JWSS - Journal of Water and Soil Science

River bend due to particular pattern, called 'Vortex Flow,' has greater erosion than straight path. Occurrence of scour around bridge abutment on curved paths is one of the main reasons for destruction of bridges. Riprap is one of the methods to control the scouring around the bridge abutment. The purpose of this study was to assess stability of the riprap around the bridge abutment at 180 degree river bend. In order to study stability of riprap around the bridge abutment, experiments were done in a laboratory flume made of Plexiglas under 180 degree bend, 2.8 m in central radius, 0.6 m in width and  R/B=4.67.  In this research, several experiments were done by placing a bridge abutment with vertical winged wall made of Plexiglas surrounded by a series of riprap.  Experiments were done by three different types of riprap with different density 1.7, 2.1 and 2.42, four different diameters 4.76, 9.52, 12.7 and 19.1 mm and four rates of discharge under pure water condition. In each experiment, flow depth was measured in terms of moving threshold and failure threshold and then the formulas were calculated by using data obtained. The results showed that the relative diameter of riprap increased with increasing Froude number in terms of moving threshold and failure threshold. Finally, the suitable formula to estimate diameter of riprap around the bridge abutment at 180 degree bend were presented in terms of moving threshold and failure threshold.

In this study, the performance of slot in the spur dike was evaluated as a way to reduce the scouring around the installed flat spur dike in a 180 degree bend and the development of scouring in the slotted spur dike was compared with that of the spur dike with no slot. To evaluate the effect of the slot on the development of scouring around it, a flat spur dike made of Plax Glass was installed in a position of 70 degrees from a bend (180 degrees) in the flume bed. After determining the maximum of scouring depth at the tip, to reduce the scouring around the spur dike, two slots with the determined height were placed in 4 different positions in the spur dike. Experiments with 4 different discharges and constant flow depth in clear water conditions were conducted. The results of the experiments showed that the created the slot reduced the scouring depth toward the spur dike. In both slots, the minimum of scouring depth was seen in a model with the closest position of the slot toward tip, and its maximum was seen in a model with the farthest position from the slot.