THE ROLE OF VORTEX SHEDDING IN THE SCOUR OF POOLS

The role of vortex shedding was investigated in the field and in two separate flume experiments. Velocities were measured with an acoustic Doppler velocimeter. The field-based project was initiated to characterize the influence of varying discharge on the strength of turbulence generated by vortex shedding. Turbulence intensities were measured at nine to 12 different profiles located downstream of a boulder constriction within a large pool on North Saint Vrain Creek, Colorado. The root mean square (RMS) of x-, y- and z-components of bed velocity along the eddy fence was significantly related to both the distance from the boulder constriction and discharge. Turbulence intensity decayed in the downstream direction and decreased at lower discharges. Flume experiments were conducted in a 6-m flume to determine the role that turbulence related to wake-zone formation plays in scouring pools. A scaled flume experiment was designed based on the geometry of the field site to determine variations in the location and strength of vortex shedding with pool elongation. The RMS of velocity is significantly related to the distance from the constriction in most pool locations downstream of the constriction. In a second set of experiments, pools are deeper and shorter when vertical free-shear layers are present. Although pools associated with non-streamlined obstructions initially present more resistance to flow, channel-bed scour aided by vortex shedding eventually lowers mechanical energy losses to levels below those in pools with streamlined obstructions. Scour primarily occurs in an effort to widen the cross-sectional area in the constricted section. Feedback between pool geometry and localized turbulence production play a secondary role in total scour but still exert a major influence on final energy slopes in the experiments. The experiment shows that pools with wake zones evolve to minimize longitudinal energy expenditure with an associated reduction in total turbulence production associated with the obstruction.