Countermeasures against local scouring at bridge piers: slot and combined system of slot and bed sill

Results are presented and discussed from two laboratory experimental campaigns specifically designed to investigate the behaviour of a slot as countermeasure against local scouring at a smooth circular bridge pier, close to threshold flow conditions of initiation of uniform sediment motion. The investigation was aimed at evaluating the effectiveness of the slot by changing its sinking depth into a sand bed, and assessing the dependence of the scour depth on different dimensionless groups. The results showed that the slot reduces the local scour at pier. The maximum reduction of the scour depth was about 30% (about 70% for both scour area and volume) in the best configurations. A combined countermeasure is also proposed and tested, consisting of a slot and a bed sill placed downstream of the pier and adjacent to it: in the best configuration, the scour reduction in front of the pier reached about 45% on average (with about 80% and 90% for scour area and volume, respectively). This last result shows that a combination of slot and bed sill may be a very effective countermeasure against local scouring at bridge piers.