Design water level for flood control of water conservancy projects at the inlet and confluence of branches in river basins: A case study from Guiping Shipping Hub, Xijiang River, Pearl River Basin

修建在河流支流入汇口处的水利工程，同时受干、支流洪水共同作用，其防洪水位不但与洪水组合特性有关，还与洪水、水利工程行洪的耦合关系有关.已有采用设计洪水重现期对应的特征洪水组合，或者直接采用经验洪水组合进行调洪计算得到防洪设计水位的方法无法有效反映洪水与水利工程的相互作用.本文结合防洪标准的本质，利用Copula-蒙特卡罗模拟方法对修建在支流入汇口处的水利工程的防洪设计水位进行计算，并以珠江流域西江支流郁江广西桂平航运枢纽水闸为例展开研究.结果表明：Copula-蒙特卡罗模拟方法可以有效考虑干、支流洪水组合特性及其与水利工程行洪的耦合关系，以其计算得到的防洪设计水位唯一且可保证达到防洪标准的要求，可有效克服利用洪水重现期确定防洪设计水位存在的不确定性；在干、支流洪水共同作用下，以设计洪水重现期的不同特征组合计算得到的防洪设计水位相差非常大，且与达到防洪标准要求的防洪设计水位相比完全偏离了合理的误差范围，不宜以设计洪水重现期确定防洪设计水位；利用经验洪水组合计算难以合理反映不同工程的洪水及工程特性.研究成果可为修建在支流入汇口处受干、支流洪水共同影响的水利工程防洪设计提供更合理的理论依据和思路.;The hydraulic engineering built at the inflow into the junction can be affected by both main and tributary floods. The control flood level has relationship not only with flood combination characteristic, but also with the coupling relation between flood and hydraulic engineering flood discharge. The existing methods of designed flood level assessment, based on the characteristic flood combination corresponding to the design flood recurrence period or the empirical combination directly, cannot accurately reflect the interaction between floods and hydraulic engineering. In this paper, the Copula-Monte Carlo simulation method is used to calculate the flood control design water level of hydraulic engineering built at the inflow into the junction. The acceptability is verified via a case study in the sluice of Guangxi Guiping Shipping Hub, Xijiang River, Pearl River Basin. The results show that, the proposed method can be used to account for the combination characteristic of main and tributary floods and coupling relationship with flood discharge of hydraulic engineering. The calculation result of the designed flood control water level meets the requirement of the flood control standard, indicating that the uncertainty of determining the designed flood control water level by using the flood return period is effectively overcame. Under the combined action of main and tributary floods, the design water level varies tremendously by using the different combinations of characteristic of the design flood recurrence period. The deviation exceeds the reasonable error range of the flood control standard, suggesting the unacceptability of the use of the design flood return period on the determination of flood control level. Otherwise, the results also indicate that it is difficult to reflect the flood and engineering characteristics of different projects reasonably by using empirical flood combination. The paper can provide a more reasonable theoretical basis and ideas for flood control design of the hydraulic engineering affecting by main and tributary floods at the inflow into the junction.