Necessary Conditions for the Langmuir Supercell Events That Dominate Sediment Transport on Shallow Shelves

Langmuir supercells (LS) are Langmuir circulations (LC) extending over full water column depth during storms on shallow shelves. LS events are characterized by bottom-origin sediment clouds resulting from upwards motion of resuspended sediment in the upwelling limbs of the LC. Associated net horizontal sediment transport in the strong mean downwind flow characteristic of LS events greatly exceeds that due to tidal flows, hence only a few such events can dominate annual net sediment transport in these environments. It is thus important to determine conditions necessary for LS events to occur. Observations from two sites are used to demonstrate that necessary conditions for LS event onset are (a) the presence of surface waves of intermediate type that “feel bottom” with amplitudes sufficiently large to resuspend sediment and (b) achievement of three conditions for full-depth LC: (i) an unstratified water column, (ii) La < ∼0.3 and (iii) |Ra| < ~10⁵, where La and Ra are dimensionless parameters derived from scaling the wave-averaged momentum equation. Event cessation is associated with failure of at least one of the three latter conditions. There is no fixed order in which conditions necessary for full-depth LC are met or fail. Circumstantial evidence from additional deeper sites suggests that full-depth Langmuir circulations may occur in unstratified water columns somewhat deeper than 30 m. Results from an October 2015 mooring deployment at 40 m depth will address whether LS occur there, subject to similar necessary conditions.