EFFECTS OF DELTA EXTENSION ON BASE LEVEL RISING OF THE LOWER YELLOW RIVER

摘要

Owing to the channel avulsion and extension of the Yellow River delta in response to sediment deposition caused by heavy sediment loads entering the estuary, the water stage at a discharge of 3000 m~3/s (denoted by H_(3000)) at Lijin station (the endpoint of the lower Yellow River) rose by 2.5 m between 1950 and 2000, with an average rate of 5 cm per year. The water stage or channel bed elevation at Lijin station acts as the local base level of the lower Yellow River (LYR), and its rise is significant for the long-term evolution of the upstream channel. Impacts of delta evolution on the changes of H_(3000) at Lijin were analysed in this study. The results showed that whenever channel avulsion occurred, the river length decreased abruptly and the channel slope increased. The channel length then increased at a decreasing rate, while the slope decreased exponentially with time and reached an equilibrium value after 5 to 6 years. This phenomenon indicates a significant characteristic of the delayed response of the fluvial system. Based on differences in the evolution of channel reaches upstream and downstream of the diversion point at the Yellow River delta, two generalized models for the longitudinal changes of the extended delta channel were proposed. An equation for calculating H_(3000) at Lijin was developed by using the delayed response model for modelling morphological responses of fluvial rivers to perturbations. H_(3000) at Lijin during the last 60 years was calculated well by the proposed method, which considers the impacts of water and sediment conditions and delta evolution on the changes of the local base level of the LYR. This study helps to increase the understanding of the interactions of delta evolution and upstream channel reaches where high sediment yield is present.