Near bed turbulence measurement with acoustic doppler velocimeter (ADV)

Acoustic Doppler Velocimeter (ADV) has become prominent device in flow and turbulence measurement. The in situ turbulence measurement requires laboratory-quality data to be obtained in the field. Thus, ADV vector type provides a better choice for flow measurement at field scale. A field study was undertaken at sand-gravel bed rivers to observe the flow field and turbulence characteristics in the presence of protuberant clast. Profile measurements were executed at near bed region to characterize the flow and turbulence characteristics at the proximity of clast reference. Four major areas were identified as sampling location relative to clast reference namely stoss side, adjacent area, wake area and the downstream area. Besides, bed load measurement was carried out concurrently to elucidate the possible relation between flow variation and transport rates. Physical meaning of flow field is expressed in term of mean velocity, turbulent intensity, turbulent kinetic energy, Reynolds stresses and horizontal-vertical interaction, local bed shear stress and quadrant analysis. It is postulated that the presence of protuberant clast modifies flow field and affect transport rates. Possible occurrences of transport rates are subject to accurate estimation of acting forces on moving particles. Estimation of critical Shields stress from Reynolds extrapolation provides better comparison with actual transport rates. Besides, contribution rate to Reynolds stress analysis compliment the importance of sweeps and ejection events to occurrence of bed load transport.