Geology Reference
In-Depth Information
Fig. 13.8
(
a
,
b
) Friction-driven vorticity (
dotted circles
)
induced by oblique flow across an elongated bump on the sea-
bed. The open
straight arrows
show the flood and ebb current
vectors. The
black arrows
show the resulting residual circulation
around the bump (i.e. the tidal ridge). The vorticity increases
toward the top of the ridge, acting to move bedload toward the
ridge crest. (
c
) Coriolis-driven vorticity induced in the northern
hemisphere (
N.H
.) by the flow across the same seabed relief.
This results in a clockwise residual circulation (counter-clock-
wise in the southern hemisphere). This effect enhances or damps
the friction-driven residual circulation. In the northern hemi-
sphere, their interplay favors the existence of ridges oriented
counter-clockwise to the flow (After Pattiaratchi and Collins
1987
)
causing ridge growth. The transport paths over the
ridge reflect the convergence of sediment toward
the ridge crest, as noted first by Van Veen (
1936
). On
the side of the ridge facing it, the dominant current is
accelerated by flow constriction and the subordinate
current is decelerated by flow expansion, so that the
residual transport by the regionally dominant current is
enhanced (Fig.
13.10
). On the ridge side facing the
subordinate current, the dominant current is weaker
because of sheltering and the subordinate current is
accelerated toward the ridge crest by flow constriction,
so that the residual transport on this side is commonly
dominated by the regionally subordinate current.
Huthnance (
1982a, b
) calculated that the ridges
must become elongated and oblique to the peak flow at
an angle of about 20°. Ridge elongation is proportional
to the elongation of the tidal ellipse, and the initial
bump from which a ridge grows does not have to be
elongated or properly oriented itself. Based on tidal
ridges in the China and Yellow Seas, Liu et al. (
1998
)
suggest that linear ridges are restricted to areas where
tidal M2 ellipticity (i.e., the ratio between the minor
and major axes of the tidal ellipse) is < 0.4 (i.e., the
tidal ellipses are significantly elongated), whereas sand
sheets occur where the currents are more rotary.
Hulscher (
1996
) showed that linear ridges are more
prone to develop in deeper water, because the 3D flow
structure is more homogenous and there is, therefore, a
smaller phase lag between shear stress and the depth-
averaged current speed. Theory indicates that ridges
Fig. 13.9
Tidal ridges and tidal-transport pathways in the
southern North Sea (ridge field 3 in Fig.
13.6a
). Note the align-
ment of the ridges at a small angle to the net sand-transport
paths, which have been determined from the asymmetry of
dunes. Most ridges are offset counter-clockwise to the tidal flow,
in response to the mechanism described in Fig.
13.8
(After
Kenyon et al.
1981
)
Huthnance (
1973, 1982a, b
). The main process involved
in the Huthnance model is that the bottom friction
associated with the bump delays the upslope current
more than it accelerates the downslope one. As a con-
sequence, the transport of sand toward the ridge crest
will be higher than the off-ridge transport, thereby
