Geoscience Reference
In-Depth Information
Tsunamis approach a coast either in the form of gradual inundation or a rapid
tide. The wave front, when running up the land, is rendered complex by various
turbulent flows, therefore, the direction of motion of the tsunami and its height may
vary very strongly even within hundreds of meters. In this connection, tsunami ef-
fects on the shore may differ even in neighbouring regions. The width of the tsunami
inundation zone depends on the relief topography, the tsunami height and its wave-
length.
Investigation of the consequences due to a tsunami traditionally includes the doc-
umentation of its run-up height, inundation height and the distance covered by
the run-up. It is also necessary to pay attention to the direction of fallen trees, pillars
and other constructions; to photograph regions of eroded land and of accumulation
of tsunamigenic deposits. The maximum inundation level often leaves a muddy sign
on the walls of houses, which also has to be recorded. The tsunami height can be
estimated from the garbage left on the branches of surviving trees and, also, by
the height, at which there are broken twigs and peeled off bark. It is necessary to
note the variation of water level with the distance from the shore, since this pro-
vides a basis for determining the wavelength. On surviving houses it is necessary
to measure the height of water at the wall open to the sea and at the opposite side
of the house, since the difference in levels permits to determine the flow velocity.
Besides the above, it is necessary to make bores at various distances from the shore
and to determine the thickness of the tsunamigenic layer and, also, to take sam-
ples (100-200 g) from this layer for a granulometric analysis. Such analysis gives
an idea of changes in the flow velocity. If possible, it is necessary to take note of
the number of strata—this provides information on the number of waves.
The main method of measuring the heights and depths of tsunami inundation
consists in gaging topographic profiles on the shore—from the sea level (perpen-
dicular to the shoreline) up to the height of maximum inundation at the maximum
distance covered by the run-up (Fig. 6.9). Gaging the topographic profile involves
registration of the time the height of the first point on the shore above the water
level was determined. Then, the table of tides and ebb-tides is applied in recalcu-
lating the profile height relative to the average sea level, and, also, with respect to
the sea level at the tsunami arrival time.
Po
Run-up length (m)
Fig. 6.9 Scheme for gaging the main characteristics of tsunami manifestations along the topo-
graphic profile
