Hydrological And Hydraulic Responses To Large-scale Water Projects In The Yangtze River Ecosystem

With the construction and operation of the large-scale water projects, the flow and sediment regimes are altered downstream, resulting in alterations in the environmental elements of aquatic habitats, i.e. hydrological, hydraulic, sediment, riverbed substrate, water quality and water temperature conditions, which are critical for the survival of aquatic creatures. In order to maintain river ecosystem health and biodiversity, it is urgent to study the alterations of aquatic habitat environment. This thesis selected the Yangtze River as study case.Firstly, this thesis introduced the components, structure, characteristics and functions of a river ecosystem. As the main component of river ecosystem, aquatic habitat is impacted by human activities at the first level, and its major environmental elements, i.e. hydrological and hydraulic variables are the main concern.Secondly, this thesis evaluated the impacts on hydrological variables downstream of the Gezhouba Reservoir, the Danjingkou Reservoir and the Three Gorge Dam. The variations in runoff and sediment load before and after these water projects constructions were compared. The results indicated that the reservoirs have caused the alterations in hydrological regimes downstream to some extent, such as more even distribution of monthly runoff and apparent decrease in sediment load downstream. The impacts of these reservoirs on flow and sediment regimes downstream varied with reservoir regulation capacity, reservoir operation pattern and the distance between target reservoir and study site. Using Principal Components Analysis (PCA), the thesis quantificationally analyzed the effects on hydrologic variables of human activities, including reservoir construction, and identified hydrological variables affected significantly by human activities.Thirdly, the impacts on the relationships between runoff and sediment load of the water projects were analyzed. The results revealed that the runoff-sdiment relationship downstream were affected by the reservoirs to some degree, depending on reservoir regulation capacity, reservoir operation pattern and the distance between target reservoir and study site, and that the alterations degree in runoff and sediment load were different.Finally, hydraulic variables were simulated and analyzed in the progenitive field of the Chinese Sturgeon below the Gezhouba Reservoir. On one hand, on the basis of the optimal flow rates for Chinese Sturgeon propagation, this thesis simulated and computed the corresponding velocity, hydraulic radius, shear, stream power and other hydraulic variables at different cross-sections in the Chinese Sturgeon propagation field. On the other hand, the variations in the hydraulic variables before and after the Three Gorge Dam constrution were compared. The results demonstrated that the value of each hydraulic variable decreased after the Three Gorge Dam constrution to some extent, which would probably affected the Chinese sturgeon propagation.This thesis could provide reference for further studying the impacts of human activities on the Yangtze River ecosystem and maintaining its health and biodiversity.