Experimental Study of Combined models of berm, side slope and slot in decreasing the bed erosion around bridge piers

In this paper، the experimental results of 12 combined bridge pier models to reduce the maximum scour depth in clear-water condition have been presented. Experimental models are classified in four groups، namely: 1) berm and semi-conical piers، 2) combination of berm and semi-conical piers with variable diameter in the bed، 3) combination of berm and semi-conical piers with constant diameter in the bed، and 4) combination of third group with trapezoidal centered slot. In this regard، 12 laboratory piers models have been examined in a flume of 12m long، 1m wide and 0. 85m deep. At first، for the reference cylindrical pier model، the experiments have been carried out for 48 hours and it has been shown that the time of 24~35 hours is usually sufficient to reach the equilibrium condition. At the beginning of the each test، the time interval to get the scour depth was chosen very short (e. g. every ten minutes)، but this interval increased gradually with continuing the experiment so that at the end of experiment it was 1 hrs. Firstly، significant decrease (up to 46 percent) in scour depth was observed for berm pier with respect to the cylindrical reference pier. This reduction was also observed for semi-conical pier، however، it was much lower than that of the berm pier. For the second group، the semi-conical combined pier with side slope angle of 2 degrees of model piers had the least bed erosion in comparison to that of the other two groups. For this model، the width of the berm is larger and diameter of the pier is smaller in bed level. Scour depth is then increased in comparison to that of the berm pier model in this group but it is decreased when compared with that of the cylindrical reference pier. Very slight decrease has been observed in the scour depth for the third group model piers by keeping the pier diameter constant in the bed. Hence، increasing the pier diameter in the bed level has intensified the bed erosion in the semi-conical combined piers. Finally، applying slot in the combined piers of third group was ineffective in increasing the protective role of model piers for the fourth group. Therefore، it can be concluded that the effect of berm model pier in decreasing scour depth was much more than the other countermeasures for the piers and their combinations. Then it was found that the different countermeasures for the bed erosion could not have positive influence on decreasing the scour depth for all the cases and a combined model of them may be less effective against scour process than a single countermeasure. The comparison of maximum scour depth has been conducted for all models with relative to those of the cylindrical reference and berm piers.