Study on Cross-shore Sediment Transport Due to Non Breaking Wave

Abstract

Cross-shore sediment transport is mainly influenced by the wave orbital motion when the sediment properties are uniform for different wave conditions. The attainment of equilibrium beach profile where cross-shore sediment movement is responsible mainly depends on sediment size and wave condition. This is a major field of interest for the researchers to evaluate the formation of natural beaches. In the Hydraulics and River Engineering Laboratory of Water Resources Engineering Department, BUET, such an experimental setup was constructed to understand the process of sediment transport in the cross-shore direction due to non breaking wave incidence. In the experiment the slope of the artificial laboratory beach profile was taken as 1:15 and the mean grain diameter was 0.125 mm. The experiment was conducted with three different water depths of 50 cm, 40 cm and 35 cm. For each water depth the wave periods were taken as 1 sec, 2 sec and 2.5 sec. On the experimental slope, 3 cm thick sand layer was placed for every experimental run and the run time was taken as 1 hr and 15 minutes. The run time was selected by carrying out different trial runs with the selected wave period before conducting the experiment. Different wave parameters such as wave period and wave height were measured for each experimental run. The mean flow velocity due to wave was measured at different locations on the slope half of still water depth from the water surface with the help of 2-D programmable Velocity Meter. After completing each run, the bed profile was measured with a point gage. The final bed profile obtained from the experimental result was compared with numerical model namely Parabolic Wave Model was found satisfactory. Again the actual velocity profile was compared with the model data, the comparison was found reasonably satisfactory. In this study, different standard non-dimensional parameters for sediment transport were reviewed and these parameters were established for this experimental setup. The parameters obtained from the experiment showed good correlation with the standard parameters. It was found that the offshore sediment transport rate was dominant for this experiment. Hence, the beach was an erosion type beach. The non-dimensional correlation established form this study shows affirmative result with the similar experiment conducted by previous researchers. This study result may be a basis to identify the parameters responsible for beach formation and construction of any coastal structures where sediment movement is a serious problem. The effect on the change of gradation for cohesionless sandy slope due to wave attack was also observed form the study. It was found that, the mean sediment grain size increased after completing a run in the onshore region. The change in sediment gradation was found significant where the incident wave breaks on the slope. But the grain diameter did not change or the change was very much negligible towards the offshore region, especially where the water depth was higher. In this study it was also found that use of CC blocks on the onshore area produce stable beach compared to that of a breakwater.