Flood Analysis And Calculation Based On Non-stationary Flood Time Series

As amounts of flood control and soil-water conservation projects have beenconstructed in Daqing River Basin, the underlying surface conditions aresignificantly changed, which leads to the inconsistency of the observed hydrologictime series. The design values which derived from the traditional frequency analysismethod under the scenario of the changing underlying surface condition are risky tobe used in the engineering hydrological design. In order to analyze the impact of thechanging underlying surface condition on the design flood hydrograph and floodrouting in Daqing River Basin and figure out the methods to deduce the design floodhydrograph and flood routing under this new scenario, the non-stationary flood timeseries of Wangkuai Reservoir, which lies in Shahe at the south branch of Daqingriver basin, were used as the basic data, and methods based on probability theory andstatistics were used to analyze the frequency of the non-stationary time series. Thedesign values were used to deduce the design flood hydrograph which was furtherapplied to the flood routing. The quantitative effects of the underlying surfacechange to the design flood hydrograph and flood routing were given as theconclusion in the end, which can be used as a reference to the design floodamendment and flood control operation in Daqing River Basin. The major researchaspects are listed as below:(1) A comprehensive method to diagnose the change point in the hydrologictime series was proposed based on the traditional hydrologic alteration diagnosissystem. This method contains three steps, namely preliminary diagnosis, detaileddiagnosis and comprehensive diagnosis. In the second step, MWP method was putforward to diagnose the change interval of the series; it can be used effectively todistinguish the possible change point combined with the results of other diagnosismethods. The results show that the change point of the flood time series of WangkuaiReservoir is1979.(2) The mixed distribution method and the probability distribution method wereused to fit the non-station flood time series of Wangkuai Reservoir, and then K-S testand AIC criteria were used to choose the optimal fitting. The design values werecalculated and utilized further to derive the design flood hydrograph based on the traditional same-frequency flood amplification method. Compared to the results ofthe traditional methods which dose not consider the inconsistency of the flood timeseries, the decreased magnitudes of flood peak and annual maximum flood volume(1-day and6-day) account for less than10%respectively, and the decreasedmagnitude of annual maximum flood volume (3-day) is about1%-15%.(3) The Copula method was used to construct the joint distribution of the floodpeak and the6-day annual maximum flood volume based on their respectivemarginal distributions which were determined in the second research aspect. Thejoint return periods and the conditional probability of the flood peak and floodvolume under certain conditions were derived and analyzed. Besides, the designvalues based on the joint distribution and joint return periods were also calculatedand applied further to deduce the design flood hydrograph using the amplificationmethod with disparate multiple proportions. The design values were compared to theresults of the univariate distribution based on the non-stationary flood time seriesand the results of the traditional method which use P-â…¢ curve to fit the originalflood time series. The results reveal that the design values of flood peak and annualmaximum flood volume based on joint distribution are larger than the respectivedesign values based on univariate distribution. Compared with the design values ofthe flood peak and the6-day annual maximum flood volume based on the traditionalmethod, the design values of the6-day annual maximum flood volume are larger,while the design values of the flood peak are larger or smaller according to theirreturn periods respectively; When the return periods are equal to or greater than200years, the design values of flood peak are smaller than the design values of the floodpeak based on the traditional method, and the comparison is converse when thereturn periods are less than200years.(4) The design flood hydrograph derived from the univariate distributionmethod and the joint distribution method were both inputted as the inflow process ofWangkuai Reservoir, based on the stipulated flood routing regulations, flood routingunder the scenario of the changing underlying surface were performed. The resultswere compared to the results from the traditional flood routing method which doesnot consider the inconsistency of the flood time series. The conclusions are asfollows: the maximum water level and the maximum discharge during the floodrouting process based on the univariate distribution method are both smaller than therespective results of the traditional method. However, as for the joint distribution method, the maximum water level and the maximum discharge during the floodrouting process are larger or smaller than the respective results of the traditionalmethod according to the return periods. When the return periods are equal to or lessthan200years, the maximum water level and the maximum discharge are larger thanthe respective results based on the traditional method, the maximum water levelincreases less than2meters and the increase rate of the maximum discharge is lessthan1%; When the return periods are greater than200years, the maximum waterlevel and the maximum discharge are smaller, the maximum water lever decreasesless than1meters and the decrease rate of the maximum discharge is less than10%.