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52 ~ ',"," . ~ .. .' ", ", . " " .- - DEPTH .', " . . . . a.... ,-."tLCf~," , . ~)J"}'",,:' - .. . DIRECTION OF FLOW --------- , ~, ~' , -------., ~ , ~'- '-~ LOCATION Figure 6.7. Scour on projecting abutments. 6.5.3 Methodology The theory and physical model testing for erosion and local scour determination has been developed mostly for structures in or along the banks of channels, The major type of struc- tures considered are bridges, embankments and abutments, However, the methodologies developed are quite applicable to the different types of structures in the flood plain. The following equations that estimate local scour are conservative. For very cohesive soils, it is likely that the scour depth will be less. For a detailed consideration of local scour and the development of equations presented here, the reader is referred to Simons and Senturk, 1976 (Reference 35). 6.5.3,1 Structures at Ground Level The local scour that occurs as flow is diverted around the corner of a building is calculated by 5 = d. [2.2 (i!.) 0,65 (V 1) 0.43] d vgd where 5 d a = depth of scour hole (ft) = depth of flow upstream of the structure (ft) = the flow length measured normal to the overall direction of flow (see Figure 6.5) (ft) = the velocity of flow approaching the structure (lt/see) = acceleration of gravity (32.2 It/sec') V g Design of the foundation wall depths and/or footings must consider the expected scoure or some means of protection against scour must be provided. 6.5.3.2 Piers, Piles, Posts and Walls The calculation for scour at piers (refers also to piles, posts and walls) is based on the shape, width and nose of the pier. Three typical shapes that are associated with elevated structures are shown in Figure 6.8.