Driving Mechanism And Risk Diagnosis And Assessment Methods Of Ice Flood Disasters In The Ningxia-inner Mongolia Reaches Of The Yellow River

The ancients said: "It is easy to prevent the summer flood,but difficult to prevent the ice flood,and the river official is not guilty on the dike-breach during ice flood season",which explains the difficulty to prevent ice flood in the Yellow River.Due to the special geolocation and the hydrometeorology conditions,the ice flood disaster in the Ningxia-Inner Mongolia Reaches of the Yellow River(NMRYR)occurs frequently and causes huge losses,which has become one of the most prominent and major natural disasters of Chinese rivers in the winter and spring seasons.Scientific prevention and control of ice flood disasters is essential.Relied on the National Key R&D Program of China “Research and Demonstration of Key Technologies for Yellow River Ice Flood Monitoring and Disaster Prevention and Control”,the article studied the evolution characteristics and driving mechanism of ice status and ice flood disasters in NMRYR,analyzed the distribution characteristics of ice flood risk,discussed the fractal characteristics of multi-dimensional river regime and its correlation with ice jam and ice dam,and then put forward the methods of ice jam risk diagnosis,division and hazard assessment of dike dangerous section during ice flood season,and the dynamic risk assessment of dike-breach ice flood,also applied these methods in practice.The main research contents and conclusions are as follows:(1)The characteristics and influence mechanism of ice status in NMRYR under the change of temperature were studied.The characteristics and driving mechanism of ice flood disaster evolution under the change of temperature,flood conditions and emergency regulation and control of ice flood-diversion zones were mainly revealed.Then,the distribution characteristics of the ice flood risk in NMRYR were analyzed.The research results are as follows.As the climate warms,the ice flood season period at the Toudaoguai hydrographic station is gradually shortening at a rate of 0.27d/a.In the past 15 years,the probability of the disappearance and recurrence of ice-run and the anomalous changes in the length of the frozen river has exceeded 50%.The ice flood risk in NMRYR has the characteristics of many danger points,long dangerous section and wide influence range.The rapid increase of velocity and quick fall of water level during the river cover break-up were the main driving factors for the dike dangerous situation.Driven by the change of erosion and silt and river facies,the river bed was raised as a whole,the river facies coefficient and the water level corresponding to the same flow were also increased,resulting in the increased risk of ice flood disaster,however,the emergency control of ice flood-diversion zones could effectively reduce the ice flood risk,and affect the distribution pattern of the danger section of ice flood disasters.The mechanism of ice flood disaster driven by multi factor coupling is more complex,the suddenness and continuity of ice flood disasters has been strengthened.The average annual number of ice dams has gradually decreased,but the number of ice jams is increasing.In the past 30 years,the impact and loss of ice flood disasters have increased significantly.Under the condition of drastic changes in cold and warm,the ice flood risk will increase greatly,and the situation of major overflow and dike-breach during ice flood season will be more severe.(2)The fractal characteristics of the multidimensional river regime evolution of cross section-longitudinal section-plane and its correlation with the ice dam were proved,the diagnosis method based on multi-combination uniform optimization of weighting,k-means clustering and random forest for the ice dam risk was proposed.And then,it was applied to diagnose the dangerous section of ice jam in typical reach,identify the main driving factors,and analyze the changing trend of the ice dam risk.The research results are as follows.The multidimensional river regime evolution in NMRYR had the multi-scale self-similar fractal characteristics.There was a positive exponential correlation between the frequency of ice dam(severe ice jam)and the fractal dimension of the main stream curvature,which shows that ice dam was more likely to occur in wide and shallow rivers with large meandering and highly developed bays.The accurate rate of ice jam risk grade diagnosis with random forest algorithm was P=97.72%,recall rate R=95.83%,comprehensive index F1=96.54%,and the diagnostic accuracy was higher than that of other methods like support vector machines.The proportions of the river sections prone to the ice dam in the reach of Shizuishan to Toudaoguai under 4 risk levels from lowest to highest were 19%,30%,26%,and 25%,respectively.The high risk areas were mainly distributed in the reach of Sanhuhekou to Toudaoguai,and the ice jam risk in the lower reaches was significantly higher than that in the upper reaches.The hazard of sudden ice jam in wide and shallow curved channels of the NMRYR was more serious.(3)Considering the difference of dike dangerous section and ice jam prone river section,the model of the division and hazard assessment of dike dangerous section during ice flood season was established based on the improved FAHP-entropy weighting and clustering algorithm,and the evaluation results were verified to be highly reasonable.Then,the spatial distribution characteristics of dike dangerous section during ice flood season in the reach of Bayangaole to Toudaoguai were studied,and the key influencing factors and changing trend of the dike hazard were analyzed.The research results are as follows.The proportions of the dike sections in the reach of Bayangaole to Toudaoguai under 4 hazard levels from lowest to highest were 50%,28%,14%,and 8%,respectively,and the dike hazard gradually increases from the upstream to the downstream with local high-risk sections.After considering the emergency regulation indicator of ice flood-diversion zones,the above proportions were adjusted to 50%,40%,4%,and 6%,and the high-hazard dike sections were significantly reduced,which indicated that the disaster reduction effect of ice flood-diversion zones was effective and the dike hazard was more sensitive to this indicator.It was also found that there were spatial heterogeneity in the distribution of dike hazard during ice flood season,and the dike hazard presented the characteristics of spatiotemporal evolution under the changing environment.(4)Considering the spatial heterogeneity of the distribution of dike hazards during ice flood season,and coupling the dike hazard degree and the vulnerability degree of dike-breach ice flood inundation,the method of combined risk analysis and dynamic evaluation of large-scale multi-breach ice flood was proposed.Coupling ice flood simulation model of river channel and flood plain was established in the reach of Bayangaole to Toudaoguai,and its calculation accuracy was quantitatively verified to be higher by the proposed index SREP.Then,the coupling process of ice-affected backwater,dike-breach flow,and ice flood inundation was simulated,and the combined risk of multi-breach ice flood was clustered and evaluated,based on which the changing trend of the vulnerability of dike-breach ice flood inundation was studied.The research results are as follows.The flooded area with water depth greater than 1m accounts for about 46.77% of 7 dike breaches in NMRYR,which indicated that the risk of dikebreach ice flood was quite high.In addition,the submergence vulnerability gradually increased with time.The results of the whole regional risk zoning coupled with the dike hazard degree reflected the correlation and difference of the risk distribution of crossregional multi-breach ice flood,which combined with the single regional risk zoning,solved the technical problem of risk assessment of sudden chain dike-breach ice flood at different spatial scales in the long river.In summary,methods of theoretical analysis,mathematical statistics,numerical modeling,intelligent algorithm and comprehensive evaluation were adopted to carry out in-depth research on the driving mechanism and risk diagnosis and assessment of the ice flood disasters in NMRYR.The research results can provide theoretical and methodological support for early identification and prediction of ice flood disaster risk,which has important theoretical value and practical significance.