Conditions for plug formation in oscillatory flow

The conditions under which plug formation may occur in oscillatory flow over beds of sediment are examined. The term plug formation is used to describe the situation where sediment starts to move as a solid block when the flow reverses. It is shown analytically that there is a lower bound on the value of the parameter S=ρU0ω/(ρs−ρ)g for plug formation. For values of S above this limit plug formation is determined by the degree of compaction of the sediment. It is suggested that the degree of compaction depends on the parameter W0/ωδs where W0 is the fall velocity of the sediment and δs is the maximum thickness of the mobile layer. Re-examination of the measurements of Dick and Sleath (1991) and Zala Flores and Sleath (1998) showed that for their acrylic sediment plugs formed when W0/ωδs was less than about 0.12. All of the tests which showed plug formation were in Bagnold’s (1954) ‘viscous’ regime. For these conditions there is good agreement between the measured velocity distribution in the shear layer below the plug and the velocity distribution calculated from Bagnold’s constitutive equations provided an appropriate value of effective viscosity is selected. Comparison of tests in which plug formation occurred with those in which it did not suggests that plug formation leads to much larger values of the effective viscosity in the shear layer close to the bed. For plug flows the measured values of effective viscosity were more than 1000 times those suggested by Bagnold (1956).