Study in mixing processes for the management of water quality of rivers is important and one of the environmental concerns. The substrates, such as dunes, are effective in control of transport, production of turbulence and flow resistance in rivers, canals and wetlands It is therefore essential that study is done in this subject. The objective of this study is to investigate the transversal mixing coefficient of dunes. For this purpose, measurements carried out on the fifth to the ninth of dunes in a series of ten dune sand of the synthetic twodimensional. The experiments carried out in a laboratory channel with dimensions of length, width and depth, 20, 0.6 and 0.6 respectively. The dunes had a wave-length of one meter, height 0.8 meters, downstream side slopes 28 degrees and width equal to the channel width. The average diameter of sand dunes used was 14 mm. To evaluate the effect of the delicately ratio on lateral mixing coefficients, two different depths were used. The results showed that dune plays an important role in increasing the lateral mixing coefficient so that in this case this rate was 2.36 times the flat bed. Complete mixing length in the channel bed dunes was 11 meters to the amount indicated on the flat bed of a third. This parameter showed a decrease with increasing depth over the dunes. The tests indicate that the self-purification power of the river on the dunes, comparing to a flat bed, with increasing water depth above of the dunes, increases.

Formation of bed forms, such as dunes, is a prominent feature in natural open channel flows with sediment transport. Knowledge of turbulent flow over sand dunes is needed to understand the interaction between the flow, the bed forms and the associated sediment transport. This study presents results of an experimental work aimed at investigating the effect of different dunes and vegetated banks on flow velocity, Reynolds stresses and turbulence intensities. Experimental measurements were made over fifth and sixth dunes in a series of ten of 2-dimensional asymmetric gravel dunes which had dune height of 4 and 8 cm, respectively. These dunes had a mean wavelength of 100 cm, mean lee slope angles of 28º, and width equal to the flume width. Rice stems with a median diameter of 2.7 mm and a stem distribution density of 400 stem/m were used to cover the flume banks for simulating vegetation on riverbanks. From experimental measurements, the effect of different heights of crest of gravel dunes on the characteristics of flow in vegetated flume; and the reattachment points for different heights of crest of gravel dunes were investigated.

Rivers have been always the main source of water for human kind and the basic element of population development. Study of the interaction between flow structure and bedforms is one way to understand the behavior of the rivers. Moreover,vegetation in natural rivers increases roughness of the main channel and flood plains which affects the geometry of channels, flow structure, bed resistance and consequently the pattern of sediment transport. Considering the role of bedforms on sediment transport, turbulence production and flow resistance, investigations on details of flow-bedforms interaction, vegetated banks and flow structure seem to be essential.
In this study, the influence of straight crested gravel bedforms and vegetation of the banks of channels on flow turbulent characteristics are investigated based on model experimentation. For this purpose, seven fixed artificial 2-D straight crested bedforms were built inside a rectangular flume of 8 m long, 0.4 m wide and 0.6 m deep. The graded gravel particles used to create the bedforms had an average diameter of d50 = 10 mm. Johnson grasses with a diameter of 2.8 mm were used to simulate vegetation cover on the flume side-walls. Since, the fully developed flow was just observed after the fifth dune, experimental measurements were performed over the fifth and sixth dunes. Overall, three runs were performed over the dunes with a wave length, height, angle of repose and flow depth of 0.96 m, 0.04 m, 28 degrees and 0.28 m, respectively. In the first case 17 velocity profiles and in the second and the third cases 21 velocity profiles were measured. All the tests were performed with a constant discharge of 0.024 m3/s. The instantaneous three-dimensional velocity components were measured using a down-looking Acoustic Doppler Velocimeter ADV. Velocities were recorded for each point with a sampling rate of 200 Hz and the sampling volume of 5 mm. The sampling duration was at least 120 seconds. Overall, about 45400000 velocity data were collected, filtered by WinADV software. Results indicated no negative velocities for both cases of with and without vegetation cover. For no vegetation case, the least value of velocity was zero at a small region on the lee side of the dune. Whereas, for the case of vegetating the side-walls, the zero value of velocity was located at the dune's trough. Negative vertical velocity value in both cases of with and without vegetation along a dune confirmed that separation is not dominant for the case of straight crested dunes compared to the corresponding sharp-crested bedforms. The Reynolds stresses increase for the case of vegetating the side-walls compared to the case of without vegetation cover. This is in part due to the increase of flow resistance, while the side-walls are vegetated.
Rivers have been always the main source of water for human kind and the basic element of population development. Study of the interaction between flow structure and bedforms is one way to understand the behavior of the rivers. Moreover,vegetation in natural rivers increases roughness of the main channel and flood plains which affects the geometry of channels, flow structure, bed resistance and consequently the pattern of sediment transport. Considering the role of bedforms on sediment transport, turbulence production and flow resistance, investigations on details of flow-bedforms interaction, vegetated banks and flow structure seem to be essential.

