ADV measurements around a cluster microform in a shallow mountain stream

A field study was conducted to examine the turbulent properties of the flow around a naturally formed cluster bedform. This was carried out on a mountain river using a 3D acoustic Doppler velocimeter (ADV). Concurrent with this objective, the ability of the ADV to make measurements in a shallow flow over a cobble bed (h(o)/d(84)= 1.75) was examined. ADV measurements in natural clear-water shallow flows around obstacles are inherently difficult to obtain due to (1) signal interference between the acoustic reflections from the boundary and the ADV sample volume; (2) regions of high turbulent intensity, such as in local detached shear layers; and (3) low concentrations of suspended particles passing through the ADV sampling volume. These processes result in velocity time series that contain a significant amount of spikes, lower-than- average signal-to-noise ratios,- and lower than average correlation values. A filtering process that optimized the removal of bad points while retaining a sufficient number of points to describe the velocity time series histogram was developed. In general, flow over the study section can be described by an inner roughness layer dominated by large roughness and detached flow, and an outer flow that exhibits a form of log scaling. It was found that the cluster acts to locally modify this structure by shifting the elevation of the roughness layer and the zone of primary production and dissipation of turbulent energy up towards the center of the water column. Mean and turbulent statistics, energy distributions, spectral properties, and a quadrant analysis are presented to characterize the flow around the cluster.