River management strategies
Land-use zoning
Land use zoning involves dividing the floodplain into areas which experience
different degrees of flood risk. e.g
Red Zone = High chance of experiencing a flood
Amber Zone = Flood possible but rare
Green Zone = Flooding very unlikely.
Once the floodplain has been divided into the different zones, an appropriate
landuse is chosen for each zone e.g
Red Zone = Golf Course, Parkland, farmland
Amber Zone = Car parks, Sports facilities, some houses but these should
have flood protection considered in their design
Green Zone = Residential housing, flood protection does not need to be
considered.
(£0.5 million if
the business
agree, could be
more if affects
trade)
This mean the river can flood naturally but will cause minimal damage and
disruption to people. Floodplain zoning is an effective method to manage floods
because it is cheap and effective at slowing the river down. Unfortunately it is
not realistic if the area is all ready built on because people would have to be
moved and homes destroyed. This would be extremely expensive and very
unpopular for those businesses or people that are forced out of their homes.
Channel
enlargement
(deepening and
widening)
Flooding can be reduced by increasing the size of the channel so that it can hold
more water. This means that in times of very heavy rainfall the river is less likely
to flood over its banks. This has the advantage of protecting the land either side
of the river. It also does not greatly affect the look of the river or spoil the
natural habitat for animals. Channel deepening and widening will need to occur
regularly as it will quickly silt up.
(£0.5 million per
year)
Afforestation
In order to increase the depth and width of the river heavy machinery will have
to be used, which is expensive. Although it may not drastically alter the look of
the river, it may disrupt the natural ecosystem. Some also argue that the natural
process of flooding helps to slow the river down. If you prevent this by making
the channel deeper and wider it will result in very high river speeds (velocity)
that could be extremely dangerous and make things worse downstream where it
has not been enlarged.
Planting trees in the upper course of the drainage basins helps to reduce flooding
because they can intercept the rain and store it. Trees store water on their
branches, on their leaves and in their roots. This helps to stop some of the water
from going into the river and slows down the rest of it. This reduces the risk of
flooding.
Afforestation does not prevent flooding from occurring and so houses could still
be in danger, but it does help to reduce the likelihood of flooding.
(£0.5 million)
Widened bridge
(£1 million per
bridge)
Afforestation is only possible if there is a lot of spare land that trees can be
planted on. It is also only possible if farmers or other land owners agree. A
benefit of afforestation is that new woodlands can improve the natural
environment by providing homes for animals. The timber can also be harvested
generating extra income.
The bridges would be widened so that debris such a trees and boulders can not
be trapped behind them creating a dam that lead to the large wall of water. The
new bridges may not be as attractive as the old arched bridges
Washlands
Washlands are areas of land in the lower course that the river can ‘wash’ or flood
into in times of flood. They are a bit like a storage area for the extra water.
When the river floods, it spills over into the washland, this not only stores the
water but also increases the amount of friction, which slows the speed of the
river down. To create a washland, a large area of land should be cleared and left
empty near to the river. This land should not be used and must be large enough
to store the floodwater.
Washlands can be effective in helping reduce the impact of flooding, but it can be
difficult to set aside enough land especially in a city. Another problem can be that
the washlands can become a dumping ground for rubbish and become an
unpleasant sight.
(£0.25 million)
Straightening
the River
A meandering river can be straightened. This helps to increase the speed
(velocity) of the river and therefore reduce flooding because the water drains
away from the flood plain more quickly. Removing the meanders also makes
using the land either side of the river much easier.
e.g for farmers, it means that their fields no longer change size because the
meanders keep changing its position. This is a particularly problem if you farm
only one side of the river
e.g for property developers it is easier to build a long straight river than one
that keeps changing its course.
(£0.25 million per
meander)
Levees and
embankments
(flood walls)
Straightening is not always successful, frequently straightened rivers quickly
return back to their natural meandering shape. There is also a large
environmental impact; fish tend to lay eggs in parts of the river where the flow is
slower. Without meanders the river flow is a lot faster and so fish are unable to
lay their eggs. Some also argue that the increase in speed just creates bigger
floods and problems downstream.
Levees and embankment walls can be built either side of the river. The walls
mean that even if the river floods above its natural riverbanks the water cannot
spread over the flood plain and damage property and land. They can be very
effective in preventing flooding.
Building the levees and embankment walls can be quite expensive although it
can be carried out without disturbing the river too much. (Especially if the walls
are set back from the river).
(£0.5million per
1km)
Dams or
reservoirs
(£3 million)
Some argue that the walls can spoil the look of the river, although this may
depend on the material used and how well they are landscaped. One major
criticism of embankments is that they can increase the speed (velocity) of the
river and so if the flood does break the embankments, the flood can often be
much more destructive because the water is travelling so fast. When a river
naturally floods it also deposits alluvium or silt on the floodplain, this makes
excellent farmland. Levees prevent this natural deposits occurring. Also if the
river does break the embankments the water cannot easily drain back into the
river after the flood.
Dams and reservoirs could be built in the upper course of a river to hold back the
floodwater. This could then be slowly released after the rains have stopped.
Dams could ensure the risk of flooding is nearly completed removed. Dams could
also be used to generate hydro-electricity power, which is a cheap and renewable
way to produce electricity. The lakes behind the dam could also be used for
recreational activities such as sailing or canoeing. However there are serious
questions over the use of dams to help prevent flooding. They are extremely
expensive. They would flood large parts of the valley. They will have a large
impact on the natural environment and finally some argue there is the potential
danger from collapse in earthquakes or terrorist attacks.