Coastal Landscape Evolution - Tsunami - page 61

In fact, the idea of uniformitarianism involves two con-
cepts (Bryant 2005 ). The first implies that geological pro-
cesses follow natural laws applicable to science. There are
no Acts of God. This type of uniformitarianism was
established to counter the arguments raised by the catastr-
ophists. The second concept implies constancy of rates of
change or material condition through time. This concept is
nothing more than inductive reasoning. The type and rate of
processes operating today characterize those that have
operated over geological time. For example, waves break
upon a beach today in the same manner as they would have
a 100 million years ago, and prehistoric tsunami behave the
same as modern ones described in our written records. If
one wants to understand the sedimentary deposits of an
ancient tidal estuary, one has to go no further than to a
modern estuary and study the processes at work. Included in
this concept is the belief that physical landscapes such as
modern floodplains and coastlines evolve slowly.
Few geomorphologists or geologists who study Earth
surface processes and the evolution of modern landscapes
would initially object to this concept. However, the concept
does not withstand scrutiny. For example, there is no
modern analogy to the nappe mountain building processes
that formed the Alps, or to the mass extinctions and sudden
discontinuities that have dominated the geological record
(Bryant 2005 ). Additionally, no one who has witnessed a
fault line being thrusted up during an earthquake or Mt. St.
Helens wrenching itself apart in a cataclysmic eruption
would agree that all landscapes develop slowly. As Thomas
Huxley so aptly worded it, gradualists had saddled them-
selves with the tenet of Natura non facit saltum—Nature
does not make sudden jumps. J Harlen Bretz from the
University of Chicago challenged this tenet in the 1920s
(Baker 1978 , 1981 ). Bretz attributed the formation of the
Scablands of eastern Washington State to catastrophic
floods. He subsequently bore the ridicule and rancor of the
geological establishment for the next 40 years for proposing
this radical idea. Not until the 1960s was Bretz proved
correct when Vic Baker of the University of Arizona
interpreted space-probe images of enormous channels on
Mars as features similar to the Washington Scablands. At
the age of 83, Bretz finally received the recognition of his
peers for his seminal work.
Convulsive events are important geological processes,
and major tsunami can be defined as convulsive (Clifton
1988 ). More importantly, it will be shown in the remainder
of this topic that mega-tsunami—in many cases bigger than
tsunami described in historic and scientific documents—
have acted to shape coastlines. Some of these mega-tsunami
events have occurred during the last millennium. In coastal
geomorphology, existing scientific custom dictates that in
the absence of convincing proof, the evidence for convul-
sive events must be explained by commoner events of lesser

magnitude—such as storms. However, this restriction
should not imply that storms can be ubiquitously invoked to
account for all sediment deposits or coastal landscapes that,
upon closer inspection, have anomalous attributes more
correctly explained by a different and rarer convulsive
process. The alternate phenomenon of tsunami certainly has
the potential for moving sediment and molding coastal
landscapes to the same degree as, if not more efficiently
than, storms. Tsunami can also operate further inland and at
greater heights above sea level. Tsunami have for the most
part been ignored in the geological and geomorphological
literature as a major agent of coastal evolution. This neglect
is unusual considering that tsunami are common, high-
magnitude phenomena producing an on-surge with veloci-
ties up of 15 m s -1 or more.

4.3 Tsunami Versus Storms

In the past, rapid coastal change, especially in sandy sedi-
ments, has been explained by invoking storms. Where
sediments have previously incorporated boulders, the role
of storms versus tsunami has become problematic. How-
ever, much of the latter debate deals with isolated boulders
or boulders chaotically mixed with sand deposited on low-
lying coastal plains or atolls (Bourgeois and Leithold 1984 ).
The pattern of stacked and aligned boulders found along the
New South Wales coast mainly lie above the limits of storm
waves or surges (Bryant et al. 1996 ). Where such boulders
lie within the storm zone, they form bedforms that have
never been linked to such events. More importantly, even a
casual reconnaissance of the New South Wales, Australian
coastline will show that storms inadequately account for the
bedrock-sculptured features dominating the rocky coast.
The uniform alignment of such forms, often not structurally
controlled, also rules out chemical weathering. Along this
coast, the largest storms measured last century in 1974 and
1978 only generated deep-water waves of 10.2 m, while the
maximum probable wave for the coast is just over 13 m
(Bryant 1988 ). The effectiveness of these wave heights
cannot be exacerbated at shore by storm surges because the
narrowness of the shelf and the nature of storms limit surges
here to less than 1.5 m. There are three parameters crucial
to many of the features outlined in the previous chapter.
Two of these, wave height and period, define the energy of
the wave; the third, duration, controls how that energy
transports sediment, erodes surfaces and generates the
landscape. Tsunami and storm waves, especially if the latter
are prolonged, effectively overlap in terms of their energy
levels and their transport capacity. It is no surprise that it is
difficult to distinguish either process in the transport of
boulders and sand. However, the duration of flow under a
storm wave cannot match that under a tsunami. The peak