Study On Flood Risk Assessment Technology And Method Of Dongting Lake Flood Storage And Detention Basin Based On Airborne LiDAR Data

After the completion of the Three Gorges Project,the flood control of Dongting Lake is still in a grim situation.Embankments' flood control ability is generally low,and the safety facilities'construction in flood storage and detention basins lags behind.Thus,recent Dongting Lake govering will be still be focused on improving flood control capacity.Due to the special flood risk in flood storage and detention area,assessing the flood risk of it can clear its residents resettlement direction and safety construction mode in different areas,and can plan the construction facilities and safety facilities of the flood storage and detention area,laying the foundation for the formulation of flood risk management policies and measures in flood detention areas.It is of great theoretical and practical significance to carry out the study of flood risk assessment in flood storage and detention basins under the background of an intensive development and an increasing risk of flood storage and detention basins.In this study,the Gonngshuangcha embankment flood storage and detention area of Dongting Lake is chosen as the experimental area.Through the collection of airborne LiDAR point cloud and high-resolution remote sensing data,research is carried out from the following four aspects:the flood disaster information extraction,the construction of high-precision flood submerged simulation model,detailed assessment of damage loss of monolithic buildings in the flood disaster and the flood risk assessment of the flood detention area.Through the establishment of a set of technology and method for the risk assessment of the flood disaster in the flood storage and detention area,it is expected to perfect and enrich the high-precision and quantitative flood disaster risk assessment theory,and provide technical support for the flood detention flood disaster risk management policy formulation and implementation at the same time.The main contents and conclusions of this paper are as follows:(1)The extraction of disasters-pregnant and disasters-bearing information of flood storage and detention basins flood disaster based on the airborne LiDAR data.This paper presents a method for the extraction of buildings based on three-dimensional fractal dimension analysis,which is applicable to the vast rural areas such as flood storage and detention areas.The method first obtained point sets of each ground featureby region segmentation on ground point cloud;and then,by three-dimensional fractal dimension analysis,the three-dimensional fractal dimensions of building point sets range from 1.38 to 1.44.The results show that there are 30551 housing in Gongshuangcha,with an accuracy of 90.04%.A method of extracting and detecting the farmland,ridges and ditches information based on high-precision digital terrain data is proposed.The method extracted target information by applying smoothing,erosion,and dilation on the airborne LiDAR-derived data.Over the entire research area,10609 farmland parcels were extracted,with a total area of 227136.41 mu.Comparing to official statistics,which is 236400 mu,our results of farmland parcels had an accuracy of 96.08%.The results show that the extracted polygons of farmland,ridges and ditches have high precision,and the small peaks such as bunkers in the digital terrain can be also removed properly.To assess dikes' damage level,A method of dike center line,crest,and slope extraction using airborne LiDAR-derived high-precision digital terrain data is proposed.Concentric circles are introduced in to avoid failure detection when the dike is broken.And then embankments parameters(e.g.crests and slopes)are detected through mathematical transformation to get regeneration of dike profile data,thus the analysis on embankment damage degree can be achieved.The results show that among 2401 sections in Gongshuangcha's embankments,there are 931 sections in the normal embankment area,accounting for 42%of the total embankment area;and more than half of the embankment areas have different degrees of damage,most of which are in moderate damage(accounting for 36%),those in very serious damage reached 8%.It is need for reinforcement and maintenance of the embankment.(2)Research on flood flooding simulation technology and method integrating high-precision digital terrain data in flood detention basin.A submerged area fast generation and block compression-tracing algorithm for large digital terrain data is proposed.It was proved to have significant advantages in both compression efficiency and running time-cost comparing to common seed growing methods.In addition,an integrated block compression-tracing and 2D flood simulation method for precise digital terrain data is proposed.Precise flood simulation results are generated by applying spatial interpolation and block compression-tracing on 2D flood evolution model.In the flood submerged extent,it indicates that the pseudo-submerged area is eliminated effectively and the flooded area is reduced comparing to the results of 2D flood evolution model.In the flood submerged quantity,both maximum submerged water depth and the local submerged water depth by this method are larger than those by the two-dimensional flood evolution model.It shows directly that using the model calculated grid makes the overall digital terrain to be raised.(3)Study on the Fine Assessment of Flood Disaster Loss of Monolithic Buildings in Flood Detention Areas.Based on the national standard "Code for Design of Residential Building(GB50096-2011)",the flood storage area buildings are divided into two types:brick and wood structure buildings and brick and concrete structure buildings,according to the height of the buildings.Among 30511 buildings over the entire flood detention area,there were 19879 brick and wood structure buildings obtained,most of which are two-storey(counted92.06%);while 10672 brick and concrete structure buildings obtained,most of which are three or four-storey(counted 93.48%).The flood loss rate curve of the two type of buildings are constructed respectively;and based on each building's submerged depth,to caculate flood loss value of all monolithic buildings in flood detention area is caculated.The results show that under a situation of 1954-type great flood,when the flood reaches the equilibrium state finally,the flood loss value of the brick and wood structure buildings in the flood storage and detention area reaches 3.358 billion RMB,and the flood damage of the brick and concrete structure buildings reaches 1.686 billion RMB,the total loss reaches 5,044 million RMB.(4)Risk assessment of flood disaster in flood detention area based on catastrophe theory.In this paper,we try to introduce the catastrophe theory and carry out the research on the assessment of the flood disaster risk in the flood detention area according to the abrupt series evaluation method.The conventional evaluation methods using administrative villages,townships as basic evaluation unit are prone to be inconsistency inside evaluation units,or the evaluation index crosses multiple evaluation units.In this paper,the flood risk assessment method is directly based on the fine grids,which effectively circumvent the above problems.Additionally,weightsbetween the indicators do not have to be considered in abrupt series evaluation method,which makes the risk assessment more objective.The results show thatunder a situation of 1954-type great flood and the flood diversion plan of Gongshuangcha the whole area of the flood storage area is at the level of heavy risk and special risk.In the eastern area,the flood risk is overall higher than that in the west,since the flood depth is deeper and the velocity is larger,and the flood disaster risk of settlements in partial area is higher significantly.