Geoscience Reference
In-Depth Information
and '100-year flood'. It is a general rule that the magnitude of a flood is inversely related
to its frequency, or probability, of occurrence (in other words, the larger the flood, the less
likely it is). A flood that is likely to occur only once in a hundred years - the 100-year flood
- has a 1% likelihood of occurring in any year, and the average interval between two floods
of that magnitude is 100 years. For engineering purposes, it is useful to know the probab-
ility of a flood of a particular magnitude so that, for example, a bridge designed to last for
50 years can be built large enough to withstand a 50-year flood, and often a 100-year flood
just in case. These are statistical probabilities, however, and there is still the chance that the
bridge may be swept away by a far larger flood.
Many of the less predictable causes of flooding occur after a valley has been blocked by
a natural dam as a result of a landslide, glacier, or lava flow. Natural dams may cause up-
stream flooding as the blocked river forms a lake and downstream flooding as a result of
failure of the dam. Earthquakes, which can cause enormous landslides, are a particularly
common cause of natural dams. For example, the Īnangahua earthquake in South Island,
New Zealand, in May 1968 triggered a huge landslide that dammed the Buller River. The
rising water backed up for 7 kilometres, raising the river 30 metres above its normal level.
Fears that the dam might suffer a catastrophic breach led to an evacuation of all the people
living in its path, but the river eventually overflowed the landslide dam, eroding it down-
ward gradually without causing serious flooding downstream.
Breaches of natural dams account for most of the largest known floods of the last 2.6 mil-
lion years, the so-called Quaternary Period. The biggest that we know about occurred as a
result of ice-dam failure after pre-existing continental drainage systems were blocked by
ice sheets during the Ice Ages that have characterized the Quaternary. Some of the largest
ever to have occurred on Earth were the Missoula floods in the northwestern USA of today.
They resulted from the repeated breaching of an ice dam that blocked the present-day Clark
Fork River between about 18,000 and 13,000 years ago. The ice created an immense lake
known as glacial Lake Missoula which spilled out to create the Missoula floods when the
ice dam periodically failed. The peak discharge of the Missoula floods is thought to have
been a gigantic 17 million cubic metres per second, more than ten times the combined flow
of all the rivers of the world today.
The evidence for the Missoula floods is convincing, but it is one of several great floods
known or suspected to have occurred in prehistoric and geological times and not all are as
well substantiated. Somewhere at the confluence of fact and fiction lie hundreds of flood
legends from cultures all across the world. These stories take their place among a far great-
er number of myths, sacred traditions, and beliefs based on the flow of rivers that form the
subject of the next chapter.
