Evaluation the effect of depth and flow velocity on the particle size and shape of surface sediments of sandy rivers using image processing methods

One of the important challenges in investigating the hydraulic, morphological and ecological behavior of rivers is to accurately determine the geometrical dimensions of river bed particles. The information extracted from the grain size curve of river bed particles has many applications in the field of river engineering, such as modeling of sediment transport, changes in sediment deposition or river bed erosion, and changes in river morphology (Hasannejad Sharifi et al., 2015).Nowadays, despite the fact that determining the granularity of the bed particles and the boundaries of rivers is important, the removal of sediments from the natural environment leads to disturbing the sedimentary bed and causes changes in the river system (Sadeghi and Qara Mahmoudoli, 2013). Therefore, it is very important to use a method that can calculate the results of granulation with less cost and quickly. Image processing methods are known as new method have been taken into consideration in order to increase rapidity and accuracy, along with the method of field measurement of granularity.
Methodology: In this research, by examining the morphological and sedimentary conditions of the rivers of the Zanjan province, as well as the ease of access, Zanjanrud river was selected as the study area. Then three cross-sections for sampling with frame dimensions of 20×20 cm were determined from the bed of Zanjanrud river and after photographing the surface of the samples, painting by spray paint in order to identify the surface grains, sampling was done from the determined sections. The samples were taken to the laboratory and the sieve test was performed. Then, in the laboratory environment, each of the samples was arranged in a metal frame with dimensions of 20×20 cm and was installed in a channel with a length of 8 meters and a width of 20 cm. By applying the conditions of stagnant water at three depths of 4, 8 and 12 cm and water flow with different depths and velocities, the samples were re-photographed and the grain size curve were obtained by Image J and Hydraulic Toolbox softwares.
Results and discussion: Based on the results of Sieve, in sample A, almost 85% of the particles have a diameter equal to and less than 31.7 mm. In sample B, almost 85% of the particles passed through the sieve of 1.38 mm. In sample C, more than 97% of the particles passed through the 38.1 mm sieve. According to the classification system of the United States, the sediments of all three sections of Zanjanrud are gravel. Also, according to the results, it can be seen that the standard deviation of the samples is less than 2, the uniformity coefficient is less than 4, and the sorting coefficient is less than 2, which shows that the examined samples are almost uniform. In sample A, atdepth of 8 cm, the average relative absolute error values were obtained similar to those at depth of 4 cm. In sample B, the average error value of the fine grains is the same \coarse-grain. The reason for this can be due to the low value of the standard deviation of this sample compared to the other two samples. The average error value at depth of 8 cm was found to be 14% for fine grains and 12% for coarse grains. In sample C, similar to sample A, the average error value in the fine grains is less compared to the coarse grains.The average relative error at depth of 8 cm was found to be 9% for fine grains and 14% for coarse grains. Results showed that Image J software has better performance than Hydraulic Toolbox software in determining the particles granulation curve of the Zanjanrud river.
The effect of flow velocity and depth on the K values of the index diameter of 50% showed that at depth of 4 cm, the average values of K in samples A, B, and C are 1.01, 1.19, and 1.08, respectively. Also, at depth of 8 cm, the values of K for samples A, B, C are equal to 1.05, 1.12, and 1.11, respectively, and at depth of 12 cm,there are equal to 1.08, 1.18, and 1.13, respectively. According to these results, it can be seen that the K values of sample A are lower than the other two samples in all three depths of 4, 8, and 12 cm. At depth of 8 cm, the K values of sample B at different velocities coincide approximately with those of sample. Also, the K values of sample C at two depths of 4 and 12 cm are lower than the K values of sample B. The average K values of the 50% index diameter at three depths of 4, 8, and 12 of sample A are 1.01, 1.05, and 1.08, respectively (with an average value of 1.046); the mean K values of sample B are 1.19, 1.12 and 1.18 respectively (with an average value of 1.16) and also the mean K values of sample C are 1.08, 1.11 and 1.13 respectively (with an average value of 1.1) (with a total average of 1.1 for all three samples).
Conclusion: In most of the hydraulic flow conditions, the granulation curve obtained by Image J software is close to the sieve granulation curve. In contrast, the Hydraulic Toolbox granulation curve is significantly different from the sieve curve and the curve of Image J software. The flow velocity and flow depth does not have a noticeable effect on the K values of the index diameter of 50%.