Experimental study of the effect of netted collar position on scour depth around of oblong-shappe bridge pier

Scouring is a natural phenomenon that arises from the erosive action of flow field in the alluvial streams . Local scour is a special type of scour. This type of scour is one of the reasons of bridge failures. One of methods of scour reduction is the use of collar and netting the collar makes it lighter. The position of collar plays an important role in reduce of scour bridge pier, therefore in this research effect of position of netted collar around of oblong bridge pier are investigated. The experiments have been in this direction on the flume with the length of 6 m, with of 73 cm and a depth of 60 cm with a slope near zero. In this research, simple collar and three netted collars(with opening parts equal to 30 percent, 50 oercent and 70 persent) in four levels including on the bed, 0.1B under the bed(B: pier’s width), 0.5B and B above the bed. The results show that levels under the bed and on the bed were the best position to install the collars. On the bed, the simple collar had 100% sufficiency and the 70% netted collar had 92% efficiency in reducing the scour. In level under the bed simple collar and netted collars had the same performance and scour depth decreased by 88%.