Introduction:
Scour around bridge piers is the main reason of bridges destruction. Hydraulic structures act as obstacles to the flow of water near the structure, so that the flow pattern changes and locally causes scour around the bridge pier. Moreover, floating debris are gradually carried by the flow and accumulate around the bridge piers. The shape of the channel as well as the position and the geometry of the bridge pier significantly affect the accumulation of floating debris and tree foliage in front of the bridge pier. The main parameters which affect the scour depth around bridge piers are flow intensity, characteristics of bed sediments, and bridge pier geometry. Previous researches indicated that the maximum scour depth for a sharp nose bridge pier located at the lateral corners of the base, between the nose and the corner. Also, wedge shape debris resulted in a lower scour depth than the rectangular ones due to the less deviation of the flow towards the base.
Materials and Methods:
The experiments were carried out in Shahrekord University laboratory. A rectangular flume was set up with galvanized floor and glass walls. The flume had a length of 20 meters, a width and height of 60 cm, and a constant bed slope of 0.1%. A total number of 52 experiments were conducted under clear water conditions, with and without debris (control sample). A square shape pier with 9 cm side was applied. The bed was covered with 160 mm of sand, with an average sediments particle size of 0.75 mm. The flow depth was kept constant at 20 cm with flow rates of 10, 20, 30 and 40 Lit/s. To simulate wedge-shaped debris, pieces of wood were placed in wedge-shaped net racks in four different sizes (varying in length and width, with a constant height). These debris were installed in front of the square base at three positions: above, below, and at the water surface. Scour depth was measured using a caliper to ensure accuracy. A dimensional analysis was conducted using the Buckingham π theorem to develop a predictive equation for scour depth around a sharp-nose square bridge pier in presence of wedge shape debris.
Results and discussions:
The duration of all experiments in this research was seven hours. To achieve the goals of the research and facilitate better analysis of the results, the initial tests were conducted without floating objects by the flow rates of 10, 20, 30 and 40 Lit/S. Subsequent experiments were carried out in the presence of floating objects in different dimensions and positions (above, below and at the water surface), in front of the square base.  It was observed that the floating objects increased the length, width, and depth of the scour hole, and consequently its volume. The level and dimensions of the floating objects in front of the pier had a great impact on the maximum scour depth. Comparison of the three positions of the floating objects in front of the pier indicated that the objects above the water surface created the minimum scour depth, while in cases of the floating objects below the water surface the maximum scour depth occurred. This trend was observed for all the experiments with different debris dimensions. According to the longitudinal and transverse profile of the scour, when the object was positioned under the water surface, the scour hole was formed upstream towards the pier, with a gradually steep slope. For the cases in which debris were located above the water level, the slope was formed more slowly. Also, according to the final changes of the bed profile, for objects with larger dimensions, a larger scour depth was obtained; therefore, more sedimentation happened across the channel. Finally, an equation was extracted to predict scour depth around bridge piers in presence of wedge shape debris, using the results of dimensional analysis and a linear regression with SPSS software. The results of the equation showed the highest R² and the lowest mean square error. Comparing the maximum measured scour depth with the calculated results by the equation confirmed the accuracyof the equation.
 Conclusion:
In this study, the scour depth in front of a square pier was investigated without (control sample) and with floating debris in three positions. For different positions of floating objects (below, above and at the water surface level), scour development during time and equilibrium scour depth were measured. The results indicated that 75% of the scouring rate occurred in the first hour, after that the scour depth slowly increased until it stabilized at 420 minutes. Larger debris dimensions resulted in the greater scouring depth around the pier. The transverse dimension had the greatest effect for wedge-shaped objects. The lowest scour depth was observed in the control sample. The accumulation of debris at the three positions of above the surface, at the same level and below the water surface were 1.4, 1.3 and 3.6 times that of the control sample, respectively. The maximum scour depth occurred in the position in which the debris were located below the water surface.