Engineering Geology

Chapter 3
windspeeds of approximately 120 km h -1 tend to be associated with central pressure drops
of around 34 mbar. Normally, the level of the sea rises with reductions in atmospheric pressure
associated with intense-low-pressure systems. In addition, the severity of a surge is
influenced by the size and track of a storm and, especially in the case of open coastline
surges, by the nearness of the storm track to the coastline. The wind direction and the length
of fetch are also important, both determining the size and energy of waves. Because of the
influence of the topography of the sea floor, wide shallow areas on the continental shelf are
more susceptible to damaging surges than where the shelf slopes steeply. Surges are intensified
by converging coastlines that exert a funnel effect as the sea moves into such inlets.
One of the most terrifying phenomena that occur along coastal regions is called tsunami, the
inundation by a large mass of water (Fig. 3.41). Most tsunamis originate as a result of fault
movement, generating earthquakes on the sea floor, although they also can be developed by
submarine landslides or volcanic activity. However, even the effects of large earthquakes are
relatively localized compared to the impact of tsunamis. As with other forms of waves, it is
the energy of tsunamis that is transported, and not the mass. Oscillatory waves are developed
with periods of 10 to 60 min that affect the whole column of water from the bottom of
the ocean to the surface. Together with the magnitude of an earthquake and its depth of
focus, the amount of vertical crustal displacement determine the size, orientation and
Figure 3.41
The northern end of Resurrection Bay at Seward, Alaska, after it had been affected by a tsunami. The epicentre of the
earthquake was 75 km distant.
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