فهرست مطالب hossein hamidi far

Flow structure around bridge pier is a very complicated phenomenon. Due to the special geometric and structural conditions in some cases, it is required that the piers be placed in pairs next to each other with special arrangements, which leads to a more complex flow structure around the piers. In this study, the variations of the flow velocity and turbulent kinetic energy around single and the twin bridge piers with a circular cross section is simulated using the Fluent model. The twin piers are placed in three configurations including tandem, side-by-side, and at inclined with the flow direction. The three-dimensional components of flow velocity, streamlines, and velocity contours have been investigated for both single and twin piers. By comparing the longitudinal velocity between measured and simulated conditions in two selected cross sections, the average error for the single tandem piers was 7.3% and 3.54%, respectively. Also, the longitudinal velocity in the tandem, side-by-side and inclined piers has decreased by 2.34% and 9.27% and increased by 87.8%, respectively, compared to the single pier conditions. In general, due to the minimum values of turbulent velocity and kinetic energy, the side-by-side model is recommended as the most appropriate arrangement of the piers with respect to the flow direction.

Several factors affecting pollutant transport in rivers. Among those, vegetation is less studied by researchers. Due to the appropriate conditions in floodplains, such as adequate moisture and fertile sediments, it is possible to develop and grow different types of vegetation in these areas. Depending on the type of vegetation, their effect on the flow and subsequently on mass transfer is different. In this paper, the effects of flow depth, vegetation arrangement, and the interaction between the floodplain and the main channel on the longitudinal dispersion coefficient, K, in a compound channel has been studied. The results show that K is greater in the main channel rather than the floodplain. In addition, the interaction of the main channel and floodplain increases the K values up to 30 and 88% for non-vegetated and vegetated conditions, respectively, which are statistically significant. Similarly, the effect of vegetation arrangement on the K values are statistically significant, and K /U*iHi is greater for the staggered arrangement rather than tandem arrangement.

One of the significant issues in bridge hydraulics and river engineering is the phenomenon of bridge pier scouring. Scour is a phenomenon caused by the flow of water in rivers and channels. Determining the maximum depth of scouring is essential because it indicates the amount of potential degradation of the flow around the structure and also plays a decisive role in estimating the dimensions and size of structures. In hydraulic structures, this phenomenon can damage the stability of hydraulic structures because water can wash sediments beneath and around hydraulic structures and carry them in the flow direction. Over time, this phenomenon drains around the bridge pier and, eventually leading to the destruction of the bridge. Generally, two fundamental procedures may reduce scour around bridge piers. First includes changing the flow pattern by using a slot through the pier, for example. The second method includes enhancing the ability of the bed material to withstand erosion by placing riprap in front of the pier, for example. In recent years, piercing a slot through the pier is a new method to control the depth of scouring. The mechanism of operation of the slot is reducing the strength of the horseshoe vortex due to the reduction of the effective diameter of the pier. Although many studies have been carried out on the scouring of bridge piers so far, the study on the effect of floating debris on the effectiveness of a slot in protecting the pier against this phenomenon has not been investigated so far. The purpose of this study is to investigate the simultaneous effect of slot and debris on the scour around bridge piers. So the results of different bridge-pier tests in clear water conditions are presented.

The experiments were performed in Sediment Hydraulics Laboratory, Water Engineering Department, Shiraz University in a glass-walled flume with a rectangular cross-section of 0.4 m wide and 9 m long with a slope of 0.002. At the downstream end of the flume, a tailgate was installed to adjust the flow depth. Uniformly graded sediment particles with a median diameter of 0.8 mm were used. In each experiment, the discharge rate was determined to maintain the clear water scour conditions according to the ratio of the average velocity to critical velocity (Uc/U=1). The diameter of the bridge pier model used in this study was 40 mm. The width of the slot used was 10 mm corresponds to ¼ of the pier diameter. Also, a 200 mm long and 12 mm diameter cylinder installed on the pier at the water surface was used to simulate the accumulated debris. The first experiment was performed with a bridge pier without slot and debris accumulation (control experiment), and each other test was a combination of the presence or absence of slot and debris.

