Experimental Prediction of the Bed Profile with the Full-submerged and Semi-submerged Debris Accumulation Upstream of the Cylindrical Bridge Pier

Background and objectives: During floods, when the water velocity is high, the flow carries the pieces of tree leaves and other solid objects with them by colliding with the built bridge's piers in the direction of the water flow and accumulating upstream. This process causes a change in the procedure and maximum scour depth around the bridge pier. If the maximum scour depth exceeds the predicted values in the design of bridge piers, it can even weaken the piers and in some cases fail and overturn them. Therefore, in this research, the effect of the debris accumulation of upstream cylindrical bridge piers in two fully submerged and semi-submerged conditions on the scour profiles around the bridge pier and their maximum values are studied in a flume of Hydraulic laboratory of Babol Noshirvani University of Technology. For this purpose, all the different parameters are determined by dimensional analysis, and the efficacy of the effective parameters such as Froud densimetric number, submergence depth, and position level of debris on the longitudinal and transverse profiles around the bridge pier is investigated. Also, one equation for predicting the maximum scour depth based on experimental data is proposed and the scope of application of the parameters obtained from the dimensional analysis for use in designs is stated.
Materials and Methods: Three sets of experiments were conducted, all of which were in the conditions of clear water and constant flow to investigate the effect of the debris accumulation on the scour values. The first set is the reference tests in the case of a cylindrical bridge pier with a diameter (D) equal to 3 cm, the second condition is for debris is buried debris (y0=0) and the third condition is for free debris (y0=D). The variable parameters in the experiments are the relative velocity of the flow (u/uc≥0.838) and the relative depth of the flow (y/D≥1.33). The distance of the bridge pier from the beginning of the apron is 3 meters and the median grain size of the sediment particles is 0.82 mm. In this research, while examining the time evolution of the maximum scour depth, to ensure the equilibrium of the scour hole in the tests, the time of the tests was considered 480 minutes.
Results: The results of the laboratory observations indicated that when the debris is buried, in y/D=1 to 1.67, the maximum scour depth increases by up to 10% compared to the reference condition. When the relative depth of the flow increases, the buried debris act the same as a collar, and at y/D≥2, the scour depth is reduced compared to the reference condition. In the case of free debris, in all relative depths (1.33≤y/D≤3), the maximum scour depth has increased compared to the reference condition, and its critical case is about 57% at y/D=1.33. The correlation of the proposed equation for predicting the maximum scour depth with the R2=0.96 and the RMSE=0.22 shows the appropriate accuracy of the proposed equation for the case of scour in the condition of debris accumulation.
Conclusion: By observing the results in general, it can be found that the debris accumulation upstream of the pier, in the case that the debris is placed above the sedimentary bed, causes a significant increase in the maximum scour depth. When these objects are at the same level as the sedimentary bed, they cause an increase in scouring in shallow flow depths, and in deeper flows, they decrease the maximum scour depth compared to the case without debris. Based on the proposed equation, the densimetric Froude number parameter has the most effective, and (y-y0)/D has the least effect on results.