Abstract:

As one of the world largest hydropower projects, the impoundment of Three Gorges Dam (TGD) after 2003 and its possible impact on downstream hydrological environment aroused world attention. The present study made an innovative research about the actual impact assessment of TGD impoundment by revealing the multiscale timefrequency characteristics of different hydrosignals upstream and downstream TGD. The daily inflow and outflow data series of TGD, daily runoff series downstream TGD in Yichang and Datong Hydrostations, daily high and low tide level series in north Yangtze River Estuary, and multiscale timefrequency characteristics of different hydrosignals for 8 years (2004-2011) were analyzed by using the improved HilbertHuang Transform (HHT) method, which was based on the Ensemble Empirical Mode Decomposition (EEMD) algorithm. Results from the improved HHT method were further compared with that from the Continuous Wavelet Transform (CWT) and Discrete Wavelet Transform (DWT) methods. Results from the improved HHT method showed the dominant 1year periodicities in the TGD daily inflow and outflow series and also in the daily runoff series in Yichang and Datong Hydrostations during years 2004-2011 after TGD impoundment, which indicated that the impact of TGD regulation on the 1year periodic features in river dynamics downstream TGD was not significant. As for the daily high and low tide level series in the north branch of Yangtze River Estuary, the dominant halfmonth periodicities were revealed by the improved HHT method, which indicated that the impact weight of upstream inflow dynamics caused by TGD regulation on estuarine river stage dynamics was lower than the impact weight of estuarine ocean tidal fluctuation. In addition, by comparing with the DWT method, which decomposed signals at the fixed frequency bands, the improved HHT method showed better self adaptability at signal decomposition because of the EEMD algorithm. By comparing with the CWT method, the improved HHT method showed more advantages in revealing signals periodic oscillations at highfrequency scales because of its capacity in avoiding frequency aliasing phenomena involved in the signal decomposition. However, the weak orthogonalities among the intrinsic mode function (IMF) components from the EEMD method would still require attention and need to be solved by further research