TRANSFORMATION OF BED SURFACE STRUCTURES AFTER A SERIES OF RELATIVELY SHORT HYDROGRAPHS

Abstract

This paper reports on a laboratory experiments in which a bimodal sediment bed was exposed to an antecedent flow hydrograph followed by a stability test. The applications of these hydrographs were intended to observe the development of bed surface structures after certain duration of exposure by the flow. Probability distributions of bed surface elevations was carried out to estimate the level of grain exposure and the variations of bed surface level. Bedload transport rates were also observed to obtain correlation with the changes in bed topography. The results suggest the changes in bed topography from original condition to the final form. It is evident that the bed surface structure has changed considerably during antecedent flow in which the average bed surface elevation decreased by almost 2.792 mm. Highly exposed areas appeared on the bed after antecedent flow test were removed and no longer exist after stability test. At this stage further erosion continued but with a mere 11.82 % decreased in the bed surface elevation. This indicates the level of erosion during antecedent flow test was significantly different than that during stability test. This study also reveals that the transported bedload during the stability test was dominated by grains in the coarse mode.