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.
