JOURNAL OF HYDRAULICS
Year:2018 | Volume:13 | Issue:2 |Papers Count:6

Knick points are normally shaped in riverbeds in the form of successive drops. Backward migration of Knick points in a canal result in instability, erosion, bed subsidence and sediment transport and may cause serious damages to the canal banks and upstream structures such as bridges. In the present research, two successive Knick points, with 10% slope and 1 meter distance are constructed at the sandy bed of a rectangular open channel with a 0.003 longitudinal slope. The bed erosion, Knick point migration and its development were studied under different discharges. The performance of four grade control structures as riprap sill, riprap sloping sill, Newbury rock riffle and cross-vane riffle were studied experimentally for the stabilization of each Knick point. All the structures were successful in controlling the bed erosion. However, the riffles operated much better than sills. Specifically, Newbury rock riffles that concentrated the flow in the middle of the channel, prevented Knick point migration and stabilized its position, completely. The experiments demonstrated that the construction of a control structure is not only effective in the stabilization of a Knick point but retarded the development of its nearby Knick point too.

Use of non-core rock-fill dams is one of the structural methods for flood control. Flood flows are usually known to have high sediment loads, and carry high content of sediment into the body of rock-fill dam with large pores that characterize the body of these types of dams. It is thus essential that the sediment concentration be determined at different points of the body so that one could determine the critical points of sedimentation, the amount of sediment passing and the amount of sediment trapped. Therefore, in the present study, a mathematical model of flow simulator was implemented in MATLAB, on the basis of Saint-Venant equations and Forchheimer equation, using finite volume method. Then the mathematical simulator model of the distribution of non-cohesive sediment concentration in the body of dam was developed, based on the model output and sediment transport equation, using F.V.M. The model determines the value of the sediment concentrations at different points of the body on the basis of the gridding from the previous stage. The experimental results were used to evaluate the output of these models. The comparison of the experimental and numerical results seemed to verify the accuracy of the numerical model. The mean value of the relative error of sediments was calculated to be 8.14 percent as determined by the comparison made between the measured data on sediment distribution and the values calculated in three sections of the body of dam.

The aim of this study is to verify the results of numerical modeling with the available physical evidence on the Miankaleh Coast. This study was made by using MIKE 21 numerical model of (SW, HD and COUPLED MODEL FM Modules) plus field studies and sediment sampling. Wave energy and current speed decreased from west to east according to the results of numerical models of MIKE in Miankaleh coast; the wave height in the Gorgan bay inlet has shown a reduction of up to 80 percent. The average current direction out of the Bay in a year is from west to east and in the bay is counterclockwise. The overall rate of sediment transport in the Miankaleh sand spit is less than 0.00002 cubic meter per second per meter. This shows low rate of sediment transport from upstream. The east coast is more sensitive than the west coast regarding the sea level change. The results of field observations and physical complications on the coast verify the results obtained from the numerical model of MIKE in the region. The average sediments size in Miankaleh coast is 0.106 mm. Miankaleh coast is a kind of dissipative coast and dissipative effect Increases from west to east. West region is sediment deposition source and east Regions of Miankaleh is sediment destination. Numerical model of MIKE is suitable for low slope coast with dissipative characteristics.

In the present study, fluid, structure and soil interactions in a pressurized water tunnel were modeled by a two-dimensional finite element method, using the “ABAQUS” software. The tunnel lining was considered as a roughening coverage to decrease the roughness coefficient and to increase the flow velocity. The mechanical parameters of surrounding rock were assumed as weak as possible compared to the tunnel concrete lining. Water flow inside the tunnel was modeled, using acoustic elements that are capable of simulating the hydrodynamic forces. Accordingly, the mechanism of crack development in the surrounding rock as the most probable place to prone this destructive phenomenon is studied applying both steady state and transient flow conditions, considering fluid-structure and rock interactions. Steady state and transient flow analyses were performed using the “HAMMER” software. The resulting loads obtained from the hydraulic analyses by HAMMER software were transmitted to the ABAQUS software for structural analyses. The stress analysis of cracked elements of the surrounding rock was carried out based on Mohr-Coulomb fracture criterion. The least values of the overburden to prevent failure due to the hydraulic jacking were evaluated by imposing the flow pressure in both steady and transient flow states. Finally, the effects of increasing height of the overburden on the hydraulic jacking phenomenon were investigated considering transient flow conditions. Results indicate that, with respect to the positions of surrounding rock elements, providing an extra overburden over the tunnel is not essentially a safe solution for the predication of hydraulic jacking phenomenon in rock around a tunnel.

Understanding the problem of saltwater intrusion under transient condition has an effective role in the management of coastal groundwater resources. In this study, the transient dynamics of the saltwater wedge and mixing zone was investigated in an aquifer with head-controlled system both numerically and experimentally. The experimental data were acquired using image processing and numerical simulations were conducted by SUTRA code. In order to study the saltwater wedge behavior in transient condition, in addition to examining the changes of the indicator of wedge toe length (that was used in most of the previous experimental and numerical studies), changes in indicator of the saltwater wedge height on the sea boundary were measured too. The results demonstrated that in the case of the advancing wedge, the wedge height reaches the steady state condition sooner than the wedge toe length, while in the case of the receding wedge, both indicators are stabilized simultaneously. Furthermore, the results for mixing zone showed that due to flushing and mixing process, the mixing zone expanded at early stages of the receding period. As the wedge approaches the steady state condition, the flushing and mixing process were stopped and mixing zone condensed gradually.

Earth dams under certain conditions are constructed as arch shaped in plane. This may have different reasons such as foundation problem, stability of the body of the earth dam and seepage. Change in the earth dam shape results in its discharge changes too. This is one of the reasons why this study was made. The main purpose of this study is to examine the numerical analysis for verifying the extent of discharge and hydraulic gradient variations of arch dams. For this purpose, a series of numerical analysis by FLAC 3D software were made, and the impact of parameters such as the shape of the valley, height of the dam and the radius of the arc were examined. The results showed that as the radius of the arc increases, discharge rate of the dam in both types of the valleies decreases. In a valley with a constant cross section at a certain height, discharge rate is more than a narrow valley. The results also showed that discharge and arc length have linear relationship. The analysis also showed that, average of hydraulic gradient downstream of arch dams is greater than the straight dams.