Effect of Floating Debris and Slot on Scour around a Bridge Pier

Introduction
One of the significant issues in bridge hydraulics and river engineering is the phenomenon of bridge pier scouring. Scour is a phenomenon caused by the flow of water in rivers and channels. Determining the maximum depth of scouring is essential because it indicates the amount of potential degradation of the flow around the structure and also plays a decisive role in estimating the dimensions and size of structures. In hydraulic structures, this phenomenon can damage the stability of hydraulic structures because water can wash sediments beneath and around hydraulic structures and carry them in the flow direction. Over time, this phenomenon drains around the bridge pier and, eventually leading to the destruction of the bridge. Generally, two fundamental procedures may reduce scour around bridge piers. First includes changing the flow pattern by using a slot through the pier, for example. The second method includes enhancing the ability of the bed material to withstand erosion by placing riprap in front of the pier, for example. In recent years, piercing a slot through the pier is a new method to control the depth of scouring. The mechanism of operation of the slot is reducing the strength of the horseshoe vortex due to the reduction of the effective diameter of the pier. Although many studies have been carried out on the scouring of bridge piers so far, the study on the effect of floating debris on the effectiveness of a slot in protecting the pier against this phenomenon has not been investigated so far. The purpose of this study is to investigate the simultaneous effect of slot and debris on the scour around bridge piers. So the results of different bridge-pier tests in clear water conditions are presented.

Methodology
The experiments were performed in Sediment Hydraulics Laboratory, Water Engineering Department, Shiraz University in a glass-walled flume with a rectangular cross-section of 0.4 m wide and 9 m long with a slope of 0.002. At the downstream end of the flume, a tailgate was installed to adjust the flow depth. Uniformly graded sediment particles with a median diameter of 0.8 mm were used. In each experiment, the discharge rate was determined to maintain the clear water scour conditions according to the ratio of the average velocity to critical velocity (Uc/U=1). The diameter of the bridge pier model used in this study was 40 mm. The width of the slot used was 10 mm corresponds to ¼ of the pier diameter. Also, a 200 mm long and 12 mm diameter cylinder installed on the pier at the water surface was used to simulate the accumulated debris. The first experiment was performed with a bridge pier without slot and debris accumulation (control experiment), and each other test was a combination of the presence or absence of slot and debris.

Results and discussion
The results showed that although the presence of the slot reduces the maximum scour depth, the accumulated debris neutralized this effect considerably. It was found that using the slot alone can reduce the scour depth upto 20%, but the debris accumulation reduces this value to 15%. In the control case, with the code NS-ND-130, the ds/b ratio is 0.85, where ds and b are the maximum scour depth and pier diameter, respectively. It was also observed that in NS-D12-130, i.e., the simple pier with accumulated debris and flow depth of 130 mm, this ratio is 1, which is 17.6% greater compared to the control test. The reason for this increase is the presence of debris. The slot guides the flow in a straight line and prevents the flow separation. Hence, in the S-D12-130 experiment, i.e., a slotted pier with accumulated floating debris, the maximum scour depth decreased by 11% compared to the control, and the ratio (ds/b) indicating maximum scour depth it reached 0.775, indicating the effect of the slot in reducing scour. Besides, it was observed that with increasing the flow depth, the effect of debris would decrease. Since no relationship has been provided so far to calculate the maximum depth of scour around the bridge piers in the presence of slot and floating debris, the ds values in the present study are compared with the results of other researchers with no slots and no floating debris conditions.

Conclusion
Various methods have been proposed by researchers to reduce scour and protect the bridge pier against scour, for example, a vertical slot through the pier. In this study, the effect of accumulated floating debris during floods on the effectiveness of a slot in reducing the scour around a cylindrical bridge pier was investigated. The results showed that although the presence of the slot reduces the scour depth, the accumulated debris neutralized this effect considerably. Finally, although using the slot can reduce the scour depth to 20%, but the debris accumulation reduces this value to 15%.