Document Type : Research Paper
PhD Student of Hydraulic Structures, Shahid Chamran University of Ahwaz
Professor of Water Sciences Engineering Faculty, Shahid Chamran University of Ahwaz
Bend outer bank region is always exposed to scour due to secondary flow. Different methods proposed to countermeasure the scour on outer bank. One of the most easiest and economic solution is using spur. As an obstacle in the flow, spur dikes increase the shear stress and scour. Studies about spur dikes show that the most critical point is spur nose. In this study riprap has been used for spur stability. Riprap design at the bend is much different from straight channel because of secondary flow and scouring pattern. In this research the effects of a diameter and the dimensions of riprap mattress were investigated, along with the main hydraulic parameters. Base on existing standards, the extent of protection required around a channel bend was determined. After placing the spurs on the determining part, riprap placed around the spurs with different arrangements. In the experiments after adjustment the discharge, flow depth was gradually decreased until the riprap failure occurred. Design equations were established based on a large number of experimental campaigns to predict riprap failure in terms of the previous set of variables. The proper sizes of riprap obtained from the proposed equations in this research study were compared with the correspondences predicted values by the other existing empirical equations and the correspondence measured values. This comparison shows that existing equations underestimate riprap stone size because ignoring secondary flow and its effect on flow pattern near spur dikes at the bend. This comparison indicates the importance of this study in developing design equations for riprap stability around spur dikes at the bends.