Flow phenomena in the benthic boundary layer and bed forms beneath deep-current systems.

The discovery of erosional and depositional bed forms in the deep ocean provides a means for estimating some properties of the benthic boundary layer and also raises questions about secondary circulation near the bed. Previously unexplained hyperbolic echoes are now seen (using a deeply towed instrument package) to be caused by furrows on the sea bed. Small furrows are 1-4 m wide, 3/4-2 m deep, and spaced 20-125 m apart; large furrows are 50-150 m wide, 20 m deep, and spaced 50-200 m apart. Analogy with shallow-water furrows and bed forms produced in deserts and in laboratory experiments suggest an origin for the furrows through interaction of large-scale helical vortices developed in the boundary layer with the sediments. The thickness of the bottom-mixed water layer should be about one-half the furrow spacing. Measurements show that mixed layer thickness (20-100 m) are of the correct order of magnitude. (A)