Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 75

huge volumes of water. Flood flows from ice dammed lakes are common in
Iceland, but not to the same degree as those during the late Pleistocene. These
types of floods are called jokulhlaups. The Pleistocene jokulhlaups occurred
around the margins of the last glacial ice sheets in both North America and
Asia. The evidence for these floods is in the form of channel scablands where
large scale erosional features including scour hollows and cavitation features
such as pot holes as well as bedrock scoured overflow channels occur. Also sed-
imentary sequences including 20 m diameter boulders and giant gravel ripple
sequences have been documented for the channel scabland area of Washington
State (Baker, 1973) and the Altai Mountains region of Siberia (Rudoy and Baker,
1993;Carling, 1996). In Washington, the largest glacial lake impounded about
2500 km 3 of water which was 600 m deep. After failure of the impounding ice
dam, the outflow was so great that the flood transported and deposited graded
sedimentary beds between 100 and 160 m high in a submarine rift valley 1000 km
from the mouth of the Columbia River (Baker, 2002). These flood flows are simi-
lar in size and velocity to the world's major ocean currents such as the Atlantic
Gulf Stream, the Kuroshio in the Pacific and the Agulhas current in the Indian
Ocean. They are also similar in dimension to the megafloods that formed the
outflow channels on Mars which were responsible for filling the Oceanus Bore-
alis (the great ocean thought to have formed across the northern portion of
Mars) and changing the atmosphere and climate of Mars from cold and dry
to warm and humid several times over the last 4.6 billion years (Baker et al .,
1991).
Conclusion
Floods are one of the most devastating of all hazards globally. More
than 75 million people are affected by floods each year, so considerable effort
and expense is directed towards mitigating against this hazard. Despite their
frequency of occurrence, many communities are still caught unaware by excep-
tionally large floods either because they have had no experience with events of
this magnitude or did not realistically believe that such an event might affect
them. Short historical records may give a false impression of the nature of the
flood hazard for a region. Palaeohydrology can help in this regard by providing a
valuable insight into the nature of the flood regime for regions as diverse as the
humid tropics to deserts where river flows may be relatively rare. Palaeoflood
reconstruction can be undertaken using a variety of techniques including slack-
water sediment analysis, flow competence measures and stage height indicators
such as scour lines, and various forms of debris. Coral luminescence banding
has also provided an unusual and accurate reconstruction of floods in the Great