基于HHT的三峡水库蓄水后坝下游水情多尺度时频特征

MULTI-SCALE TIME-FREQUENCY CHARACTERISTICS ANALYSIS OF RIVER DYNAMICS DOWNSTREAM THE THREE GORGES DAM AFTER ITS IMPOUNDMENT BASED ON HHT

利用基于噪声辅助经验模态分解（EEMD）的改进型希尔伯特-黄变换（HHT）,对三峡蓄水后8a的三峡日入库、出库流量和坝下各水文站日水情信号进行多尺度时频分析,并与连续小波和离散小波变换进行异同性比较。结果表明,三峡蓄水后2004-2011年三峡日入库、出库流量和宜昌、大通站日径流量均具显著的1a尺度主周期振荡规律,三峡季节性调蓄对坝下流量1a尺度的主周期振荡规律无显著影响。三峡蓄水后河口北支江水位的半月主周期振荡特征始终占主导地位,表明三峡调蓄影响下上游来水量变化对河口北支水位波动的影响权重小于河口自身海潮波动的影响权重。与离散小波变换固定频带下的信号分解比较,基于HHT的水文信号多尺度分解有更好的自适应性;与连续小波变换比较,HHT方法在水文信号高频部分周期振荡特征的揭示和频域混叠现象的避免方面更具优势。

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 hydrcrsignals 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.