عنوان مقاله [English]
Background and objectives:
One of the highest losses due to natural disasters is bank erosion. Rivers affected by erosion and sedimentation are subject to various changes such as change of direction, movement in transverse and longitudinal directions, bed level changes and transformation of geometric features. There are several methods to protect the bank in the outer arc, spur dikes are one of the best and most economical methods used in most parts of the world.
In this study, flow and sediment patterns in a 90 degree mild bend with and without spur dikes by changing the position and length of the spur dikes with Configurations 15%, 20% and 25% of the flume width was examined using numerical model CCHE2D and by considering the hydraulic performance of spur dikes the optimum combination of parameters, length and position of spur dike in the bend were investigated.
Materials and Methods:
The bend used in the numerical simulation in this study is a 90 degree mild bend flume with 70 cm width in hydraulic laboratory of Shahid Chamran University . In order to simulate the flow and sediment pattern the numerical model CCHE2D was used in this study. This model is a two-dimensional hydrodynamic model which has been prepared and developed at the International Science and hydraulic center of Mississippi School of Computational Engineering in America.
Hydraulic and sedimentary results were verified by comparing the simulated results by model with experimental results.
The results indicate the numerical models high capability in simulating the flow and sediment pattern in the bend. Comparison of the model and experimental velocity results indicates an accuracy of more than 94 percent. The sediment validation also indicated that the models accuracy in estimating the sediment erosion and deposition is over 90%.
The results show that by increasing the length of the spur dikes, the maximum velocity and depth of erosion and sedimentation also increases. For example, the maximum scour depth of spur dikes with length equal to 15 and 20 percent of flume width compared to spur dikes with 25 percent of flume width length decreases 44% and 33% respectively. Also the maximum velocity of spur dikes with 15 and 20 percent of flume width length decreases 9% and 16% respectively compared to spur dikes with 25 percent of flume width length. The results show that the erosion and sedimentation pattern of spur dikes with length equal to 20% of flume width is preferred. Because, firstly they divert the flow to the center of the main channel with causing the least changes in the channel bed.
Secondly, the spur dikes with 15% and 25% of the channel width length cause a massive volume of sedimentation in the inner arc of bend.