مجله هیدرولیک
سال دهم شماره 2 (تابستان 1394)

Multidimensional spillways are important hydraulic structures for regulating water level and flow in canals, rivers and reservoirs. The main hypothesis for the development is to increase the conveyance capacity through the spillway crest length at a given width. Finding an appropriate geometry has always been an important issue for designers in achieving maximum water productivity of hydraulic structures. In this study, several plans were simulated numerically considering effective length of spillway crest. Results showed that the highest reduction in the effective length of spillway crest occurs for plans with greater interference area at normal and lateral flows. Also, spillway model No. 5 was recommended as a sufficient plan in terms of flow capacity because of having just one interference area for normal and lateral flows and insignificant compaction during falling streams due to long lateral arms.

One of the vital structures in the land transportation cycle is the bridge. Constructing this structure over the rivers results in an intensified 3-D flow pattern around piers which in turn causes the bed sediments to be eroded and scoured from the immediate vicinity of the pier and its foundation. It is likely that the bridge will fail particularly during a high flood event, if the foundations or piles are not constructed deep enough. Hence, besides the previous works on the study of the factors affecting scour at bridge piers, considering appropriate countermeasures against bridge scour is of primary importance. In the present study, the effect of bed sill location on the temporal evolution of the scour around an inclined pier group was experimentally investigated under various hydraulic conditions and foundation levels. A physical model of pier group with two inclined 2.5x3.5 cm rectangular piers and 28 degree inclination angle constructed on a 10x16 cm foundation was considered in the experiments. The experiments were carried out with different locations of sill (front, middle, and rear of the foundation), various flow velocities and depths, and normalized installation levels of the foundation (distance of the top of the foundation to the bed surface normalized by the pier width) equal to -1.0, -0.5, 0.0, and +1.0. The results of the present study indicate that the bed sill location has a significant effect on the temporal evolution of the depth of scour hole. Based on the comparisons made in this study, it is found that the installation of the sill in front of the foundation has a better performance in the scour mitigation rather than other arrangements. It is also concluded that the average reduction in the scour depth in all foundation installation levels is equal to 22, 18, and 15 percent for front, middle, and rear sill configurations, respectively.

In this research, a simulation-optimization model is developed for minimizing flood damage downstream of multi-reservoir systems by using operation of spillway gates in absence of any flood forecast. The operation policy is based on a multi-stage method (MSM) used to control floods of various magnitudes in reservoirs with gated spillways. It is assumed that the system has no flood forecast and thus shapes and sizes of the inflow hydrographs are not previously known. In this method, the rate of flow released in each stage level is decided according to the current reservoir water level and determined in such a way that flood damage risk for downstream areas is minimized while the dam’s safety is maintained. For this purpose, an optimization algorithm is introduced in which the expected annual flood damage downstream of the river system is the objective function and the spillway discharges in stage levels of spillways gates are the decision variables. A continuous genetic algorithm is utilized to solve this problem. As a case study, the Dez river system including multi-reservoir of Dez and Bakhtiyari is analyzed. Finally the current research result is compared with another research. Results obtained from the proposed model indicate dramatic decrease in the expected annual flood damage. Comparison of the results with other studies shows that the proposed model has a better ability in flood control and minimizing the damage costs.

The current induced by removing a gate separating two fluids of close densities is simulated using a developed Smoothed Particle Hydrodynamic (SPH) model. The simulation is performed for fluids with density ratios between 0.9 and 1.0. To develop the two-phase model, no significant changes are applied to the basic SPH formulations, regarding the low density difference between the two fluids. The classic SPH equations are used with a small change: the speed of sound and the reference density are changed in order to produce the same reference pressure for both phases. In addition, density re-initialization is applied for each phase separately. The resulted inviscid flow has characteristics very close to or the same as the experimental results. The flow depth and the flow front speed are two parameters selected to validate the simulations. The effects caused by the variations in the density differences to the phenomenon and effectiveness of the applied method are evaluated by performing a series of simulations of different density ratios and resolutions.

Extremal hypotheses without bank stability constraint typically over-predict and under-predict alluvial channel width in large rivers and natural streams, respectively. While this process may appear inversely for the depth. In general, results obtained from unconstrained extremal hypotheses are indicative of inappropriate agreement between computed and observed dimensions of the rivers. Such discrepancies between regime model predictions and observed channel widths have been used to argue that optimizations such as MSTC do not describe the behavior of alluvial systems. However, extremal hypothesis models that explicitly consider bank stability exhibit no such bias and can predict alluvial channel widths quite accurately. One of the important factors in disparity of the data may be lack of appropriate relationship to assess bank vegetation of the rivers. For this reason, a modified analytical model has been developed to reduce the effect of bias by considering boundary shear stress, bank stability and vegetation. The model takes into account channel shape factor, bed load equations in the form of excess shear stress and vegetation quantification (i.e. using bank material friction angle) which enables one to predict optimal channel geometry dimensions. Finally, developed model was calibrated using the field data of the rivers in the United Kingdom and Iran. In addition to indicating the effect of bank stability and vegetation on estimation of the geometric characteristics of the channel, the results obtained also confirmed the efficiency of the constrained model in comparison to the unconstrained one.

Hydraulic jump dissipates destructive energy downstream of spillways and chutes. Stilling basin materials have considerable impacts on hydraulic jump properties. As the roughness of stilling basins increases, the length and conjugate depth of jump reduces, and this will affect the dimensions of stilling basin and as a result the cost of structure. One of the methods used for this purpose is to use riprap on the bed of basin that will in turn create a continuous roughness. In the present study, the hydraulic jump characteristics were experimentally investigated over two sizes of ripraps used to furnish the stilling basin bed in a rectangular flume that has the dimensions of 0.4 m wide, 0.4 m deep and 12 m long. The results showed that the length, depth and rolling length of hydraulic jump of riprapped basins were reduced by 35, 50 and 49 percent respectively when compared to those with smooth bed. In addition, the results showed that the variations in bed riprap sizes from 4.45 to 5.75 millimeters reduced the length of jump by 13.5 percent compared to smooth basin. However, there is no significant impact of riprap size on conjugate depth.