设计洪水峰量最可能组合法的计算通式

For conventional multivariate frequency analysis methods,there are an infinite number of combinations of flood peaks and volumes satisfying the flood prevention standard under the given joint return period(JRP);however,different joint design values would lead to differences of flood hydrograph and maximal water level which occurs in reservoir routing.Hence,how to select reasonable design values under given JRP is very important. In this paper,the Copula functions were used to construct the multivariate joint distributions of flood peaks and volumes,and the OR JRP was adopted as flood prevention standard.The normalized joint probability density function value was adopted to measure the relative occurrence likelihood of flood events,and the general model of Most Likely Composition (MLC) of flood peaks and volumes were established subject to the flood prevention standard.The Lagrange multiplier method was adopted to solve the model.The trivariate and four-dimension MLC design floods of Danjiangkou reservoir in Hanjiang basin were estimated using the proposed method,and the quantiles estimated by univariate identical frequency (UIF) and multivariate identical frequency (MIF) methods were compared with those calculated by MLC.The results indicated that the UIF estimations could not satisfy the flood prevention standard;the UIF method assumed that the flood peak is fully dependent upon the flood volume and all the variables are characterized based on univariate distributions.The UIF method cannot take the correlations of hydrologic variables into consideration,and it does not satisfy the inherent rule of flood events. It is also indicated that the MLC method had a smaller flood peak and larger long-duration volume than those by the MIF method.The Danjiangkou reservoir has the capacity of multi-year regulation and the flood volumes are mainly responsible for flood prevention safety,and thus the MLC estimation values are safer.The joint design values of MLC method are more rational according to the application requirements in Danjiangkou reservoir, and consequently the proposed method had a strong statistical basis and could reflect the inherent rules of hydrological events,and it could be used to estimate the design floods of reservoir.