The influence of grading on concentration calculations

The main objective of this study is: investigation of the influence ofthe composition of the bed material on the composition of the suspended material at the reference level and on calculating the reference concentration, based on recent measurements. This objective was achieved by a study of literature, the investigation of data from six measurements and by the use of the Van Rijn-formula for the reference concentration. A method to calculate the suspended sediment transport rate is integrating the product of the vertical velocity profile and the time and bed-averaged vertical concentration profile. Most of the expressions for the time-averaged concentration profile have as input parameter, besides many other parameters, the median bed particle size dso and do therefore, pure theoretically considered, only fit for perfectly uniform bed material. A possible grading of the bed material, which will normally be found in nature, is not taken into account. Two calculation-possibilities to take the grading into account are the d-method and the fraction-method. The measured data, on which this study is based, come from the results of experiments carried out by Jacobs and Dekker (2000). The main conditions which are used in this study are a constant significant wave height H and peak period Tp and a varying depth-averaged current velocity. Two types of bed material are used in this study: uniform and graded bed material. Using the particle diameters dso (= ds) and d90 of the bed material as input parameters in the Van Rijn-formula gives in all considered cases an underestimation of the actual (measured) reference concentration. The outcome of the Van Rijn-formula can also be multiplied with a factor to repair the gap between the actual (measured) concentrations and the calculated concentrations. This factor is 1.70 in case of the uniform bed material and 1.90 in case of the graded bed material. The situation with waves only and uniform material seems to need a higher factor (ca. 6.3). Analysis of the fraction-method made clear that this concept, as treated in this study, seems to be interesting as starting point for further analysis.Civil Engineering and Geoscience