Keywords : Local scour
journal of kerbala university,
2015, Volume 11, Issue 2, Pages 1-12
Local scour is the erosion of natural bed material around bridge piers by the vortexes action of flowing water. It can result in structural collapse, loss of life and property. Local scour is live-bed if the flow upstream bridge is transporting bed material, otherwise it is clear water. In this paper, the local scour around the piers of al-Hindiya second bridge during the year 2013 was studied. From field surveys at the bridge site it was observed that the cross section area of river upstream bridge is concentrated to the left side as a result of the presence of middle bar. Also there is a scour hole with depth about 4 m around the 4th pier from left bank of the river, while no scour effect was found at the other piers. The field scour depth was compared with depth calculated from the seven empirical equations using bridge geometry, bed materials properties and hydraulic characteristics of the stream flow at approach channel. Laursenꞌs equation was used to find critical velocity of bed materials which showed that there was clear water scour around bridge piers and the scour formula by Sheppard & Melville 2006 had a good agreement with measured scour depth, while other equations were underestimation. Depending on Sheppard and Melville equation a new empirical formula was proposed for al-Hindiya second bridge to predicate the scour depth around the pier corresponding to the hydraulic conditions similar as the study region.