The results showed that although the presence of the slot reduces the maximum scour depth, the accumulated debris neutralized this effect considerably. It was found that using the slot alone can reduce the scour depth upto 20%, but the debris accumulation reduces this value to 15%. In the control case, with the code NS-ND-130, the ds/b ratio is 0.85, where ds and b are the maximum scour depth and pier diameter, respectively. It was also observed that in NS-D12-130, i.e., the simple pier with accumulated debris and flow depth of 130 mm, this ratio is 1, which is 17.6% greater compared to the control test. The reason for this increase is the presence of debris. The slot guides the flow in a straight line and prevents the flow separation. Hence, in the S-D12-130 experiment, i.e., a slotted pier with accumulated floating debris, the maximum scour depth decreased by 11% compared to the control, and the ratio (ds/b) indicating maximum scour depth it reached 0.775, indicating the effect of the slot in reducing scour. Besides, it was observed that with increasing the flow depth, the effect of debris would decrease. Since no relationship has been provided so far to calculate the maximum depth of scour around the bridge piers in the presence of slot and floating debris, the ds values in the present study are compared with the results of other researchers with no slots and no floating debris conditions.

Various methods have been proposed by researchers to reduce scour and protect the bridge pier against scour, for example, a vertical slot through the pier. In this study, the effect of accumulated floating debris during floods on the effectiveness of a slot in reducing the scour around a cylindrical bridge pier was investigated. The results showed that although the presence of the slot reduces the scour depth, the accumulated debris neutralized this effect considerably. Finally, although using the slot can reduce the scour depth to 20%, but the debris accumulation reduces this value to 15%.

Bridges constructed across rivers are among the most important structures, especially during and after flood events. Bridge piers are exposed to some hazardous factors such as local and contraction scour hydrodynamics. The lower prediction of maximum scour depth can cause  bridge failure, while over-prediction leads to a very costly project. The appropriate estimation of maximum scour depth around bridge piers is, thus, necessary. In some bridges, more than a single pier is considered during the design phase, and the interaction between the adjacent piers makes the situation very different. The flow structure and scour process around a group of bridge piers are, indeed, very different from a single bridge pier. Many researchers have studied the scour around single and/or multiple-column bridge piers. For instance, studying the scour depth for a group of piers, Mahjub et al. (2014) reported that the maximum scour depth around the second bridge pier was less than that of the upstream pier, while the scour depth around the third pier was lower than that of the first and second piers. Moreover, Daneshfaraz et al. (2014) investigated the effect of pier slots on the maximum scour depth around the two-column bridge piers. The results that the slot reduced the maximum scour depth compared to the piers without a slot. In this paper, the flow and bed topography around a group of piers was studied experimentally.

River bank erosion imposes sever damages to the adjacent lands annually and changes the river morphology considerably. In this study, the effectiveness of the Vetiver grass root system in enhancement of the soil shear strength and reducing the river bank erosion is examined. Several experiments were carried out to find the effect of Vetiver grass root system on soil cohesion and internal friction angle. Also, variations of morphological characteristics of Vetiver grass root system including RAR, RDR, RDDI and RLD in different depths have been investigated as well as the lateral and vertical distribution of roots. Three different spacing between Vetiver grass plants have been studied and the optimum distance has been proposed. The results showed that Vetiver grass root system increases the soil cohesion and internal friction factor upto 104% and 83%, respectively. Also, it was found that soil cohesion and internal friction factor are best correlated with RAR and RDR, respectively. Finally, an attempt was made to compare the Vetiver grass with several native trees. It was concluded that Vetiver grass is more suitable than common plants and can be considered as an alternative for river bank protection against erosion.

Scouring and variations of bed topography in meandering river is one of the most interesting issues in river engineering. Vegetation is a significant feature of rivers. Various studies have shown that the presence of vegetation may have an important impact on flow field and river morphology. Due to the complex geometry and hydraulics of flow in natural rivers, physical or mathematical models are often useful to evaluate morphological variations. The purpose of the present study is to investigate the morphological changes in alluvial meandering rivers with compound cross section to gain insight into the physical processes involved. Hence, the bed topography in an alluvial meandering asymmetric compound channel is assessed with vegetated and non-vegetated floodplain conditions. Cylindrical rods of 10 mm diameter were used to simulate rigid vegetation. The rods were installed over the floodplain in two parallel patterns with one row and two rows of vegetation, respectively. The results showed that the point of maximum scour depth was located near the outer bank. Due to high flow velocity entering the bend, the scour depth at section entrance of the bend was greater than other sections. Also, the thalwege was moved away from the interface after planting the vegetation.

