Precipitation and evapotranspiration - Fundamentals of the Physical Environment - page 98

Floods

HUMAN IMPACT

Floods are the most common of all environmental hazards. Each year many thousands of people die as a result of
flooding and millions are affected by indirect consequences such as damage to crops, housing, transport, etc. Although
coastal floods do occur, often the result of tropical cyclone surges as in Florida or Bangladesh or tsunami, as in the
Indian Ocean in 2005, it is on riverine flood plains where most flood problems are found. Floods can occur in both
developed and developing countries, though the impact is often greater in the latter, where high populations are to
be found on extensive and often relatively fertile flood plains such as those of the Ganges and the Yangtse. In this
section we concentrate on the meteorological factors behind flooding whilst in Chapter 14 the geomorphological
aspects will be examined.

Most floods are the result of severe meteorological or climatological conditions ( Figure 5.15 , see also box on p. 324).
They may follow severe local thunderstorms or more widespread rain falling on to a saturated landscape as in the
June 2007 floods in Yorkshire and the July 2007 floods in the Severn Valley, UK. These two dramatic events were
the result of the summer jet stream following a more southerly track than normal, allowing frequent active
depressions to pass across England and Wales rather than Scotland. In parts of the West Midlands the May to July
rainfall was more than 280 per cent of average (see additional case study 'Jet streams and the summer of 2007
in the UK' on the support website at www.routledge.com/textbooks/9780415395168 ). Snowmelt, particularly when
associated with further rain, can cause major flooding. Storm surges on to the coast as a result of tropical cyclones
can cause even more damage when reaching deltas where rivers are in flood. Floods may also occur as a result of
individual disasters such as landslides or dam bursts. Although these conditions may be very important, major floods
are usually the result of flood-intensifying conditions which worsen the original meteorological problem. For example,
the basin characteristics may aid the movement of rainwater by having steep, unvegetated, impermeable slopes,
variable altitude and a basin shape which focuses the tributaries on to a particular part of the catchment, as at York
on the Ouse in northern England. Floods often occur where there is a sudden change of channel gradient, causing
the flow of water to decrease its velocity and perhaps spill over the flood banks. In general, floods caused by convection
are relatively local, whilst cyclonic storms, snowmelt and orographic floods can cause more widespread damage.

Severe floods have occurred in most parts of the world at some time or another. In Mozambique, in south-east Africa,
abundant rains in the rainy seasons of 1999/2000 and 2000/2001 associated with a La Niña state in the Pacific Ocean,
topped up by a tropical cyclone, caused the major rivers flowing from Transvaal and Zimbabwe to discharge vast
volumes of water on to the coastal flood plains of Mozambique. By international standards the loss of life was not
great, but nearly half a million people were displaced and the infrastructure of the country was severely affected.
Some of the most extensive floods occur on the vast river plains of China, Bangladesh and the Mississippi basin in
the United States. The rains, levee collapse and a tidal surge associated with Hurricane Katrina caused devastation
in New Orleans in 2005, causing an estimated 1,840 deaths and damage amounting to $81 billion. It has been
estimated also that some 5 M Chinese lost their lives in floods between 1860 and 1960 (Smith 1996).

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sea level. In contrast, in temperate areas a large proportion
of the rainfall comes from deep stratiform clouds that
extend through a considerable part of the troposphere.
Here the effect of altitude on rainfall is more marked,
though the increase on windward slopes is usually greater
than on leeward slopes. Comparisons are difficult, how-
ever, because some of the precipitation on the mountains
in temperate areas falls as snow and, as we have seen, snow
is impossible to measure accurately.

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Snowmelt
Rain
Combined rain
and melt
Icemelt

Estuarine interactions
between streamflow
and tidal conditions
Coastal storm surges

Earthquakes
Landslides
Failure of dams and
other control works

Figure 5.15 Conditions which may be significant in intensi-
fying river flooding.