Check calculation and verification of design floods for Danjiangkou Reservoir based on multiple approaches and schemes
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Abstract:
Original design flood and the check flood for the Danjiangkou Reservoir are 79 000 m3/s and 118 000 m3/s respectively, which may be significantly overestimated. The overestimation of design floods would lead to a lower flood limited water level (FLWL) in the reservoir, a key parameter for balancing flood control and comprehensive utilization during the flood season. Simultaneously, the hydrological regime of the downstream Han River has changed significantly, and the original design floods for the Danjiangkou Reservoir cannot meet the new demands of efficient water resources utilization and eco-environment protection. Consequently, it is imperative to recheck the design floods and characteristic water levels of the Danjiangkou Reservoir to enhance the comprehensive utilization benefits under the condition of the flood prevention standards unchanged. The observed flood data series at the Danjiangkou Reservoir dam site was restored and extended to 2023, and the historical floods of the Han River basin were re-evaluated. The discontinuous data series consisted of seven historically investigated floods in the years of 1583, 1693, 1724, 1832, 1852, 1867, and 1921, along with 95 years of annual maximum peak discharge and flood volume series recorded from 1929 to 2023, while the 1935 observed large flood had been treated as an extraordinary flood event. The P-Ⅲ distribution and curve fitting method were selected for the frequency analysis, and the sample statistical parameters were initially estimated using the linear moment’s method, followed by adjustments using the sum of squared deviations criterion to finalize the P-Ⅲ distribution parameters. Five different schemes were proposed to recheck the design floods of the Danjiangkou Reservoir and described as follow: Scheme-1, Extend both the observed flow data series (1929-2023) and historical investigation period (1583-2023), while the historically investigated floods used were the same as in the original design; Scheme-2, Revaluate historical investigation period (1368-2023), while the others were the same as Scheme-1; Scheme-3, Treat the floods of 1583 and 1724 as indetermined historical floods, while the others were the same as Scheme-2; Scheme-4, Exclude the historically investigated floods of 1583 and 1724, while the others were the same as Scheme-2; Scheme-5, Extend and use the observed flood series data only (1921-2023), while remain the 1935 observed large flood being treated as an extraordinary flood event. Only the historically investigated 7-day flood volumes from the years 1583, 1832, 1852, 1867, and 1921 are available, while the other two, which lacked magnitudes, are treated as indetermined floods that only consider for their ranking order. The other set of discontinuous data series is similarl to the above five schemes mentioned above. Multiple approaches, including palaeoflood estimation in the Han River basin, empirical formulas for the largest floods on record globally, and International Commission on Large Dams (ICOLD), are used to calculate probable maximum floods. After comprehensively analyzing and verifying the results estimated through multiple approaches and schemes, it is concluded that the checked design floods for the Danjiangkou Reservoir are significantly smaller than the original design values; The 1 000-year design and 10 000-year check peak discharges at the Danjiangkou Reservoir dam site are 56 950 m3/s and 70 000 m3/s, which have decreased by 27.91% and 40.68%, respectively. The 1 000-year annual maximum 7-day flood volume has reduced 2.1 billion m3, while the construction of upper stream reservoirs increases 1.7 billion m3 flood prevention storage. Therefore, it is suggested that the FLWL during the summer flood season be raised to 163 m, which can effectively enhance the flood water utilization rate and reservoir refill rate, and maximize comprehensive benefits, and ensure the safety of flood control operations.
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