Mean flow and turbulence around experimental spur dike

An acoustic Doppler velocimeter was used to measure flow and turbulence around an experimental spur dike in a flat and a scoured bed. Differences of mean velocity, turbulent intensity and Reynolds stresses between these two flow fields were analyzed. Upon the formation of scour hole, mean flow velocities in the downstream and lateral directions were reduced, but increased in the vertical direction. The turbulence intensities (u′u′ and v′v′) are much larger, and the vertical component (w′)(w′) is smaller than that in the flat bed. Among three Reynolds stresses, the -ρu′w′¯ and -ρv′w′¯ components are much smaller than the -ρu′v′¯. Bed shear stress near the dike can be 6 to 8 times as large as that of the approaching flow so that a local scour is developed near the dike without the shear stress of approaching flow exceeding the critical shear stress of bed material. The local scour initiated at the upstream of the dike and then extends to downstream from the dike tip. These results indicated that the development of local scour synchronizes to high shear stresses resulted from the horseshoe vorticies on a mobile bed surface.