Modelling of flood risks to people and property in a flood diversion zone

Abstract

Extreme floods often occur in the middle Yangtze River. The Jingjiang flood diversion zone needs to be operated during these events to protect the safety of the levees along the Jingjiang Reach. Therefore, it is important to be able to predict the potential flood risks to people and property in such an area for the purpose of flood management. In this study, an integrated numerical model for estimating the flood risks in a flood diversion zone is proposed, including a module for predicting the 2D hydrodynamic processes of flood inundation in a study area with complex topography, and a special module for estimating the flood risks to people (children and adults) and property (vehicles, buildings, and crops) using newly developed safety criteria. The proposed model was used to predict the flood inundation process and variation in hazard degrees of people and property, based on a hypothetical discharge hydrograph during the operation of the Jingjiang flood diversion zone. The model predictions show high flood loss rates for various flooded objects such as people, vehicles, buildings, and crops, with a mean loss rate for these subjects of 75% after 140 h. This suggests that the operation of a flood diversion zone should be cautiously considered, as it would likely result in a huge loss of people and property. Furthermore, an investigation was conducted into the effects of different roughness coefficients and people stability criteria on the model predictions. The results show that variable Manning’s roughness coefficients need to be used in the hydrodynamic module according to different underlying surface conditions, and a mechanics-based criterion for the stability of people in floodwaters should be adopted to assess the potential hazard degrees.

中文概要

目的

分洪工程的启用具有非常重要的防洪效益,但同 时也将严重威胁分洪区群众的生命财产安全。为 定量计算洪水中人体(成人与儿童)、车辆、房 屋、农作物(水稻和棉花)的洪水风险与洪灾损 失,考虑受淹对象的失稳机理,提出分洪区群众 生命与财产的洪水风险模拟模型。

创新点

1. 基于力学过程中的洪水中人体与车辆失稳的计 算公式,建立相应洪水风险等级评定的新方法, 并提出4 类受淹对象平均损失率的计算方法;2. 结合二维水动力学模型的计算结果,分析4 类受 淹对象洪水风险的时空变化情况,同时讨论根据 不同下垫面类型取不同糙率值以模拟洪水演进 过程的必要性,并比较文献中提出的洪水中人体 风险等级计算结果的差异。

方法

1. 分析现有洪水中人体、车辆、房屋和农作物风 险或损失的计算方法,提出相应洪水风险计算关 系或计算曲线(公式(3)~(6),图1 和2);2. 参考1954 年荆江分洪工程北闸第一次的分洪情 况,通过计算分洪区140 h 的洪水演进过程和4 类受淹对象洪水风险的时空分布(图8),同时得 到4 类受淹对象平均损失率随时间的变化情况 (图10);3. 在荆江分洪区洪水演进过程模拟中, 讨论根据不同下垫面类型确定相应糙率值的方 法与计算区域糙率统一取值0.04、0.05 或0.06 的 3 种工况下洪水要素变化的差异(图11 和12), 并采用文献中提出的洪水中人体风险等级计算 方法,比较洪水中人体风险等级变化的异同(图 13)。

结论

1. 一旦荆江分洪工程启用,截止至北闸开启140 h 时,洪水中人体、车辆、房屋、农作物的平均损 失率达到75%以上,即分洪工程的启用将造成重 大的生命财产损失;2. 糙率取值方法的不同,导 致洪水演进过程不同,进而影响各类受淹对象的 洪水风险评估,因此需要根据不同下垫面类型确 定相应的糙率值;3. 文献中提出的洪水中人体风 险等级计算方法考虑了人体失稳的力学过程,综 合考虑了水深和流速的影响,可以更安全可靠地 应用于实际洪水中人体的风险等级评价。