Effect of Local Roughness on Local Scouring Reduction at Bridge Abutment

Bridges are one of the most important structures which are under attention from many years. Bridge is a structure to cross over obstacles such as rivers or valleys. In recent decades, despite using modern materials and technology, and spending lot of costs for designing and building bridges, failure of them are commonly reported due to flood and local scouring. Local scouring occurs by placing obstacles such as abutments in flow direction. Different methods of reducing scour hole have been investigated by researchers. (Johnson et al. (2001) and Kayaturk et al. (2012)).
The methods of scour reduction are divided to two main groups, known as direct and indirect methods. In direct methods or bed resistant methods, the resistant of bed against scouring is increased by adding different materials to the bed and near groin. In these methods, flow patterns remain constant but resistant of bed materials increases. Morales et al. 2008 investigated these kinds of methods by using of gravels and rocks. Korkut et al. 2006 used sacks of sands (geo bag). In indirect methods which are known for changing flow pattern, changes are made to flow pattern to reduce the power of destructive vortexes or remove them from head of abutment. Examples of investigating these methods are using parallel walls in abutments by Li et al. 2006, using submerged plates by Johanson et al 2001 who investigated a short vertical abutment in flood plain of a river. They investigated effects of different cases of placement submerged plates in main channel on water scouring. They reported that these plates eliminate vortexes from abutment and scour hole will transfer to the middle of the river; however scour depth will increase in this method.
One way of controlling and reducing local scouring, is to create roughness on abutments which are prominent spherical and angled rings concrete. In this study, by creating roughness on semi-circular abutment, their effects on reducing scour depth were investigated