The sediment transport is one of the most important processes in watercourses. Smaller, lighter grains are transported more often by the water flow, while larger and heavier grains are transported only during larger flows or during major flooding events. It is important to know the information about the grain size distribution, because is used in the sediment capacity calculations. It is also essential for the assessment of channel roughness. River bed consists of two layers. It is important that the analysis of sediment grain size is undertaken separately for both layers since different characteristic grains are used for different analyses. Data on the grain size of the top layer is used to estimate the roughness, and a representative grain of the base is used to calculate the sediment transport. As part of the diploma thesis, we determined the grain size distribution of five natural gravel bars in the area of the middle Sava. We used 3 different methods for each location and finally we compared the results obtained. Grain size was determined by sieving analysis, line analysis and optical analysis using the Basegrain software. The result of each analysis was a grain size curve, with the help of which we determined the characteristic grains, d10, d30, d50, d70, d90 and the mean arithmetic grain dm. The values of characteristic grains were compared with selected statistical tests. The results of the statistical tests used for all methods give similar results, but certain results are more comparable than others. In most cases, the results of line and optical analyses are the most comparable, while the results of sieving and line analysis often deviate the most. This is also expected, since different samples are analyzed with different methods. In the sieving method, only the upper covering layer of the sediments was sampled, while in linear and optical analysis, the data is converted into a volumetric sample, which also represents data on the grain size of the base and not only the top layer. We also found that the grain size is not the same even within an individual gravel bar but changes all the time with changing flow conditions and depends on the micro location. Since the data on the granular distribution is quite important, with a small financial and time investment, measurements could be carried out at certain locations at certain time intervals, thereby obtaining data on changes in the granular distribution over time. With this information, we could better understand and predict what is happening in watercourses. Based on the results of this thesis, we recommend determining the grain size distribution by a combination of linear and optical analysis, as they provide comparable results, both are relatively quick, cheap and do not require special access to the location, such as e.g., sieving analysis.
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