Geoscience Reference
In-Depth Information
The removal of forests from a catchment area will
increase the propensity for a river to flood and also
increase the severity of a flood event. Conversely the
planting of forests on a catchment area will decrease
the frequency and magnitude of flood events. Fahey
and Jackson (1997) show that after conversion of
native tussock grassland to exotic pine plantations
a catchment in New Zealand showed a decrease in
the mean flood peaks of 55-65 per cent. Although
data of this type look alarming they are almost
always taken from measurements at the small
research catchment scale. At the larger scale the
influence of deforestation is much harder to detect
(see Chapter 8).
normal discharge expected to flow through it. When
the river channel is altered in some way it can have
a detrimental effect on the flood characteristics
for the river. In particular,
channelisation
using
rigid structures can increase flood risk. Ironically,
channelisation is often carried out to lessen flood risk
in a particular area. This is frequently achieved,
but in doing so water is passed on downstream at a
faster rate than normal, increasing the flood risk
further downstream. If there is a natural floodplain
further downstream this may not be a problem, but
if there is not, downstream riparian zones will be at
greater risk.
Land drainage
Urbanisation
It is common practice in many regions of the world
to increase agricultural production through the
drainage of 'swamp' areas. During the seventeenth
and eighteenth centuries huge areas of the fenlands
of East Anglia in England were drained and now are
highly productive cereal and horticultural areas. The
drainage of these regions provides for rapid removal
of any surplus water, i.e. not needed by plants.
Drained land will be drier than might be expected
naturally, and therefore less storm runoff might
be assumed. This is true in small rainfall events but
the rapid removal of water through subsurface
and surface drainage leads to flood peaks in the
river drainage system where normally the water
would have been slower to leave the land surface.
So, although the drainage of land leads to an over-
all drying out of the affected area it can also lead
to increased flooding through rapid drainage.
Again this is scale-related, as described further in
Chapter 8.
Urban areas have a greater extent of impervious sur-
faces than in most natural landforms. Consequently
the amount of infiltration excess (Hortonian) over-
land flow is high. In addition to this, urban areas are
often designed to have a rapid drainage system,
taking the overland flow away from its source. This
network of gutters and drains frequently leads
directly to a river drainage system, delivering more
flood water in a faster time. Where extensive urban-
isation of a catchment occurs; flood frequency and
magnitude increases. Cherkauer (1975) shows a
massive increase in flood magnitude for an urban
catchment in Wisconsin, USA when compared to a
similar rural catchment (see pp. 169-170). Urban-
isation is another influence on flooding that is most
noticeable at the small scale. This is mostly because
the actual percentage area covered by impermeable
urban areas in a larger river catchment is still very
small in relation to the amount of permeable non-
modified surfaces.
Climate change
River channel alterations
In recent years any flooding event has led to a
clamour of calls to explain the event in terms of
climatic change. This is not easy to do as climate is
naturally so variable. What can be said though is
that river channels slowly adjust to changes in flow
regime which may in turn be influenced by changes
Geomorphologists traditionally view a natural river
channel as being in equilibrium with the river
flowing within it. This does not mean that a natural
river channel never floods, but rather that the
channel has adjusted in shape in response to the
