Analysis and calculation of inconsistent flood and its influence on flood control of water conservancy projects

【作者】 曾杭；

【导师】 冯平；

【作者基本信息】 天津大学 ， 水文学及水资源， 2015， 博士

【摘要】 极大洪水事件是威胁人类最大的自然灾害之一。在工程设计中,频率分析计算是风险评估和风险缓解必不可少的一部分,它通过统计模型计算极值水文事件的发生频率,从而为防洪工程的设计提供依据。标准的频率分析一般假定实测水文序列满足一致性要求,但是目前越来越多的研究表明,气候变化和下垫面变化显著地影响降雨和洪水形成的物理基础,导致实测水文序列“一致性”假定遭到严重质疑。因此传统的频率分析方法不再适用,为提供更精确的风险分析,探索非一致性极值水文序列频率计算方法并定量估计环境变化对极值水文序列频率的影响是极其重要的。本文以受气候和下垫面显著变化影响的大清河流域为例,承担整个流域防洪重任的上游各大型水库入库洪水序列形成的物理基础和条件发生改变,入库洪水序列的一致性要求不再满足,若继续采用传统的洪水频率分析计算方法,对水库调洪以及白洋淀水系区域防洪会产生一定的风险。本文在对白洋淀上游大型水库入库洪水序列进行非一致性洪水序列频率分析的基础上,研究大清河流域现状气候变化和下垫面变化对白洋淀上游水库设计洪水成果、水库调洪和区域防洪的影响;另一方面,基于气候预测因子,对入库极值洪水和极值降雨频率进行预测,为环境变化条件下白洋淀水系上游水库设计洪水的修订和库淀防洪调度提供技术支撑和科学依据。主要研究内容及成果如下:(1)对大清河流域白洋淀上游四个大型水库的入库洪水极值序列进行变异诊断,分为初步诊断和详细诊断。其中详细诊断中洪水序列突变性诊断(即变异点诊断),首先采用Pettitt检验法得到变异点范围,再结合其他变异点诊断方法结果确定可能变异点,最后结合各水库控制流域内土地利用变化情况、水保工程资料和降雨序列变异点结果进行综合判断,得到各水库变异洪水序列的最可能变异点均为1979年,且均呈现显著的趋势变异。(2)基于各水库入库洪水序列的变异点诊断结果,分析条件概率分布和混合分布两种分布模型下的非一致性洪水序列频率计算形式,混合分布优于条件概率分布。选取混合分布模型对变异洪水系列进行非一致频率计算,得到各非一致性洪水序列不同重现期下的设计洪水值,再采用同频率放大法得到非一致性洪水序列的设计洪水过程。将各水库考虑环境变化的设计洪水成果,与不考虑洪水序列变异情况的设计洪水成果进行比较,结果表明不同重现期下,西大洋水库年最大1日、3日和6日洪量设计值减小程度均低于20\%,王快水库年洪峰流量、年最大1日、3日和6日洪量设计值减小程度均低于16\%,龙门水库年最大1日、3日和6日洪量设计值减小程度均低于13\%。(3)将基于非一致性洪水序列混合分布得到的设计洪水过程和不考虑环境变化的传统P-Ⅲ分布计算得到的设计洪水过程分别作为水库入库洪水过程,按各水库给定的调洪规则进行调洪计算,并比较调洪成果:非一致性条件下,各水库最高库水位减小幅度均小于1m;各水库最大下泄流量,西大洋水库除50年一遇设计洪水过程计算得到的成果因水库调洪规则其减小比例为81.68\%外,其余设计标准下减小比例为0.07\%~32\%;王快水库各设计标准下最大下泄流量减小比例均低于6\%;龙门水库各设计标准下最大下泄流量的减小比例在0\%~7.19\%范围。(4)考虑环境变化和不考虑环境变化两种条件下,白洋淀上游各水库不同重现期下的下泄流量过程,经各自的河道洪水演进,再叠加各重现期对应的河道区间设计洪水过程,得到两种条件下不同重现期的白洋淀入淀设计洪水过程,结果表明:考虑环境变化的50年一遇设计洪水过程在洪峰流量附近减小程度有明显差距,20年一遇设计洪水过程在设计洪水退落阶段也有明显减小,10年一遇设计洪水过程两种情况相差不明显。两种假设条件下,各不同重现期的白洋淀入淀设计洪水过程经淀区调洪计算得到对应重现期的白洋淀调洪成果,将两种条件下的成果进行比较:基于入库洪水序列变异的最高淀内水位有一定减小,减小程度不超过0.2m,但相应的白洋淀蓄水容积减小程度达1.3亿m3,白洋淀最大下泄流量的减小比例均低于10\%。(5)以西大洋水库入库年最大1日洪量序列和年最大30日降雨序列为例,通过相关性检验、主成分分析、广义线性模型的建立和物理意义分析,确定与西大洋水库控制流域的洪水和降雨相关性最好、影响最大的气候预测因子为位于北印度洋和热带西太平洋区域的MJJ SSTa时间序列(海表温度异常的5、6、7月的平均值)第一和第六个主成分PC1和PC6。建立3种假设条件下的模型:一致性模型、时间趋势模型和考虑PC1和PC6气候预测因子的非一致模型对非一致性洪水/降雨序列进行频率分析,通过模型之间的比较,以气候预测因子为协变量的时变矩模型最优,且能充分描述洪水/降雨频率的变化趋势,避免一致性模型对大洪水/降雨事件风险的低估和对小洪水/降雨事件的高估问题,定量估计气候变化对非一致性极值洪水/降雨频率的影响;并基于贝叶斯模型,得到洪水风险/降雨估计值的不确定性区间,为将来的非一致性洪水/降雨设计值进行预测提供依据。

