Geoscience Reference
In-Depth Information
6.2 Geomorphological Consequences of Tsunami:
Deposits of Paleotsunamis
Detailed investigation and documentation of the effects of the tsunami influence on
the coast is a very important task, which permits to study more profoundly the nature
of this phenomenon, to develop necessary recommendations for tsunami zoning, to
improve schemes for evacuating the population, to determine the local influence of
the coast morphology on the tsunami effects and much more.
A tsunami drags along a whole number of geomorphological consequences. The
most widespread of them consists in the deposition of sea sand, silt and uprooted
material far from the shoreline. The most strong tsunamis move material over dis-
tances up to 5 km, and, when the water draws back, the territory, that was flooded,
turns out to be covered with a layer of 'tsunamigenic deposits' of thicknesses from
centimeters up to meters. Tsunamigenic deposits, as a rule, consist of the mate-
rial making up beaches and the shallow-water part of the coastal zone. Moreover,
tsunamis shift a large amount of torn-up and broken trees and bushes, if there were
any on the shore. In inhabited places a tsunami carries away a lot of garbage and
fragments of destroyed buildings. Thus, for example, the front of the Indonesian
tsunami of December 26, 2004 carried so much material with it in the region of
Banda Aceh city that it looked more like a mud flow or lahar (Fig. 6.7).
One more, very widespread, effect, caused by tsunamis on shores, consists in soil
erosion and abrasion of hillsides and terraces. As a rule, these effects accompany
strong tsunamis with high flow velocities. In such cases, a tsunami strips the layer of
soil and vegetation away from the earth's surface, washes away coastal swells, river
sandbars, ledges of terraces and washes out niches along slopes, made up of sedi-
mental or weakly consolidated rock (Fig. 6.8). Investigation of the consequences of
Fig. 6.7 City Banda Aceh (North of island Sumatra) destroyed by the tsunami of December 26,
2004. The photograph was taken at a distance of 3 km from the shore, in the very beginning of
the zone, where material and fragments of buildings displaced from the shore were deposited
(Photo by T. K. Pinegina) (see also Plate 10 in the Colour Plate Section on page 318)
Search WWH ::




Custom Search