Effect of geometric parameters of spur dike and depth-placed riprap on its failure threshold in a 90° flume bend

摘要

Spur dikes are one of the most effective and economical structural methods, which are generally used to protect riverbanks against erosion. Nowadays, in addition to protection of rivers' bank, spur dikes are also used for improving the environmental conditions along a riverbank. One of the important topics regarding this type of river structure is their protection against scouring around their pier noses that has been less studied. Riprap is a simple and economic way for protection of spur dikes against scouring. The purpose of this study was to investigate the effect of geometric parameters of spur dike and depth-placed riprap for dike protection on the threshold failure of riprap and the consequent stability of spur dike in a 90° bend. In order to achieve the research objectives, one riprap size of 3/8 inch diameter was used. Ripraps were placed in 3 depths (Y/D50 =2,4,6), also 3 lengths of spur dike(15%,20% and 25% width of the flume), three spur dike angles (60,90 and 120 degree) and three dike spaces (3,5 and 7 times of spur length) were used as different variables in this study. The standards of Federal Highway Administration (FHWA) and U.S. Army Corps of Engineers were the main base for selecting the variable ranges. The results showed that the critical velocity at the riprap threshold failure increased by increasing the depth-placed riprap and the spur dike angle and decreased by increasing the spur dike length and space between dikes.