This research presents recent advances on morphodynamic modeling over gravel dunes. Experiments were carried out on flow over 4 different dunes developed in 12m flume at Institute of Hydraulic Engineering and Water Resource Management of Technical University of Graz، Austria. Dunes are developed in the height of 4،6 and 8 cm and in wavelength of 1m with 2 different particle sizes (6 and 13 mm) and also two different lee angles (38 and 8 degree). The separation of flow over gravel fixed dunes is investigated by Acoustic Doppler Velocimetry (ADV) and Particle Image Velocimetry (PIV) and to compare the experimental results with numerical models over dunes، the 3D numerical model (SSIIM) is applied in this study. The results show that for the same two dunes under the same velocity but different flow depths، larger separation occurs under lower flow depths and a decrease in dune height causes a reduction in length of separation. Moreover the separation zone has a strong relation with lee angle and in the situation of low lee angle no flow separation was detected. Finally the result reveal that numerical model successfully simulate flow over dune in compared with ADV and PIV data. Although ADV application near the bed indicates probably weak performance for high discharge and low water depth of dune No. 4، PIV results present more agreement with the numerical model in this region.

The formation of bed form in alluvial rivers due to sediment transport has a significant effect on the hydraulic parameters of the flow such as bed shear stress. The formation of the bed form and its shape and geometry depends on the bed shear stress. Therefore, the relationship between bed form and flow parameters (such as bed shear stress) is complicated. In the present study; the effect of dune bed forms with different heights on bed shear stress has been investigated. Artificial dunes made by sand-cement mortar with a length of 25 cm and heights of 1, 2, 3, and 4 cm were used. In the tests of this research, flow discharge of 10, 15, 20, 25, and 30 l/s and bed slopes of 0, 0.0001, 0.0005, 0.001, and 0.0015 were used. The results showed that with increasing the relative submergence and Δ/λ, the bed shear stress increased in dune-covered beds. The formation of the dune bed form and the increase in its height leads to an increase in the bed shear stress. The bed shear stress in dunes with a height of 1, 2, 3, and 4 cm was, on average, 39, 80, 141, and 146% more than in plane beds, respectively. Moreover, form shear stress for dunes with a height of 1, 2, 3, and 4 cm was, on average, 27.37, 43, 57.11, and 58.74% of the total shear stress, respectively.

