مجله هیدرولیک
سال دوم شماره 2 (تابستان 1386)

In this paper the bed evolution in channel bend under different hydraulic conditions is presented. Experimental tests were conducted in 180 channel bend with movable bed. In each test the bed level at various sections were measured. Experimental results showed that in all experiments, the point bar was formed in the first half of the inner bank and scour hole was formed in front of the point bar. At the outer bank, in the second half of the bend, the point bar was formed in the inner bank and some scour holes were formed in the outer bank. The position of these scour holes was found to be dependent on Froude number. By increasing the Froude number, these positions are shifted toward the apex of the bend. Verification of bed level in cross sections shows that the slope of bed increases as the Froude number increases. In this study, a relationship is presented that predicts the transverse bed profile.Also, the velocity measured in section with maximum deposition show the existence of secondary flow only in some parts of channel cross section due to the lateral balance between the upslope force induced by the helical circulation and the down slope gravitational force in the lateral direction. The oscillatory manner of the bed evolution result from oscillatory manner of depth averaged velocity across the channel.

In this paper, the generalized likelihood uncertainty estimation (GLUE) method is evaluated for updating rainfall-runoff model parameters in Gharehsoo-Karkheh sub basin. In two scenarios, observed discharges in continuous (from the beginning of the flood hydrograph) and discrete periods were used to change the parameters and update the forecasts in each time step. The continuous scenario showed better performance. Therefore, lead time and forecast error were computed in the continuous scenario. It was assumed that precipitation ends in the next time step while rainfall intensity and observed discharge in the current time step were used for updating. The results showed that GLUE was capable and flexible in correcting the forecast discharge error due to no forecast of rainfall. Lead time was represented by the difference in the time at which the peak discharge distribution becomes stable and the actual observed time to peak discharge. A lead time of 8 hours was achieved for two floods in Gharehsoo sub-basin with some 20 hours of time of concentration. The forecast error remains the same after the time when the peak discharge distribution becomes stable. The error in the simulation of the peak discharges of two floods were 21 and 16 percent which are caused by the uncertainty in the model structure and input data.

Sensitivity analysis approach should be regarded as a new means which was introduced recently for flow assessment of irrigation systems. In this paper, three hydraulic sensitivity indices for channel reach were developed, and the algorithm for analyzing flow based on these indices was presented. For evaluating the efficiency of indices and the proposed algorithm, a real case was simulated by SOBEK model. Using the results of two alternative operational scenarios, i.e. the minimum and maximum operated discharge values of the canal, the quantified values of indices were calculated. The results show a successful application of the reach sensitivity indices in analyzing and predicting flow conditions in the system. A proper evaluation of the system response to hydraulic perturbations, which was achieved by using the proposed convenient means, could improve the network operation, and increase the overall water use efficiency. The proposed indices and algorithm could be applied in design process and also in operational period of irrigation systems.

In order to adapt the hydraulic conditions of super critical flow in the chute spillways, variable width may be designed by designers. For most of the cases, the design of contracting chutes would help reducing water depth at upstream of the chute which reduces the dam height and increasing water depth at downstream of the chute may provide better smaller stilling basin for hydraulic jump. Super critical flow in the chute canals with contracting width may associate with the oblique steady shock waves. There are some difficulties in accurate numerical solution of water surface and velocity discontinuities due to shock waves. In this paper a novel algorithm for Overlapping-Cell-Vertex Unstructured-Finite- Volume solution of shallow water equations is used in which the numerical fluxes on the boundary edges of each Control-Volume are computed using an Upwind-Flux-Averaging formulation. Using this formulation in combination with Multi-Stage Runge-Kutta Time-Stepping provides accurate and oscillation free explicit solution of super critical flow with sharp shock waves. The accuracy of the computed result for analytical and experimental steady horizontal test cases is compared with reported data. The efficiency of the solution algorithm for the test cases is presented in terms of convergence history plots and CPU time consumption for the convergence of the solution to the steady state.

Different approaches have been proposed for estimation of the number and the correct amount of critical depths in compound cross sections and these ideas resulted in the definition of the Froude Number. These models include unit cross section, modified unit cross section, energy method based on energy correction coefficient; momentum method based on characteristics lines and weighted Froude Number method. The researchers have tried to present a method based on the Froude number that is able to determine the points of changes in flow regimes of specific energy curve correctly. It has been stated in the previous researches that among existing methods, only momentum and energy methods have high capability in estimation of critical depths in compound cross sections, and therefore are reliable. However, an investigation of correctness of estimation of the number and the amounts of critical depths in these cross sections using all Froude Number methods in comparison to specific energy methods shows that the proposed relationships do not have the capability yet. This study shows that in non-homogenous compound cross sections with high relative roughness and also in compound cross sections with wide flood plain, the number of estimated critical depths with these two methods is more than the number of points of changes in flow regime of specific energy curve. Therefore, use of these two methods is not recommended under all conditions.

Scouring phenomena is one of the most important issues in coastal and river engineering. Construction of protective structures such as sea walls on shoreline can cause scouring and determination of scouring depth in front of such structures is one of the important problems in the design of their foundation. Effects of destructive erosion under coastal structure induced by wave reflection and the problem of stability of these structures are important too. Using Caspian sea coasts data, the potential of scouring at different points in front of sea walls and in case of natural coast without protective structure are estimated and compared. This can be useful in coastal engineering studies used in the design and management of coastal defense projects.