Water level variation characteristics under the impacts of extreme drought and the operation of the Three Gorges Dam

Under the influence of a climate of extreme drought and the Three Gorges Dam (TGD) operation, the water levels in the middle and lower reaches of the Yangtze River in 2006 and 2011 changed significantly compared with those in the extreme drought years of 1978 and 1986. To quantitatively analyze the characteristics of water level variations in 2006 and 2011, a new calculation method was proposed, and the daily water level and discharge from 1955-2016 were collected in this study. The findings are as follows: in 2006 and 2011, the water level in the dry season significantly increased, but that in the flood season obviously decreased compared with the levels in 1978 and 1986. Here, we described this phenomenon as “no low-water-level in dry season, no high-water-level in flood season”. Based on the calculation method, the contributions of climate variability and the Three Gorges Dam operation to water level variations in the middle and lower reaches of the Yangtze River were calculated, and the contributions indicated that climate variability was the main reason for the phenomenon of “no low-water-level in diy season, no high-water-level in flood season” instead of flood peak reduction in the flood season and drought runoff implementation in the dry season, which are both induced by TGD.

Erosion-deposition patterns and depo-center movements in branching channels at the near-estuary reach of the Yangtze River

Channel evolution and depo-center migrations in braided reaches are significantly influenced by variations in runoff.This study examines the effect of runoff variations on the erosion-deposition patterns and depocenter movements within branching channels of the near-estuary reach of the Yangtze River.We assume that variations in annual mean duration days of runoff discharges,ebb partition ratios in branching channels,and the erosional/depositional rates of entire channels and sub-reaches are representative of variations in runoff intensity,flow dynamics in branching channels,and morphological features in the channels.Our results show that the north region of Fujiangsha Waterway,the Liuhaisha branch of Rugaosha Waterway,the west branch of Tongzhousha Waterway,and the west branch of Langshansha Waterway experience deposition or reduced erosion under low runoff intensity,and erosion or reduced deposition under high runoff intensity,with the depocenters moving upstream and downstream,respectively.Other waterway branches undergo opposite trends in erosion-deposition patterns and depo-center movements as the runoff changes.These morphological changes may be associated with trends in ebb partition ratio as the runoff discharge rises and falls.By flattening the intra-annual distribution of runoff discharge,dam construction in the Yangtze Basin has altered the ebb partition ratios in waterway branches,affecting their erosion-deposition patterns and depo-center movements.Present trends are likely to continue into the future due to the succession of large cascade dams under construction along the upper Yangtze and ongoing climate change.