Scour is a natural phenomenon that is the result of the erosive action of flowing water, excavating and carrying away material from the bed and banks of streams and from around the piers and abutments of bridges. Different materials scour at different rates. Loose granular soils are rapidly eroded by flowing water, while cohesive or cemented soils are more scour-resistant. Failure of bridges due to scour at their foundations, which consist of abutments and piers, is a common occurrence. Based on the mode of sediment traort by the approaching flow, local scour is stroked="f" filled="f" path="m@4@5l@4@11@9@11@9@5xe" o:preferrelative="t" o:spt="75" coordsize="21600,21600" sa ), identified based on dimensional analysis, including abutment Froud number (Fr), armor-layer thikness to armor particle diameter ratio ( ), and armor particle to bed particle diameter ratio ( a ). The experimental data of clear water scour condition in thinly armored beds under the stability of surface particles limitation were used to determine an equation for equilibrium scour depth. Finally two relations for forecasting equilibrium scour depth for both semi-circular and semi-eliptical abutments were achived. Key Words Local scouring, abutment, armoring, sediment.
