Experimental investigation of the effect of debris accumulation on the local scour at bridge pier and abutment

Accumulation of floating debris around the bridge's piers and abutments causes reduction of river flow area, flow diversion, flow accelerating and altering of scour pattern. The investigation of potential impacts of debris on the local scour processes is one of the main factors in design of bridge structures across the rivers. These wooden floating debris may have different shapes in terms of accumulation and position, often have rectangular shape in the nature. Although, the effect of debris on piers scour has been studied by different researchers, to the author's knowledge, no investigation has been conducted to study the effect of debris on flow pattern and scour hole characteristics in the case of combinative presence of pier and abutment. Therefore, in this study, the effect of debris with different geometrical characteristics on the pier and abutment scour and flow behavior was investigated experimentally. in this study, the effect of distance between bridge pier and abutment, geometrical characteristics of debris (including thickness, diameter and shape) on the scour was investigated experimentally. The experiments were conducted at the hydraulic and water structures laboratory of department of water engineering of Shahid Bahonar university of Kerman. The experimental flume has a rectangular cross section with 8 m length, 80 cm width and 60 cm depth. Model of bridge pier (diameter 3cm) and bridge abutment (6cm*12cm) was selected by stainless steel. Sedimentary bed with thickness of 16 cm, was composed of sediments with d50=0.91 mm. To avoid undesirable erosion of sediment, false bottoms were installed at the upstream and downstream parts of the study reach. Prismatic objects with different shapes of rectangular, triangular and semi-circular were used as debris. (The range of relative thickness of debris (T_d/D) was from 1 to 3 and the relative length of debris (D_d/D) from 4 to 10). The sediment threshold velocity and the maximum velocity of experiments of this study are 0.4 and 0.2 m⁄s respectively which shows that, all experiments were carried out at the clear water condition. The results showed that by decreasing the relative distance between bridge pier and abutment (G/D) from 6.66 to 3.33, the maximum scour depth at pier and abutment increased 8.1 and 12.5%, respectively. Also, the rectangular debris caused the most scour depth in comparison with the other debris shapes. By increasing the relative thickness of the semi-cylindrical debris (T_d/D) from 1 to 3, the maximum scour depth around the pier and abutment was respectively increased 7.64 and 24.21. In addition, the experimental results showed that the effective length of debris has a significant influence on the dimensions of scour hole, so that, the maximum scour depth in the presence of semi-cylindrical debris with relative effective length (D_d/D) of 10, increased 50.8 and 58 percent compared with that of the reference test, for the bridge pier and abutment, respectively. According to the results of this study, There was a direct relation between the scour depth and the debris thickness, so that, by doubling the relative thickness of rectangular debris, the scour depth around bridge pier and abutment is became 1.2 and 1.05, respectively. With increasing the relative length, the scour depth increased at first, thereafter reached to a constant value. For example, for rectangular debris, by changing the relative length from 4 to 10, the scour depth around bridge pier and abutment was increased 22.4 and 10.2 %, respectively, but for larger relative lengths, no change was observed in the scour depth. In addition, by decreasing the distance between pier and abutment, the maximum depth of scour hole was significantly increased compared with the reference test.