Geoscience Reference
In-Depth Information
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Fig. 6.41
Amphidromic tidal gyres of the North Sea and surrounding areas. Each of the the three systems has anticlockwise sense of rota-
tion. Full lines are co-range lines with tidal range in meters. Dotted lines are cotidal lines indicating the level of high water at the stated
number of hours lapsed since the Moon passed over the Greenwich meridian.
6.5.3
Storm set-up and wind-forced geostrophic
London), in the Bay of Bengal, and in the Venetian
Adriatic (where in both places the inhabitants are not so
lucky). The very low barometric pressures during storms
cause a sea-level rise under the storm pressure minimum.
The magnitude of this effect is about 1 cm rise per mil-
libar decrease of pressure. So passage of the eye of a trop-
ical storm of pressure 960 mbar might cause a few tens of
centimeters of sea-level rise. The very low core pressures of
coastal tornadoes are particularly effective at raising the
setup of shelf waters, sometimes up to 4 m or more above
currents
Let us examine the effects of storm winds in more detail,
for, as we shall see later, major shelf erosion and deposi-
tion result during such episodes. As in lakes, wind shear
drift causes set-up of coastal waters; should this coincide
with a spring high tide, then major coastal flooding
results. The effects are well known in the southern North
Sea (where the Thames Barrage now protects low-lying
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