The Effect of Height and Vertical Position of Slot on the Reduction of Scour Depth around Cylindrical Bridge Pier in Uniform and Non-uniform Sediments Beds

Bridge scour is one of the most important challenges in river engineering. Every year, thousands of bridges around the world are destroyed due to scouring. Hence, the accurate prediction of maximum scour depth for saving financial resources and the safety of human lives is of paramount importance. One way to reduce scouring is to use slot at the pier of the bridge. Research on the effect of the slots, in reducing scour around the bridge pier, has been done bed sediments of uniform type and in most cases the depth of flow have been considered as a constant factor. The aim of the present study is to investigate the effect of sediment non-uniformity in the presence or absence of slots in the bridge pier and the relative effect of increasing the depth of flow (with increasing flow rate) on the scour depth around the cylindrical pier.

The present experiments have been conducted in hydraulic laboratory at Bu-Ali Sina University, Hamadan (Iran). The pier model with a diameter of 4 cm was put inside a rectangular flume with dimensions of 0.5 m wide, 10.5 m long and 0.5 m height. Three different types of slots were used along with the pier without slot. In order to obtain the effect of sediments on scour depth, two types of bed were used. The bed of the first and second types had uniform and non-uniform sediments, respectively. In this experiment, flow rates of 8, 10 and 12 l/s were used with corresponding flow depths of 5.7, 6.6 and 7.4 cm, respectively, Melville and Chiew (1999) chose a time as equilibrium after which scour depth changes of less than 0.5% of the pier diameter over a 24-hour period. According to this criterion, the time of all experiments was considered to be 6 hours.

The percentage of scour depth reduction varies according to the height of the slot and its location. Comparison of S1 and S2 models for two slot models in the same dimensions showed that the closer the slot is to the bed, the greater its effect in reducing scour depth. Also, increasing the height of the slot under the bed (S3 compared to S2) is more effective in reducing the scour depth. Comparison of the first and second beds showed that by converting uniform to non-uniform sediments under constant particle diameter conditions, the maximum scour depth in models S0,S1,S2 and S3 at a flow rate of 8 l/s was 76, 78, 76 and 79%, is respectively reduced. The reason for this can be the formation and expansion of a layer of coarse particles on the bed surface (armor layer). As the flow rate increases due to the increase in flow depth, the scour dimensions increases. According to the obtained results, the scour hole in the non-uniform bed reaches equilibrium with less time than the uniform bed. The formation of an armor layer in a non-uniform bed is the reason for less equilibrium time in similar conditions than in a uniform bed. With increasing the Froude number of flow in all base models, the depth and dimensions of the scour hole have increased. Increasing the Froude number makes the downflow current stronger, and as a result, horseshoe vortices are formed and act with less time. In clear water conditions, the maximum scouring depth will occur in the state u/〖 u〗_c =1 (Melville and Chiew 1999.( The results showed that with increasing the ratio u/〖 u〗_c of clear water scouring conditions, scouring depth increases in a variety of models and beds. According to Guo (2012) if the ratio of flow depth to pier diameter is less than 6. By increasing this ratio, the scour depth in uniform and non-uniform sediments increases.

The results of this research showed that the use of slots in Cylindrical bridge pier was more effective in reducing scouring than the use of bridge piers. With the conversion of uniform to non-uniform bed due to the formation of the armor layer, the scour depth has decreased by an average of 77% in different base models and also the time to reach the scour equilibrium has been reduced. Scour depth has increased with increasing flow rate, Froude number and ratio u/〖 u〗_c in different pier and bed models. In general, the closer the slot is to the bed, the more it reduces the scouring depth and the non-uniformity of sediments due to the formation of an armor layer around the pier causes a significant reduction in scouring depth.