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Showing 4 results for Shafai Bejestan
Minimization of Reservoirs Sedimentation by Optimal Operation of Bottom Outlets
E. Valizadegan, M. Shafai Bejestan, H. Mohammad Vali Samani,
Volume 15, Issue 55 (spring 2011)
Abstract
Reservoir sedimentation is an unavoidable problem which has unsuitable effects on reservoirs such as decreasing of reservoir useful volume, decreasing of dam stability, unsuitable operation of operational gates and penstocks and decreasing of flood control volume. The minimization of reservoir sedimentation is a nonlinear and constrained optimization problem. Constrains imposed include reservoir storage level and releases in each time, and reservoir storage level in the end of operational period. In this study, after calibration of GSTARS3 software, one of the newest mathematical model for simulation of river and reservoir sedimentation developed by USBR, for region of Voshmgir dam, results of running of software were converted as a part of data file to an optimization model by a mediator computer program. After running the optimization model, results were converted to GSTARS3 by another mediator computer program. Then, GSTARS3 was run again with new data file, obtained from running the optimization model. Results of running of GSTARS3 were converted to the optimization model again. The continuation of this process (loop) finished when the desired accuracy was obtained. In other words, the optimum condition was obtained when the running of this loop finished. The constrained optimization problem changed to unconstrained problem using penalty function method. The Powell method, a method of direct search methods, was used to solve this unconstrained optimization problem. Capabilities of the model were demonstrated through its application to the Voshmgir dam in Gorgan for a 12 month period to obtain the optimal operation policy for minimization of reservoir sedimentation.
Determination of Stable Riprap Diameter Around Cylindrical Bridge Pier in the River Bends
E. Nohani, M. Shafai Bejestan, A. R. Masjedi,
Volume 18, Issue 68 (summer 2014)
Abstract
Local scour around piers is the major cause of their foundation failure in the river bends that endangers the stability of the structure and its efficiency. Riprap is commonly placed around the bridges piers for local scour protection. The aim of this study was to present an equation for estimating stable riprap diameter around a cylindrical bridge pier in river bends. In this study, using an experimental model with a 180 degree bend stability, four different riprap diameters under different flow conditions and clear water flow were studied. Empirical relationships based on dimensional analysis for stable riprap design around the bridge foundation was presented. The experimental results were compared with equations provided by other researchers, including Lauchlan (1999), Parola (1995) and Chiew (1995). Results showed that the presented equation in this paper has a good precision. The simple equation presented in this study included all factors important to the instability of the riprap, and recommends designing ripraps around the bridge pier in river bends.
Experimental Study of the Effect of Ripple Bedforms with Fine and Coarse Sediments on Bed Shear Stress
M. Heydari, M. Bahrami Yarahmadi, M. Shafai Bejestan,
Volume 26, Issue 2 (ُSummer 2022)
Abstract
Bed shear stress is one of the most important hydraulic parameters to determine the amount of bed and suspended load and the bed and bank scouring in rivers. Bed shear stress depends on bedforms (ripples, dunes, and anti-dunes) in alluvial rivers. In this study, the effect of artificial ripple bedforms on bed shear stress has been investigated. Two types of uniform granulation with average sizes (d50) of 0.51 and 2.18 mm were used to roughen the surface of the artificial ripples. The bedform length and height were 20 and 4 cm, respectively. The angles of its upstream and downstream to the horizon were selected equal to 16.4 and 32 degrees, respectively. Different flow rates (Q= 10, 15, 20, 25, and 30 l/s) and different bed slopes (S= 0, 0.0001, 0.0005, 0.001, and 0.0015) were examined. The results showed that by increasing the particle size on the bed surface, total shear stress (tb ), grain-related bed-shear stress (t¢b ), and form-related bed-shear stress ( t²b )  increase. The value of tb , t¢b , and t²b in bed form roughened by sediment size of 2.18 mm were, on average, 22.38, 30.86, and 22.3% more than the bed form roughened by sediment size of 0.51 mm, respectively.

Experimental Study of Bed Shear Stress in Straight Channels Covered with Dunes of Different Sizes
M. Badzanchin, M. Bahrami Yarahmadi, M. Shafai Bejestan,
Volume 27, Issue 1 (Spring 2023)
Abstract
The formation of bed form in alluvial rivers due to sediment transport has a significant effect on the hydraulic parameters of the flow such as bed shear stress. The formation of the bed form and its shape and geometry depends on the bed shear stress. Therefore, the relationship between bed form and flow parameters (such as bed shear stress) is complicated. In the present study; the effect of dune bed forms with different heights on bed shear stress has been investigated. Artificial dunes made by sand-cement mortar with a length of 25 cm and heights of 1, 2, 3, and 4 cm were used. In the tests of this research, flow discharge of 10, 15, 20, 25, and 30 l/s and bed slopes of 0, 0.0001, 0.0005, 0.001, and 0.0015 were used. The results showed that with increasing the relative submergence and Δ/λ, the bed shear stress increased in dune-covered beds. The formation of the dune bed form and the increase in its height leads to an increase in the bed shear stress. The bed shear stress in dunes with a height of 1, 2, 3, and 4 cm was, on average, 39, 80, 141, and 146% more than in plane beds, respectively. Moreover, form shear stress for dunes with a height of 1, 2, 3, and 4 cm was, on average, 27.37, 43, 57.11, and 58.74% of the total shear stress, respectively.

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