【Abstract】 Extreme floods are one of the most threatening natural disasters for human beings. Frequency analysis is an integral part of risk assessment and mitigation in engineering design and commonly relies on the assumption of identically distributed observations. However, increasing evidence demonstrate that large-scale modes of climate variability and land use change exert a significant influence on hydrological extreme events in various regions worldwide. In light of the substantial change of climate, land use/land cover and increased number of soil-water conservation projects in Daqing River Basin, the assumption of stationarity in annual maximum inflow flood series of reservoirs should be reconsidered; otherwise reservoirs will undertake risks and which will further influence the flood-control system in Daqing River Basin. Baiyangdian Lake Basin, which is the principal flood-control part in Daqing River Basin, was taken as the study area in this thesis. Annual maximum inflow series of four upstream reservoirs were analyzed using non-stationary flood frequency method to obtain the significant effect of climate and land use/land cover change on design flood, reservoir flood routing and regional flood control. Meanwhile, climate predictors were identified to predict the extreme flood/rainfall. The main conclusions and innovations are summarized as follows:(1)This thesis proposed two steps to detect the variation in annual maximum inflow series and the second step is named detailed diagnosis to detect the trend and change points of flood series. For change points detection, Pettitt test was employed to identify the variation range and was associated with other methods’ detection results, land use/land cover change and change point of annual maximum 30-day accumulated rainfall series in reservoirs catchments to confirm the most possible change point. The results reveal that the most possible change point of all non-stationary flood series are occurred in 1979 and they have significant temporal trend.(2)Based on the change point detection result, the conditional probability distribution and mixed distribution were analyzed for nonstationary flood frequency calculation. And the conditional probability distribution is inferior to the mixed distribution. Hence, the mixed distribution was employed to analyze the nonstationary flood frequency and the results demonstrated that(a) the mixed distribution provided a more appropriate and superior fit than conventional distribution(P3 distribution);(b) Compared with the design flood values estimated by P3 distribution, design flood values of nonstationary flood series estimated by mixed distribution have different extent of decrease under cases of different return periods. Each reservoir has the similar conclusion.(3)The two design flood hydrographs under series’ non-stationarity and taking no account of series’ non-stationarity were regarded as inflow hydrographs of each reservoir. Then they were used to implement flood routing and to obtain the flood routing results. Compared with flood routing results regardless of series’ non-stationarity, namely, maximum reservoir water level and maximum discharge, the results under environmental change have different degrees of decreases corresponding to different return periods in each reservoir. This thesis estimated the effect of environmental change on reservoir’s flood routing.(4)Similarly, there were two cases for considering series’ non-stationarity and taking no account of series’ non-stationarity. On the basis of discharge hydrographs of each reservoir with two cases, the design flood hydrographs of rivers were added correspondingly and the inflow flood hydrographs of Baiyangdian Lake under two cases are obtained. Furthermore, two inflow flood hydrographs are routed through Baiyangdian Lake to investigate the influence of environmental change on regional flood routing.(5)The Xidayang Reservoir catchment is the largest catchment in Daqing River Basin. The annual maximum daily inflow(AMDI) to the Xidayang reservoir was analyzed to identify the climatic drivers of this variability. Since there were limited land use change data, the annual maximum 30-day rainfall(AMR) series was used to assess whether the AMDI trend can be ascribed to climate. A Bayesian nonstationary model was developed for the AMDI and AMR using lognormal and generalized extreme value distributions with climate predictors. We find that the average May-June-July Sea surface temperature anomalies in the Northern Indian Ocean and Western Pacific Ocean show a high negative correlation with the AMDI and the 30-day AMR series. Next we compare models of AMDI and AMR under three assumptions: a) time-invariant, b) linear temporal trend and c) climate informed, and find that the climate-informed models exhibit the best performance according to Deviance Information Criterion(DIC) and Bayesian coverage rate using 90 th percentile for both AMDI and AMR. Leave one out cross validation(LOOCV) is used to demonstrate that these models can provide useful prediction guidance for flood control and flood/rainfall risk management of reservoirs in Daqing River Basin, before the monsoon season begins, thus facilitating adaptation to a changing climate.

【关键词】 非一致性； 气候变化和下垫面变化； 混合分布模型； 时变矩模型； 气候预测因子； 调洪计算； 水库防洪和区域防洪；
【Key words】 Non-stationarity； Climate change and underlying surface change； Mixed distribution； Time-varying moment model； Climate predictors； Flood routing； Reservoir and regional flood control；