Predicting the spread of pollutants is essential for rivers protection and management as well as human health and public safety perspectives. Longitudinal dispersion coefficient is conventionally predicted using costly field tracer tests, empirical equation or analytical formulation. However, the results obtained by these traditional methods are valid for only the reach examined or the flow and geometry condition under which the formula presented. In this study, an innovative method is introduced to predict the longitudinal dispersion coefficient in waterways using the digital image processing technique.
The experiments were carried out in a recirculating glass-walled laboratory flume of 18m length, 0.9m width and0.6m height with an asymmetric compound channel section. The ratio of flood plain width to main channel width was 1 and the overflow depth was 0.14 cm. Flow velocity measurements were taken using three-dimensional Acoustic Doppler Velcimeter (ADV). Three different tracers including color powder, food color and potassium permanganate solution were tested to find the most suitable tracer and. The tracer was injected using a half-tube filled with the dye solution and released uniformly and instantaneously across the flume width. The injection section was taken sufficiently far downstream of the start of the flume such that the flow was fully developed determined by measured velocity profiles. The spreading of the tracer cloud was recorded at three locations 4.00, 6.44 and 8.88 m downstream of the injection point using three digital video capturing Fujifilm JX420 cameras. Then, the captured videos were used to extract image sequences.
Using the Beer-Lambert law of absorption, which correlates the absorbance to both, the concentrations of the attenuating dye as well as the thickness of the material sample, the depth-averaged concentration of the tracer across the flume width was determined. The longitudinal dispersion coefficient was calculated by the standard method of change of moments. The results showed that the image processing technique could be used as a reliable, accurate and economic method in studying the longitudinal dispersion coefficient. Three different tests were conducted with different relative depths including 0.15, 0.25 and 0.35 in the compound channel.
As the magnitude of the relative depth increases from 0.15 to 0.35, the non-dimensionalized longitudinal dispersion coefficient increases 65 and 56% in the main channel and floodplain, respectively. Finally, an equation is proposed to calculate the longitudinal dispersion coefficient in compound channels based on the experimental data. More researches are needed to extend this method to the field condition.

Water quality control is very important for people, animals and plants. Predicting the spread of contaminants is important for managing and protecting rivers and streams to the balance of the ecosystem. Pollutants are introduced into waterways, though a variety of sources such as point and non-point sources. Under steady state conditions, where longitudinal mixing is not significant, studying the lateral mixing is essential in evaluating the influence of pollutants on water quality. Lateral or transverse mixing is the hydraulic process by which a plume of contaminant spreads laterally and dilutes. In water quality management, the transverse mixing is more significant than either vertical or longitudinal dispersion, especially, when dealing with the release of wastes from point sources. Hence, a wide range of field, laboratory and numerical modelling approaches, including laboratory and field measurements, and analytical and numerical investigations have been developed, to quantify the lateral mixing coefficient. However, most of the researchers have ignored the effects of vegetation on the lateral mixing process in their studies. Many studies have shown that the flow characteristics through vegetation are different from those in non-vegetated waterways. For example, laboratory studies have revealed that flow velocity and large-scale turbulence tend to be greatly decreased within a plant canopy, because the resistance to flow by the vegetation. Also, vegetation affects the transport of dissolved and particulate material, such as sediment, nutrients and pollutants. In this study, the effect of the floodplain vegetation on lateral mixing coefficient in compound channels is investigated experimentally. Also, a comparison is made between the results of the present study with those obtained by previous researchers.
Experiments were carried out in a laboratory flume 18m long, 0.9m wide and 0.6m high with an asymmetric compound channel section. Three different densities of cylindrical PVC elements of 1 cm diameter were used in addition to the case without cylinders. Three-dimensional flow velocity measurements were taken using a down-looking four beam Acoustic Doppler Velcimeter (ADV). A highly concentrated solution (C0=10 g/L) of red dye (KMnO4, Potassium permanganate) was injected as a tracer sufficiently far downstream of the beginning of the flume such that the flow was fully developed determined by measuring velocity profiles. Variations of tracer concentration at three locations 4.00, 6.44 and 8.88 m downstream of the injection point were determined using image processing technique. In this technique, digital cameras are used at specified cross sections to capture the pixel intensity before and during the passage of the dye cloud. Using the Beer–Lambert Law, the pixel intensity is related to the dye concentration after a simple calibration. Afterward, the images could be used as input files for MATLAB’s Image Processing Toolbox.
The results showed that due to the strong secondary currents and unstable vortexes in the compound channel, the tracer cloud is periodic. The transverse mixing coefficient in the main channel is also always greater than that in the floodplain and its value increases with relative depth. Another factor that was found to affect the lateral mixing coefficient was the vegetation density. The non- dimensioed transverse mixing coefficient increases with vegetation density in the main channel as well as the floodplain. As vegetation density increases from 0.26 to 0.88%, the non- dimensioned transverse mixing coefficient increased up to 40% of the flow relative depth of 0.15. For low density vegetation (0.26%), the lateral mixing coefficient in both the main channel and floodplain was increase upto 30%. Also, for the vegetation density of 0.88%, the lateral mixing coefficient increases up to 80 and 107% for the floodplain and main channel, respectively. As the flow relative depth increase, the effect of the vegetation on the transverse mixing coefficient decreases on both the main channel and floodplain.
It can be concluded that floodplain vegetation affects the transverse mixing coefficient in the main channel and floodplain, significantly. Also, the flow relative depth and vegetation density are two important factors that control the mixing process in compound channels. The results of the present study were in good agreement with those obtained by Lin and Shiono (1995), Siono and Feng (2003), Shiono et al. (2003), Zeng et al. (2008) and Zhang et al. (2010). More researches are needed to extend the findings of the present study to the field applications.

