Floodplain hydrodynamics and connectivity in a natural compound channel during unsteady flow events

Abstract

This study investigates the hydrodynamics features and the floodplain connectivity of a natural compound channel under unsteady flood conditions using a two-dimensional shallow water equation model and a Lagrangian particle tracking method. Two flood events in 2019 in the middle reach of the Ganjiang River in China were simulated. The results show that during the rising stage, flow first passed through the low-lying areas of the floodplain. The floodplain discharge ratio increased almost linearly with the depth ratio between the floodplain and main channel when the floodplain was not fully inundated. When the floodplain was fully inundated, a second linear relationship was found between the floodplain discharge ratio and depth ratio. During the falling stage, flow first moved back to the low-lying floodplain and main channel before fully receding from the floodplain. The sequence of peak velocity, discharge and stage in unsteady flow lee to higher velocities, lower depths, and shorter residence times during the rising limb compared to those in the falling limb at the same discharge. The threshold discharge for floodplain inundation was during the rising stage larger than during the falling stage. The shortest particle residence time was observed at the flood peak, while the residence time in the rising stage was longer than in the falling stage. The particle travel distance was similar at different stages. The exchange flux between the river and floodplain increased with inflow discharge following a power law relationship. The ratio of exchange flux to inflow discharge also increased with inflow discharge up to an upper limit of 65.5 %. Particle residence time was negatively correlated with discharge following a power law with a lower limit of 2630 s, while particle travel distance is positively correlated with discharge following a power law with an upper limit of 2325 m. These findings shed light on the complex hydrodynamic processes and connectivity patterns in natural compound channels during unsteady flood conditions.