Experimental investigation of the collar effects on reduction and delaying of local scour at rectangular bridge abutments

Scouring phenomena around bridge abutment is one of the most significant causes of bridges failure. Investigation on Erosion around bridge abutments and present a reliable method to protect them against scouring is necessary because of significant and vital rule of these structures in transportation. There have been many studies to limit the maximum depth of scouring at abutments such as using a protective collar, riprap mattress, geo bag and etc. But most of them are not economical and efficient to countermeasure the abutments with different shapes and length. Using protective collar at abutment is one of the simplest and most applicable techniques to countermeasure of abutments in comparison with other protective methods. Simple application and appropriate performance of collar lead to widespread investigations on it. To study the performance of collar, shapes and lengths (La) of the collar are the most effective parameters in reduction of maximum depth of scouring. While, the performance of collar varied for different flow intensities. For lower flow intensities, using collar may lead to diminish the scouring at tip of the abutments. But for upper flow intensities (near the threshold of the bed material), the efficient of collar reduces and flow can be able to scour below the collar and reach to tip of the abutment. As whole, there are two types of abutment (short and long abutments) in terms of length. In short abutments, the ratio of the length of the abutment to the flow depth is less than one ( ), while for long abutments this ratio is more than unit ( ). In the previous investigations, the applicability of using collar in reduction of scour depth at short abutments has not been studied widely. Also, there are no many studies on the effects of various flow conditions on efficiency of collar for both short and long abutments and no comparing between them. In this study, scouring around two kinds of rectangular abutments (short and long abutments) were investigated. All experiments were conducted in a 1 m width, 1 m depth and 14 m length rectangular flume located at the Porous Media Laboratory of Amirkabir University of Technology. The test region, where the abutments were placed in the flume, was selected in a way that a uniform flow was established and fully developed turbulent flow was reached. The bed materials are uniform with a median size of 0.91 mm and a geometric standard deviation of 1.38. The experiments were conducted under the clear water and various flow conditions (U/Ucr=0.95, 0.85, 0.75). The discharge of the flow was adjusted by an inlet valve and measured by a rectangular weir at the end of the flume. In this investigation, long duration (70 hour) tests were conducted to determine the equilibrium depth of scouring. Equilibrium depth of scour was defined as a depth with less than 1 mm change after 2 hours of experimental run. According to the conducted tests and obtained time duration data, 40 and 50 hour were adopted as the equilibrium time for short and long abutment experiments (with and without collar), respectively, in which 95% maximum depth of scouring was occurred. The experiments were continued until reach the relative equilibrium state. The experimental results for scouring of the bed materials for the various flow intensity of U/Ucr=0.95, 0.85 and 0.75 showed that the use of collar with 2La in length for short abutment and La for long abutment at bed level, causes the reduction of scouring about 46%, 57% and100% for short abutment and 26%, 24% and 60% for long abutment, respectively. The start time of scouring at tip of the long abutment in the same flow condition and using collar with the same sizes, took place 405, 490 and 760 minutes after experiments run, respectively. Also, the start time of scouring at tip of the short abutment protected by collar at bed elevation took place 300 and 255 minutes for U/Ucr=0.95, 0.85 flow intensity and no scouring on tip of abutment during 40 hour of test run. Furthermore, applying collar on abutment led to reduction of scouring volume around the abutment equal to 37%, 64% and 25% for short and 34%, 43% and 44% for long abutment in U/Ucr=0.95, 0.85 and 0.75 flow intensity, respectively.