Experimental Investigation of Shallow Recirculating Flows

Experiments were conducted to study the bed-friction influence in the recirculating zone of a shallow open-channel flow. Velocity measurements were made using a hot-film anemometer. The recirculating flow rate and the length of the recirculating zone were determined from the measurements, and they are related to a horizontal length scale and a bed-friction length scale. For deep flows, the results are independent of the bed-friction length scale. For shallow flows, the results are independent of the horizontal length scale. Turbulence in the recirculating flows is generated, on one hand, by the bed friction and, on the other hand, by the transverse shear. The bed-generated component is proportional to the bed-friction velocity. The transverse component is confined to a transverse, mixing region. In the limit of shallow water depth, when the flow is under strong bed-friction influence, both the transverse mixing region and the recirculating zone have the same length of about one friction length scale.