On way to understand the behavior of rivers is to study of flow structures and bedforms. Most rivers have rough beds, that are called bedforms. These shapes have different types depending on the hydraulic conditions that cause the resistance to flow, so sufficient knowledge of the hydraulic resistance of the flow is necessary to calculate discharge, depth, water velocity, flood prediction and sediment transport to reduce the damages that are caused by rivers. Despite years of research and experimentation on bedforms, there is still no adequate and accurate equation to predict the geometry of bedforms and their interaction with flow. The RANS turbulence model is not sufficiently accurate in detecting flow separation in high-altitude and high-angel lee side dunes, but it is more appropriate in other dunes. The LES turbulence model can be more appropriate in detecting flow separation in the vicinity of the bed. But in the layers away from the dune, it produces relatively severe turbulences. The DES turbulence model is more accurate in detecting the flow separation in large-scale dunes and has less execution time than the LES, but this model produces irregular vortices in smaller dunes near the bed. In this research, with the aim of investigating the effect of dune geometry on flow structure, numerical simulation of flow motion on dunes in open channel duct was investigated. In this regard, 29 simulations were performed to study the effect of the geometry of five types of dune with different angles and heights in different hydraulic conditions and different bed roughnesses with RANS and DES turbulence models. The STAR-CCM+  was used in order to simulate the numerical model in this research. This software provides highly realistic results by providing a seamless environment with high network production capability and extensive simulation tools, which helps professionals in the process of working with fluids.In this research, VOF method is used to calculate free surface area, and the solution method is Implicit Unsteady. In order to sensitize the numerical model to the number of computational cells as well as to optimize the runtime and output accuracy, several blocks of lattice dimensionality change in data points and dune locations were used. To select the appropriate networking dimensions, 8 models were run to optimize the time and accuracy of the model. To evaluate the accuracy of the simulations, the numerical model results were compared with those of previous researchers. The results of comparison of the numerical model and the experimental model showed that only about 9.5%, 15.5%, 14.5%, 9.4%, 12.2% and 7.4% were different. This error rate indicates good numerical model accuracy. Also the results of numerical model + STAR-CCM were compared with SSIIM numerical model. then, by using the dimensional analysis, effective factors on the interaction of dune geometry and flow structure were investigated and finally, a formula was developed to predict this interaction. The results of the evaluation of the obtained formula for investigating the effect of dune geometry on the hydraulic flow showed that the presented formula with error of 11.25% and 0.86 R2, due to the completely random nature of bedform formation, is very accurate.

Rafsanjan plain, being about 4000 Km2 in area is located at the southeast of central Iran in a tectonic depression trending NW-SE. Due to the erosion of Sarcheshmeh and Davaran mountains (respectively at the south and north of the plain), sediments carried by floods to this depression have created bahadas comprised of alluvial fans at the north and south of the plain. At the central part, where the floods cease, a clay flat playa is present. Aeolian landforms lie mainly at the surface of both northern bahada and the central playa

Fine-grained materials of Clayey Loess deposits of Gorgan region were mixed with sand dune in the lab with various rations and their shear behavior was measured and evaluated by direct shear test. The result of the study showed that the increase of fine-grained clay in sand dune was directly related to the adhesion force and inversely to the internal friction angle. The increase or reduction in shear strength was affected by the fine-grain percentage. Dry condition and saturation of the test and sand dune had a less influence on shear parameters but in the mixed samples in dry condition and saturation, shear parameters revealed a considerable difference. The results of this study can be used in sand stabilization, erosion prevention, slope stability, embankment construction, compaction increase, and environmental pollution, and dust and water channels avoidance.

In order to investigate the flow formation on dunes, the experimental data from a flume 12 meters long, located in Hydraulic Lab at Technical University of Graz (Austria), were collected. In this study, dunes (particle sizes of 13 and 6 mm) in a 2-D plan were developed with the wavelength of 1 meter, the lee angle of 8 degree, and the crest heights of 4 and 6 cm; these were uniformly installed across the width of flume. The analysis of the experimental flow velocity profiles measured by ADV and PIV technology and the numerical profiles modeled by SSIMM showed that in the same hydraulic conditions, there was no significant relation between drag coefficients of particles on dunes and flow discharge variation, while the water depth reduction caused a sudden increase in the drag coefficient up to 66%. Also, reducing particle size of the dune increased the drag coefficient and there was a significant relation between particle size (diameter) and dune formation, so that in smooth crested conditions, as compared with the sharp crested dune, the drag coefficient was increased up to 32%.