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The impact of silt trapping in large reservoirs on downstream morphology: the Yangtze River

机译：大型水库淤泥淤积对下游河床形态的影响

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The sediment load of the Yangtze River (China) is decreasing because of construction of dams, of which the Three Gorges Dam (TGD) is the best known example. The rate of the decline in sediment load is well known, but changes in the sediment grain size distribution have not been given much attention. The TGD mostly traps sand and silt while clay is flushed through the reservoir. A large amount of sand is available in the Yangtze River downstream of the reservoir, and therefore the pre-dam sand concentration is not substantially reduced. The availability of silt on the Yangtze River bed is limited, and it is expected that most silt will be removed from the riverbed within one to two decades. In order to evaluate the impact of the change in grain size distribution on the tidal flats of the Yangtze Estuary, a highly schematized tidal flat model is setup. This model broadly reveals that the observed deposition rates are exceptionally large because of the high sediment concentration, the abundance of silt, the seasonal dominance of waves (shaping a concave profile), and the offshore tidal asymmetry. The model further suggests that deposition rates will be limitedly influenced by reductions in clay or fine silt but strongly impacted by reductions in median to coarse silt. The response of the downstream morphology to reservoir sedimentation therefore strongly depends on the type of trapped sediment. As a consequence, silt-dominated rivers, such as the Yangtze River and the Yellow River may be more strongly impacted than sand-dominated systems.
机译：由于修建大坝，长江（中国）的泥沙负荷正在减少，其中最著名的例子是三峡大坝（TGD）。泥沙负荷的下降速度是众所周知的，但是泥沙粒度分布的变化并未引起足够的重视。当粘土通过储层冲洗时，TGD大部分会捕获沙子和淤泥。在水库下游的长江中有大量的沙可用，因此大坝前的沙浓度并未显着降低。长江河床的淤泥供应有限，预计大多数淤泥将在一到二十年之内从河床清除。为了评估粒度分布变化对长江口滩涂的影响，建立了高度图解化的滩涂模型。该模型广泛地揭示出，由于高的沉积物浓度，丰富的淤泥，波浪的季节性优势（塑造凹形剖面）和近海潮汐不对称，观测到的沉积速率异常大。该模型进一步表明，沉积速率将受到粘土或细粉土减少的有限影响，但受到中值至粗粉土减少的强烈影响。因此，下游形态对储层沉积的响应在很大程度上取决于捕获的沉积物的类型。结果，以泥沙为主的河流（如长江和黄河）可能比以沙为主的系统受到的影响更大。

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