Hydraulic jump has been used widely for dissipating excess energy downstream of hydraulic structures in irrigation and drainage networks. The characteristics of hydraulic jump in triangular sections have not been studied as much as those in rectangular, trapezoidal and circular sections. In this paper, the characteristics of hydraulic jump in a triangular channel with broad crested end sill is studied. The experiments are carried out in a laboratory flume of 9 m length, 0.5 m width and o.6 m height with glass side walls. Different flow discharges and sluice gate openings with issuing jet Froude number in the range of 2.5-12.5 was studied. The results showed that for a given Froude number, the required tail water depth for the triangular section is up to 70% lower than that in a rectangular section. Based on the regression analysis, several empirical equations are proposed for determining sequent depth ratio and dimensionless sill height as a function of inflow Froude number. The proposed equations can be used in designing controlled hydraulic jump in triangular sections were tail water is not adequate for a classic hydraulic jump.

The aims of this study were to evaluate the treatment efficiency and effectiveness of Vetiveria zizaniodesof two samples of unconventional water of Golgohar Mining and Industrial Company. The experimental samples were included mine wastewater and saline ground water. This research was conducted in a greenhouse at Central Water Research Laboratory of University of Tehran. This experiment was designed as Randomised Complete Block Design (RCBD) including two salinity levels with three replications during four weeks. Results showed that treatment efficiencies of Total Dissolved Solids (TDS) and Electrical Conductivity (EC) were equal 31.5 and 28.3 percentages for mine wastewater and 33 and 28 percentage for ground water on fourth week respectively. Also results indicated that among Cations and Anions, SO4 and K parameters have shown highest and lowest rates at the end of fourth week respectively. It concluded vetiver treatment system include valuable potential for unconventional water treatment.

Floodplain vegetation affects the flow and pollutant transport in waterways. In this paper, the effect of submergence of floodplain vegetation on the transverse mixing coefficient (ε) in a compound channel with asymmetric section is investigated experimentally. The experiments were carried out with different vegetation densities and relative flow depths. Tracer concentration was measured using MATLAB’s image processing toolbox in three sections downstream of the injection point. The transverse mixing coefficients were calculated by standard method of moments according to the variations of variance of tracer concentration. The results showed that ε increases in the emergent vegetation compared to the bare conditions up to 127 and 114% in the main channel and floodplain, respectively, while these values are 43 and 37% for the submerged vegetation. Also, ε increases with the flow relative depth for both submerged and emergent vegetation conditions. As the relative depth increases from 0.15 to 0.25, the lateral mixing coefficient in the floodplain and main channel increases to 97 and 45% for submerged and to 91 and 42% for non-submerged vegetation, respectively. For a specific relative depth, ε increases with the vegetation density. Finally, it was found that as the vegetation density increases, the difference between ε values for submerged and emergent vegetation conditions increases too. In general, the results showed that both submerged and non-submerged floodplain vegetation affect the lateral mixing coefficient significantly.

Because of the complexity of the physical processes in the vicinity of the hydraulic structures due to the separation of the flow, traditional methods for for prediction of maximum scour depth downstream of hydraulic structures are mostly based on empirical approaches. Hence, only a few theoretical works have been reported to study this phenomenon. The present paper describes a new approach based on the momentum principles to estimate the maximum local scour depth downstream of a submerged sluice gate flowing over horizontal or adverse stilling basin. A control volume of the fluid in the equilibrium state of the scour hole was considered and based on momentum principles, some equations are derived to estimate the scour depth at equilibrium state. To verify the proposed equations, large numbers of experiments were planned and conducted under wide range of characteristic parameters such as, incoming Froude number, sediment size, tailwater depth, length and slope of the apron. It was found that the proposed equations fall in a good agreement with experimental results. It was also observed that, in the case of horizontal apron, a specific tailwater depth exists with which the local scour depth attains a minimum value. However, in the case of adverse basins when the tailwater depth takes a specific value, the maximum depth of the scour hole reaches to its maximum and then decreases to a constant value as the tailwater depth increases. This critical tailwater depth was formulated using a semi-theoretical equation.