Study of different scouring areas relative to flow velocity in Downstream Base in pairs of bases and Impact of cable protection method on these areas

Study of different scouring areas relative to flow velocity in Downstream Base in pairs of bases and Impact of cable protection method on these areas Abstract Bridge failure is a common phenomenon all around the world. Bridges are one of the most important structures which are under attention from many years ago. Bridge is a structure to cross over obstacles such as rivers or valleys. Investigation of scouring in water structures especially in bridges is absolutely important in river engineering. Failure of several structures in all over the world are usually due to structural consideration and giant scales on piers. Created procedure of scouring by group of piers are more complicated than one single pier. Increasing the resistance of bed materials and decreasing the power of erosion factors are the ways to stand against local scouring. To decrease the power of erosion factors (horseshoe and wake vortex), equipment such as collars, submerged vanes and etc. are being used. This study was conducted with freshwater on cylindrical piers. The experiments were with a constant discharge during 6 hours in hydraulic laboratory of Ferdowsi University of Mashhad using a flume of 10 m length, 0.30 m width and 0.50 m depth. Sands with median diameter of 0.72, special gravity of 2.65 grams per cubic centimeters and geometric standard deviation of 1.12 are used in the experiments. Range of flow rate was from 8 to 18 l.s-1. An adjustable weir in the downstream regulates the water depth in the channel. The area for conducting the experiments in the channel has 1 m length and 10 cm bed height, which is 6 meters away from the beginning of the channel. Scouring procedure of downstream pier was investigated base on flow velocity. Results show that depth of local scouring of downstream pier can be categorized in 4 zone based on flow velocity: 1- No scouring occurrence zone, 2- Synchronized scouring zone, 3- Transitional zone, and 4- Deviance zone. Following previous investigations, effects of application of rolled cable over piers on reduction of scouring around two piers and zones of downstream pier (zones related to velocity) were examined. In this study, the efficiency of using rolled cable around the piers to decrease scouring is investigated. Results show that rolled cable, scouring will decrease. In fact, rolled cable around the piers will decrease the power of down flow, horseshoe and wake vortexes. Results show that scouring was reduced around piers due to application of rolled cable. So that cable at its best state reduces the maximum scour depth by 50 percent for downstream pier in the situation distance 3D and 54 percent for downstream pier in the situation distance 5D. Reduction of scouring depth of downstream pier was less than upstream piers. Scouring depth of downstream pier with rolled cable was 7 and 26 percent reduced in comparison with piers (No rolling cable was used) located at 3D and 5D from it. Because of scouring reduction as consequences of rolled cable, downstream zones were significantly changed that finally caused synchronized zone removal. Keywords: Local Scouring, Base group, Protective method, Downstream Base, Scouring zones