Experimental study of turbulent unconfined groundwater flow in a single fracture

Laboratory experiments have been carried out to study groundwater flow in a single fracture under the conditions of different surface roughness and apertures. We found that the gradient of the Reynolds number versus the average velocity in a single fracture was almost independent of the change of fracture surface roughness, and it decreased when the aperture decreased under the same surface roughness. The experimental results showed that the average flow velocity (V) could be approximated by an empirical exponential function of the hydraulic gradient (I), and the power index of the exponential function was close to 0.5 when the hydraulic gradient is around 0.003 to 0.02. Such a V–I relationship indicated a non-Darcian turbulent flow in the fracture even though the Reynolds number was relatively low (between 333.26 and 1413.62). This finding supports the claims of non-Darcian flow observed in fractures by many recent studies under relatively fast flow condition but disagreed with the Darcian flow and local cubic law assumptions used in some previous studies.