Dynamic and delayed channel geomorphic responses to changing hydrological processes following the Three Gorges Dam operation

Abstract

AbstractThe characteristics of dynamic and delayed channel geomorphic responses to changing hydrological processes are still insufficiently understood, especially in large dammed rivers and segments farther downstream from the dam. Geomorphic effects farther downstream from the dam could be easily underestimated. This study investigated the hydrological changes and channel adjustments in the Chenglingji–Jiujiang Reach (CJR), ~450 km downstream from the Three Gorges Dam. The post‐dam evolution developed from early low‐intensity alternating erosion/deposition to later intense erosion. This is because the increasing sediment starvation caused by the dam and attenuated upstream sediment replenishment produced streams with excess transport capacity for the farther downstream channel, where the early mild evolution and fine‐sandy riverbed guaranteed adequate sediment availability. Additionally, the increasing geomorphic contribution of low‐to‐medium flows, coupled with revetment resistance to lateral adjustments, promoted migration of the major adjustment area from above the bankfull channel to below the medium‐flow channel and characterized erosion by one‐way incision with no evident channel narrowing/widening. The net increase in reach‐averaged thalweg incision depth during a later period (2.85 m) was 35 times larger than that during an earlier period (0.08 m). Channel morphogenetic intra‐annual hydrological processes were identified to contain all discharges smaller than 39 000 m3/s, and discharges of 30 000–39 000 m3/s exerted maximum geomorphic effects. On this basis, a method for estimating the cumulative erosion volume at equilibrium was proposed and integrated into the Delayed Response Model. The encouraging simulation and prediction results indicate that geomorphological adjustments within the CJR were greatly influenced by the hydrological conditions of current year and previous 4 years, while the impacts of extreme and rare events depended more on the consistency of their hydrological behaviours with channel evolutionary trends than on their temporal distances. This work facilitates the understanding and prediction of channel self‐adjustments farther downstream from the dam.