Experimental Study of the Effect of Mining Materials Downstream of Bridge Pier on Scour Profile with optimizing Distance Approach

Background and objectives: Human encroachment in rivers, such as the construction of a structure (bridge) or deformation of the bed, has affected its natural flow, which leads to the phenomenon of scouring and the creation of holes in the natural bed river. After deformation of the bed due to sedimentation and erosion, these holes can move in the longitudinal direction of the river and cause various damages to the structures and facilities in the riverbed as well as the bridge pier. Bridges are one of the most important access structures whose failure has irreparable social effects, so the effect of mining material holes on the bridge piers can be very important. The extension of mining material holes along the riverbed and its movement towards the bridge piers and accompanied by the scouring phenomenon and turbulent flows formed around the piers causes the destructive effect of erosion and overturning of the pier. Therefore, in this study, the effect of mining material downstream of the cylindrical bridge pier on the surrounding scour profiles is investigated with changes in the depth and distance of the mining material hole from the pier .
Materials and Methods: For this purpose, on an experimental model and in clear water conditions and constant flow, changes in the profile of the hole and the effect of the depth of the hole and its distance from the pier to the scour profile around the bridge pier were evaluated, by mining holes with H/D= 0.8, 1.6 (ratio of hole depth to pier diameter) and with I/D= 10, 20, 30 (ratio of hole distance from pier to pier diameter) downstream in sediment bed with depth 0.1 meters.
Results: The results of experimental observations showed that in the case without mining hole, the lowest maximum scour depth related to the experiment was obtained with a Froude number of 0.26 at 9 mm; While the highest maximum scour depth was related to the experiment with a Froude number of 0.46 of 60 mm. Compared to the case without a mining hole at the Froude number of 0.43, the lowest maximum increase in scour depth in the hole harvesting experiment was with H/D=0.8 and at I/D=20 by 8%. While the highest increase in maximum scour depth in the experiment with hole removal was 26% with H/D=1.6 and I/D= 10. In order to find the optimal distance for mining materials downstream, experiments with I/D=30 show the maximum values of scour depth equal to the case without mining material holes. Therefore, by increasing the distance from the pier I/D=10, 20, and then 30, the maximum increase in scour depth compared to the case without a mining hole decreased from 26% to 15% and then to zero%.
Conclusion: By observing the general results, it is possible to comprehend that mining material from the channel bed downstream, affects the amount of scour depth and increases it, and by increasing the mining hole depth and reducing its distance to the pier, the scour depth around the pier increased. Also, in order to predict the maximum scour depth, relationships in the case without mining hole and also for the case with it have been proposed using the obtained data based on regression analysis. The accuracy and also the scope of application of these relationships are mentioned with statistical indicators.