مجله هیدرولیک
سال دهم شماره 1 (بهار 1394)

The reservoir sediment siphon dredging methods have been widely studied in recent decades. These methods do not have many disadvantages of other methods like extra energy consumption, reservoir operation interruption, water loss, releasing sediment with extra concentration. Sediment evacuation by burrowing type suction pipe method is an important method in which sediments are sucked through holes located at the bottom of the suction pipe, using only the potential difference of the reservoir upstream and downstream water levels. In this study the influence of pipe bottom holes extension is surveyed, using a 61 mm internal diameter pipe. The holes layouts consist of two holes diameters of 16 and 30 mm and three different spacing of holes. The results showed that the holes extension has a dramatic effect on the performance of the method, and, for both diameters, the method has its maximum performance if the holes are only located on the pipe bend part. Also, spacing of the hole affect the performance. Finally, the hole's layout with hole diameter of 30 and spacing of 90 mm with five bottom holes on the bend section has the maximum performance with average sediment concentration of 3.41 percent by volume.

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.

Considering the complexity of groundwater media and high cost of groundwater monitoring, new innovative methods help us in the improvement of groundwater systems. In this research for minimizing overall data loss in total of optimized network, particle swarm optimization algorithm was used. In order to verify the effectiveness of the algorithm, one monitoring network with 57 observation wells was chosen. Later by reducing the number of monitoring wells to 42, the amount of RMSE was measured. Considering 0.3 as the threshold of error, the optimized network was obtained using the remaining 45 observation wells. A comparison of groundwater contours with original network showed that the solution of proposed algorithm is effective. In the remaining 42 observation wells there were some minor discrepancies. In comparison with genetic algorithm, the solution of PSO algorithm is better and has higher efficiency. Comparison of their RMSE also shows a faster convergence for PSO algorithm. Prediction of groundwater level contours in both models were possible and comparable too. Distribution of the omitted observation wells supported this comparison

Mesh-free particle (Lagrangian) methods, such as moving-particle semi-implicit (MPS) and smoothed particle hydrodynamics (SPH), are the newest methods in computational fluid dynamics, which have been applied in flow problems with large deformations and inconsistency. The aim of this research was to develop and improve the simulation of open-boundary free-surface flows, using the new method of weakly compressible MPS (WC-MPS). Most studies in the field of Lagrangian models are concerned with closed boundary conditions. This study, introduces a comprehensive method for MPS modeling of cases with inflow and outflow boundaries. Inflow and outflow boundaries are considered and the boundary conditions and recycling of particles are prescribed. The algorithm not only improves the inlet and outlet boundary conditions, but also reduces the pressure fluctuations at boundaries. To evaluate the proposed method, the famous applied flow problem of ogee spillway and steady shallow flow over curved bed is analyzed. The program is written in C language and validations are performed for this code. Comparison of the results of proposed model and physical models showed acceptable accuracy in modeling the free surface flows with open boundary conditions.

Due to hydraulic complexity in confluences, the study of different features of these locations such as sedimentation, erosion, and environmental considerations are of high interests to many researchers. Although many laboratory researches have been carried out regarding erosion and sedimentation in confluences, little researches have been done numerically. In this research the features and performance of FLOW-3D numerical model to simulate the flow, erosion, and sediment transport patterns in a junction of 2 canals with different flow rates and constant sediment flow, are investigated by comparing the results obtained from the numerical model with those obtained from a laboratory model. The widths of the canals of the physical model were 15 and 50 centimeters. The comparison between numerical and physical models shows an average of 6.3% error between maximum sediment depth in the junction of lateral and main canal between two models. Moreover, according to the measured results of erosion depth at the right bank of the main canal at the junction, the numerical model can estimate the erosion at the bank with the average of 26% error. Furthermore, the numerical model was able to predict the water surface profile of the flow in the junction with an average error of 1% when sediment was not injected.

In order to protect hydraulic structures against scour, it is necessary to know what causes scouring. Local scour is the process whereby sediment is removed from the base or toe of a structure due to change of local flow patterns. Local scour at a bridge pier principally results from the down flow that forms along the upstream face, and the resulting horseshoe vortex at the base of the pier wich cause an increase in sediment movement leading to scour. The studies showed that sediment transport near the bed depends on bursting event. Quadrant analysis is a method to quantify the contribution to the Reynolds shear stress production by the coherent eddies during the events involved near-bed bursting in the turbulent boundary layer. In this study, the instantaneous velocities of flow in three directions were measured using a Micro ADV in an equilibrium pier-scour hole around a circular bridge pier in a rectangle experimental flume. The results reveal that the dominant event is sweep (Q4), followed by ejection (Q2), then outward (Q1) and inward (Q3) in the entire flow depth except at some points close to the bed level and within the scour hole (z/h< 0) where the contribution of ejection is more important than that of the sweep and Q2 events gradually exceed Q4 events with an increase in z. Also, the contribution of sweep and ejection event diminishes away from the bed level and in contrast, the contribution of outward and inward (Q1 and Q3 events) continues to have insignificant contributions and increase close to water surface becoming approximately equal in the water level. Furthermore, it is also found that the difference in distances from the pier have no significant effect on contributions of events.