Abstract: Hyporheic exchange is one of the important physical processes that affect the flow conditions of river-groundwater belt, the water environment restoration, and the ecological balance. However, the mechanism of the influence of clay lens on subsurface flow in riverbed has not been well revealed. In this study,the hyporheic exchange under the influence of clay lens is investigated by means of physical modeling with appropriate dimensions with reference to actual geological conditions.Under controlled water head and discharge, the hyporheic exchange is simulated in the presence of varied sizes of clay lens at different depths, and the groundwater flow field in the hyporheic zone is rendered according to the measured pressure data. For each simulation case, the gradient of flow field is calculated using the contours drawn at the same distance from the clay lens in four directions. The velocity field near the lens can then be obtained, and is further transformed to flow field in line with Darcy’s law. The study reveals that hyporheic discharge decreases with the increase of the size and depth of single clay lens.As the water head difference between the upper and lower surface water increases, the flow velocity increases at the upstream face of clay lens with an attenuating growth rate, but reduces at the back side of the lens with an aggravating reduction rate. In addition, a method for studying small scale subsurface flow exchange processes is also provided.

Key words: clay lens, physical model, hyporheic exchange, Darcy’s law, velocity of flow