Experimental study of the effect of the length of gabion spur dike On flow pattern and topography of bed in a channel with movable bed

spur dike is a hydraulic structure that is created in open channels and rivers to protect the walls against erosion or deviations and water guidance in the desired directions. The most important problem of spur dikes is scour at around it that makes breaking and overturning the spur dikes.Considering that the flow pattern and topography of the bed is significantly affected by the presence spur dike, it is important to consider these issues.in this purpose, the effect of variations in the length of the gabion spur dike with porosity of 30 & 50% was investigated in this study. For measuring 3D velocities and bed topography, Vectrino was utilized. 3D velocities were measured in movable bed state. Finding show that increasing the length of the spur dike, the maximum depth of scour and scour hole dimensions increases. According to the results of flow pattern experiments in this field, increasing the section constriction and consequently, increasing the flow rate in cape of spur dike with developments secoundary Flows increases the maximum depth of scour and scour hole dimensions. By increasing the length of the gabion spur dike, the range of the effect of this structure on its upstream is increased, so that the velocity changes through the canal and approaches the opposite wall. The highest rate of velocity in all layers, after a gabion spur dike in the range near the side, extending towards the middle channel.