity combination of disaster-causing factors,which can provide some references for future cities to formulate bank protection projects with higher flood resistance and detailed urban disaster prevention plans. |
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