Geology Reference
In-Depth Information
Fig. 13.6
Map showing the distribution of the major tidal
ridges (
a
) on the Western European shelves and (
b
) the East
China and Yellow seas. All the ridges are linear, offshore 'en
echelon' tidal ridges, except those in location 5, which are ban-
ner banks associated with a headland. In (
a
), ridge field 1
occurs on a headland-associated shoal-retreat massif (Swift
1975
). Ridge field 2 might also occupy a shoal-retreat massif;
alternatively, it may represent the redistribution of an east-west-
oriented barrier complex that existed earlier in the post-glacial
transgression. Ridge fields 3 and 4 may represent an embay-
ment-head type of occurrence (cf. Dyer and Huntley
1999
). In
(
b
), note the train of tidal ridges along the retreat path of the
Changjiang River (ridge field 2). Ridge field 1 may occupy the
retreat path of the Huanghe and/or Han River
Fig. 13.7
(
a
) Tidal ridges around the Portland Bill headland,
English Channel (see location in Fig.
13.6a
- ridge field 5).
These are typical 'banner banks' that form in the lee of coastal
promontories (After Bastos et al. (
2003
). (
b
) Numerical model
of tidal residual circulation (After Pingree and Maddock
1979
).
Bedform migration directions on Shamble Bank are consistent
with the counterclockwise circulation of the modeled eddy. The
observed convergence of bedload transport toward the centre of
the eddies is explained by a centripetal reduction of bottom shear
stress (see detailed explanation in Dyer and Huntley
1999
). The
process is efficient for small eddies only (ca. 10-20 km in diam-
eter), implying that this process cannot generate longer ridges
by two vorticity forces that increase toward the ridge
crest, one due to the increase of bottom friction in the
shallower water over the crest, and the other due to an
enhanced Coriolis effect (Fig.
13.8
). The Coriolis
effect either dampens or enhances the friction-driven
circulation, depending on whether the ridge is oriented
clockwise or counter-clockwise, respectively, to the
peak tidal flow. This explains why ridges are mostly
skewed in a counter-clockwise sense relative to the
peak tidal flow in the northern hemisphere (Fig.
13.9
).
The growth of a linear shelf ridge from a small
bed perturbation (bump) was first modeled by
