OBSERVATIONS OF THE INSTANTANEOUS NEARBED STREAMWISE FLOW VELOCITIES IN CONSTANT FLOWRATES
Seven series of laboratory experiments were carried out in a re-circulating glass sided tilting flume to investigate the instantaneous nearbed streamwise flow velocities in different duration of constant flowrates. Each series were observed at different time elapsed to represent the flow condition at different time of bed exposures. The instantaneous nearbed streamwise flow velocities were recorded using an Acoustic Doppler Velocimeter (ADV), which was positioned in such a way the probe could easily traverse the grid points to collect measurements. The experiments suggest that the instantaneous nearbed streamwise flow velocities within the grid varied throughout the constant flowrates with lower range of variations occurred in the shorter tests and wider variations in the longer tests.This means the constant flowrates produced different level of turbulences in a relatively small area even at adjacent points. With a relatively stable standard deviations regardless of the time elapsed of measurement it was also found that the average values of instantaneous nearbed streamwise flow velocities are slightly lower particularly for the shorter time interval between measurements and generally decreased towards the end of each test. This indicates that there was always a spatial variation in the average instantaneous nearbed streamwise velocities throughout the test.