Underlying gravel layers in a large sand bed river and their influence on downstream-dam channel adjustment

Due to the changes in environmental factors during a riverʹs historical development, underlying gravel layers are found in many large plain rivers. When the buried depth of this gravel layer is within the reach of down-cutting by clear water scour after reservoir construction, it may be exposed and exert a far-reaching influence on channel adjustment. In Hanjiang River, the longest tributary of the Yangtze River in China, down-cutting has been greatly reduced and even stopped, due to the total change of bed material composition. The exposure of a gravel layer results in a dramatic increase in the bedʹs hydraulic roughness, so the channel slope may increase after a decline during the period prior to the exposure of the gravel layer. Moreover, the exposed gravel layer increases the erosion resistance of bed material, making a lower relative erosion resistance of bank to bed material. Where the bankʹs erosional resistance is weak, this may lead to a tendency towards channel widening.