ESTIMATING SHEAR STRESS OF STREAM BED USING ACOUSTIC DOPPLER VELOCIMETERS

Fluid shear stress, an index of fluid force per unit area on the stream bed, is a very important parameter in flume/river studies, especially in the areas of sediment transport and biofilm erosion. This parameter is a linkage of flow conditions to sediment mobilization and transport along with biofilm development and erosion. Therefore, the estimation of shear stress on stream bed, which is fairly complex in natural flow fields, plays a key role in sediment and biofilm studies. Generally, instead of being measured directly, shear stress is determined by the velocity profiles near stream bed using logarithmic methodology. Impact of shear stress in erosion and development of biofilm has been extensively investigated using three dimensional physical/mathematic models. The accuracy of shear stress significantly depends of the measurement of velocity profiles. The objective of this research is to estimate shear stress for future investigations of biofilm erosion and sediment transport in stream/river using flume experiment and mathematical modeling methods. A straight hydraulic flume with unidirectional turbulent flow is used to simulate a natural river in the laboratory. The 20 feet long flume was marked at equally divided 4 feet interval to obtain 5 points where Acoustic Doppler Velocimeters (ADVs) are placed to measure velocities in three dimensions. The logarithmic equation is used to translate velocities to shear stresses at the bottom of the flume. Similar experiment will be conducted in the south fork of Tule River for comparison purpose. A selected 20 feet stretch of Tule River was marked at 5 points equally separated by 4 feet and velocity profiles at each point will be measured using ADV as well. For the verification purpose, an Acoustic Doppler Current Profiler (ADCP) mounted on a moving boat will be used to validate the velocity profiles obtained by ADV. Summaries of the estimated values for the bottom shear stress, shear velocity, and the coefficient of friction are presented. Confidence intervals about the shear stress estimates are provided.