عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Trees are destroyed by natural phenomena such as landslide, wind or flood, … and are floated in flow direction on the river surface. Floating debris accumulates in front of hydraulic structures, such as bridges, and increases the flow velocity as a result of decreasing the flow cross section. Consequently, both scour around the bridge pier and hydraulic forces acting on the bridge pier are increased. So the floating debris may have catastrophic effect on the bridge. The recognition of flow field around bridge piers may be useful to prevent the damages on these structures. Researches showed that horse shoe vortexes are formed in front of pier and waking vortexes are formed in the back of pier. In addition, in front of the bridge pier downward flow has an important role on the scour depth, the woody debris will intensify the downward flow and therefore the scour depth is significantly increased.
In this study, the effect of woody debris accumulation on the scour depth in correspondence of the bridge piers is experimentally investigated. The experiments were performed in clear water condition as well as different hydraulic conditions in an experimental flume with a 20 m length, 60 cm width and 60 cm height in hydraulic lab of Shahrekord University. Bridge pier was a sharp nose square pier which comprises a square with a 9 cm side length and a sharp triangular nose with a 10 cm height. The nose of the triangle was a 48-degree angle. Rectangular woody debris with different lengths and widths is used for the experiments. These woody debris are located in front of bridge pier and in different depths with respect to the water surface. The experiments were performed with 15, 20, 30 and 40 l/s flow rates. All the experiments were performed at a 20 cm flow depth. The average size of sand particles was d50=0.78 mm, and in all experiments the flume slope was 0.001. The scour depth was measured by a caliper with 0.1 mm accuracy. The scour depths are read at time = 1, 15, 20, 30, 45, 60, …, 420 minute. The results showed that the maximum scour depth occurs at t=7 hour.
In order to perform the clear water condition, in which there should be no sediment transport in the flume except around pier, the threshold of sediment incipient motion must be determined. Therefore, the incipient motion particles velocity, Uc, based on Richardson and Davis's equation is estimated (Uc = 0.388 m/s). In all experiments the flow velocity was greater than Uc. In each experiment, the flume bed was balanced. After fixing the woody debris in front of the pier, the flume was filled with water at a low flow rate (Q ? 0.001 m3/s) and then the flow rate was increased to reach desired discharge value. Dimensionless parameters are determined using dimensional analysis.
The results showed that woody debris has a significant effect on the scour depth and the position of the woody debris is also effective on the scour depth. When the woody debris is placed over the surface, at the surface, and below the surface, respectively, the ratio of scour depth to the scour depth of control sample is 1.7, 1.85 and 2.2. Moreover, when the percentage of blockage (ratio of the area occupied by floating objects to the flow cross section) is 30% and debris is located below the water surface, the maximum depth of scour (2.2 vs. control sample) will occur. Furthermore, the scour hole profile around bridge pier was measured and plotted, the results showed that the woody debris dimensions has significant effect on the width and length of the scour hole. The effect of roughness was also investigated, and the results showed that the roughness does not affect the scour hole dimensions, but can increase the dynamic force acting on the pier. The results of dimensional analysis were used in order to find an equation for predicting the maximum scour depth in present of woody debris accumulation. The parameters that have less effect on the scour were not considered in the regression. An equation was presented for calculating the maximum scour depth around a sharp nose pier in presence of woody debris accumulation. The predicted results have good agreement with the measured one.