Assessment of a River Reach for Environmental Fluid Dynamics Studies

Abstract

Turbulence is the fundamental mechanism governing energy transfer in river flows that was conventionally examined in laboratory flumes. Recently, a trend has been observed for constructing larger scale and outdoor facilities that tend to avoid the problems of upscaling of experimental results. This paper presents the results of an experimental study performed on a river reach used as an environmental field laboratory. The study is focused on the understanding of the spatial arrangement of the flow structure and its dependency on the temporal variability of the flow. Detailed measurements were taken using acoustic Doppler velocimeters and their analysis was completed applying the theory of open-channel flows. The obtained results reveal that the flow structure on the river reach resembles characteristics of a typical three-dimensional open-channel flow. Away from the riverbanks, the flow behaves as a quasi-two-dimensional fully developed turbulent open-channel flow thus providing conditions favorable for field experimental studies of shallow mixing layers and flows over patches of submerged aquatic plants. An interesting observation in the seasonal dynamics of turbulent shear stress patterns was that the height of the roughness layer was reduced in the central part of the flow, though the overall roughness coefficient was increased. At the same time, the structure of the secondary flow near the banks was also substantially altered as the secondary circulations observed at low water levels were replaced by flow separation and internal boundary layers at medium water levels.