Environmental Engineering Reference
In-Depth Information
Figure 2
Conditions which may be significant in intensifying river
flooding.
Most floods are the result of severe meteorological or climatological conditions
(Figure 1, see also box on pp. 291-3). They may follow severe local thunderstorms or
more widepread rain falling on a saturated landscape. 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 (Figure 2).
Severe floods have occurred 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. It has been estimated
that some 5 million Chinese lost their lives in floods between 1860 and 1960 (Smith
1996).
