Sediment Concentration due to Currents and Irregular Waves

Abstract

Coastal changes are often the result of changes in the sediment transport rates along a coast. For the prediction of coastal changes, it is therefore necessary to accurately compute the transport rates. For the computation of longshore transport rates, a number of models is available, for example the TRANSPOR model by Van Rijn [1993]. In most of these models, a Single Fraction approach is used, in which the fall velocity of the sediment is based on the 0 50 of the bed material. This implies a more or less uniform size distribution of the bed material. In many coastal areas, non-uniform size distributions of the bed material occur. However, not many experimental data about the behaviour of graded bed material are available in literature. For this research, experiments with both uniform and more graded sediments were therefore conducted to study the influence of the grading on the sediment transport due to currents and irregular waves. In this M.Sc. thesis of the Hydraulic Engineering Section of the Faculty of Civil Engineering and Geosciences at Delft University of Technology, the results of the measurements are analysed and compared to results calculated with the TRANSPOR model. The analysis of this report is confined to the suspended sediment transport. From the measurements it can be concluded that the sediment concentrations under currents and irregular waves in case of graded sediment are larger compared to the sediment concentrations in case of more uniform graded sediment. A second conclusion is that the average size of the suspended sediment in case of graded bed material is smaller than the size of the suspended sediment in case of uniform bed material, even though the average size of the bed material is approximately the same for both graded and uniform bed material, and sometimes even larger for the graded bed material. Apparently, the finer fraction in the graded sediment is more easily brought into suspension. A comparison between the results from the experiments and the TRANSPOR model with a Single Fraction approach shows that the model can predict the sediment transport rate within a factor 2, which is often assumed to be acceptable. For the graded sediment, the model is not able to reproduce the shape of the concentration profile. Especially in levels above a quarter of the waterdepth, the concentrations calculated with the model are too low compared to the measured concentrations. A comparison between the results from the experiments and the most recent version of the TRANSPOR model with a Multiple Fraction method shows that the model produces better results for the computation of the concentration profile, compared to the TRANSPOR model with a Single Fraction method. The model does not yet correctly compute the size distribution of the suspended sediment over the height. Apparently, the hiding and exposure influence in the model, which is often applied in river situations, is not applicable without modification for the calculation of the suspended sediment transport due to currents and irregular waves. More research is recommended to gain more insight in the processes, which influence the interaction of the different fractions in current and wave situations.