Hyporheic exchange in a compound channel under unsteady flow: Numerical simulations

Hyporheic exchange (HE) within compound channels under unsteady surface water flow conditions plays an important role in the river-floodplain ecosystem. Numerical simulations were applied to investigate the HE in a compound channel under unsteady flow. High velocity regions were observed in the hyporheic zone near the banks of the floodplain and of the main channel, and the distribution of the pore water velocity in this region was seen to significantly change under the unsteady conditions. The velocity peaked during the rising and falling phases and had the smallest value when the inlet discharge of surface water had the maximum and minimum values. The changes in the relationship between surface groundwater recharge and discharge were observed in the context of a lateral water level threshold of approximately 1.2 m. About 75 % of the hyporheic exchange flux was due to the dynamic pressure. Comparing the hyporheic exchange in the compound channel driven by static pressure with that in a rectangular channel, it was found that the floodplain decreased the hyporheic exchange. These findings provide a better understanding of the HE driven by unsteady flow in a compound channel and may be applied in combined flood management and river ecological restoration.