Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 269

mean high-water level, as in Hurricane Carla on Padre
Island, Gulf of Mexico.
The magnitude of wind shear setup can be roughly esti-
mated by assuming that the shearing stress,

nature of storm surges enables prediction for vulnerable
areas like the North Sea and the Adriatic by reference to
monitored upcurrent changes in sea level during storm
development. Offshore, the large wave setup during
storms means that a compensatory bottom flow occurs out
to sea, driven by the onshore to offshore pressure gradient.
Such geostrophic or gradient currents (which are also
turned by Coriolis forcing; Fig. 6.42) have been proven
by measurements during storms to reach over 1 m s 1 ,
running for several hours (a fact suspected by submariners
since 1914, see Fig. 6.42). They are a major means of off-
shore transport from coast to shelf.

, due to the
wind balances the pressure gradient due to the sloping sea
surface,

p / x , that is g h p / x , where h is water

depth and is water density. Solving for the slope term for

storm winds of 30 m s 1 acting on 40 m water depth yields
about 2.2 10 6 for the 600 km long North Sea, leading to

a superelevation of about 1.3 m. This is 50 percent or so
less than the observed surge height because we have neg-
lected important effects due to the Coriolis force, which
pushes the current against adjacent shorelines where it is
further amplified by resonance and funneling. In the case
of the major southern North Sea storm of 1953, the
southerly directed wind drift was first forced westward
onto the Scottish coast with the southward traveling
(anticlockwise) Kelvin tidal wave, where it ultimately gave
rise, some 18 h later to a

6.5.4 Shelf density currents

Density currents are also important in shelf transport.
Hypopycnal (positively buoyant) jets of fresh to brackish
water with some suspended sediment issue from most
estuaries and delta distributary mouths. In higher

3.0 m superelevated surge
along the Dutch and Belgian coasts. The Kelvin wave

(a) (c)

Storm wind

Mid-depth
geostrophic
flow

Setup
MSL

Gradient
current

Oscillatory
boundary
layer

Bottom
flow

Bottom
flow

(b)

Coriolis
force

Force balance and uniform
steady flow

Resultant
force

Pressure
force

Friction
force

Coriolis
force

The earliest recorded direct impression of storm waves (and
?gradient currents) from the sea bottom occurs in the log of
HM submarine, E10, in 1914 in the southern North Sea, off
Heligoland. After torpedoing a German cruiser the sub
bottomed to 30 m and thereafter a very bad storm grounded
and shifted her despite over 10 tons of negative buoyancy

Pressure
gradient
force

Fig. 6.42 Shoreface to shelf geostrophic gradient currents. (a) Section; (b) force balance; (c) plan.