Simulation and study of riparian groundwater exchange processes in the Shishou reach of the Middle Yangtze River

Riparian zones in alluvial rivers have important functions such as hydrological regulation and environmental purification. This study investigated the groundwater flow processes of the Shishou reach in the Middle Yangtze River (MYR), which can provide an important basis and reference for planning and management of the riverbanks. Soil gradation measurements and groundwater level monitoring were firstly carried out in the Shishou reach in 2021. Afterwards, numerical simulations were conducted of the groundwater flow processes under the influence of the varying river stage. Based on these numerical tests, the influencing range of riparian groundwater flow was investigated, and variation characteristics of riparian groundwater level at locations with different topographic features were quantitatively analyzed. Measured data and calculation results show that riparian groundwater level was significantly affected by river stage, given that the hydrological synchronization at 10 m away from the bank marginal was almost the same, and the peak at 20 m had a delay of only one day. With the increasing distance from the bank marginal, the response of groundwater level to in-channel water level became slower, with the peak being flattened. The impact zone of groundwater flow variation could extend to about 1 400—1 600 m away from the bank marginal. The annual variation of groundwater level outside this range was less than 5% of the maximum variation of in-channel water level. The unit channel-length water volume seeping into bank was about 5 000 m3 / m during the 2021 rising and flood-peak periods, and the water volume discharging from the bank was about 3 000 m3 / m during the 2021 recession period. Compared with the straight reach, the response of groundwater level at the convex side to in-channel water level was faster with a higher peak value and an opposite phenomenon was observed at the concave side. As the process of rainfall infiltration was considered, the calculated runoff coefficient was about 0. 80 in the study area. Compared with the case without rainfall infiltration, variation characteristics of groundwater level were almost unchanged, but the unit channel-length water volume seeping into and discharging from the river banks were both equal to about 4 000 m3 / m in 2021. © 2023 China Water Power Press. All rights reserved.