Geography Reference
In-Depth Information
fertile floodplains of the world means that less floodplain can be uti-
lised by a river for this purpose. Flooding in lowland areas is therefore a
bigger problem as the extra water is brought downstream more quickly
and in greater quantities than ever before. Solutions therefore need to
be mixed and include land management solutions as well as flood
defence solutions. Dealing with the flood problem means that we need
to build resilience into the built environment in lood-prone areas. An
example of resilience might be designing buildings that have very flex-
ible ground floor use such as parking areas for cars. When there is a
flood warning then the parking area can be evacuated of cars and flood-
ing allowed to occur without any damage to residential dwellings on
higher levels.
Flood risk is often interpreted by examining the historic frequency
of flooding. If a water level greater than 10 metres in height occurred
five times in the past ten years we would say that the return frequency
of the 10 metre flood is on average once every two years. However, this
does not mean that a 10 metre high flood will occur once every two
years. It may be that in one year a 10 metre sized flood occurs three
times. Looking at past records might not be the best guide to predicting
future flooding, particularly if land management change has taken
place in the catchment or if climate change is likely to change the pre-
cipitation patterns or vegetation cover.
River channel change
Rivers often have channels that look very different from one
another. The shape of river channels also varies within the same
river along its course. Most rivers have a long profile (slope of a
river from its source to mouth) which is concave with progres-
sively lower gradients downstream. The long profile varies with
geology, tectonics and variability in runoff. Other profile shapes
also occur when there is interruption by lakes or very resistant
rocks (which often result in waterfalls) or through large changes in
sea level. If sea levels fall then the whole river may start to erode its
bed downward in response. If sea levels rise the river may deposit
more sediment and bed levels will be raised along its length as
downstream processes have knock-on effects upstream, as well as
upstream processes delivering water and sediment downstream.
 
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