Abstract
The retention lake often plays an important role in flood mitigation through the water storage and the lake–river interactions. However, the evolution of real-time flood regulation capacity remains poorly characterized. Using wavelet decomposition and flood peak removing ratios, this study presents a comprehensive evaluation of the characterization, mechanism, and impacts of the flood regulation capacity in Dongting Lake. The results indicate that the change of flood regulation effect of the lake can be well reflected by the multi-year changes in the variances of the inflow and outflow runoffs. The wavelet decomposition indicates that the flood regulation of the lake is mainly functioned on the high-frequency floods with durations less than 32 days. The average yearly flood peak removing ratios range from 0.13 to 0.56, but no significant tendency changes on the effect of the flood regulation capacity has happened during the study period. The changes in maximum regulation volume reveal that the flood regulation of the Dongting Lake is mainly a passive process decided by the complex river–lake relationship and the interactions among different processes of discharge and sediment. The impacts from the large volume reduction caused by sedimentation in the lake is compensated by the increased flood controlling water level, which in turn have resulted in the new phenomenon of “normal discharge, high water level and disaster” in the lake regions after the 1990s. The significant impacts on the lake–river relationship caused by the sediment reallocation from the operation of the Three Gorges Reservoir (TGR) have further changed the hydrological regimes between the lake and the Yangtze River. Influenced by the new lake-river interaction pattern the discharge passing capacity downstream the outlet of the lake is becoming a key factor that affects the flood regulation capacity, which is leading to a shift of the flood pressures from the lake region to the downstream of Yangtze in the near future.
Highlights
Lake–river systems are among the most diverse and dynamic ecosystems benefitting from the natural fluctuations of water level rhythms [1,2]
For a large retention lake like Dongting Lake, its real flood process is often formed from the superposition of several flood wavelets with different wavelengths and amplitudes [21,30]
The actual flood regulation characteristics of the lake, and their evolution, can be analyzed by using the wavelet decomposition based on the variances of the inflows and outflows of the lake
Summary
Lake–river systems are among the most diverse and dynamic ecosystems benefitting from the natural fluctuations of water level rhythms [1,2]. Hydrological connectivity and flood pulses, as the central aspects for the integrity of a lake–river system, have been largely altered by river regulations [3,4,5]. Hydrological regimes of lake and river are extensively altered. Water 2020, 12, 2853 by damming and water diversion [9,10], as an enlarged channel storage lake plays an important role in alleviating flood and maintaining wetland ecology [11,12]. Retention lake affects flood peaks and smooth flow pulses, and affects ecosystems by complex water level fluctuations [3]
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