Internal seiches and subaqueous landforms in lacustrine cohesive sediments

Abstract

The introduction of sidescan sonar to surveying the floors of Scottish Lochs has permitted recognition of low-relief blister-like slides on the slopes of major basins. Here we report that these features occur discontinuously, but in regularly distributed zones, in the lochs examined, at the level of the summer thermocline. Internal seiches on the thermocline create circulation patterns dominated by periodically reversing horizontal currents in nodal positions, and vertical motion at antinodes. Although the currents are not strong enough to entrain the deposited flocculent cohesive sediments, we suggest that they exert sufficient shear stresses at nodal regions to cause breakdown of floe structures. This gives rise to 'shear thinning' of the surficial sediments and a consequent deepening of the weakened near-bed concentrated suspensions which, because they are situated on 8–18° sloping surfaces, begin to move downslope. As water escapes, shear strength is regained and the sediment ceases to move, leaving zones of blister-like slides centred on the nodal sections of the basin slopes. At the antinodes, no slides are developed.