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Evaluate how well humans control river flooding (1300 Word Limit)
Throughout history humans have tried to control the effects of river flooding by using
many different strategies. Flood walls, Embankments and Levées are all structural methods
which have been used around the world to protect cities surrounding rivers. There also been
general adjustments to the river channel this was done to the Mississipi river to reduce the
effects of flooding. Scientists have also tried to predict when a river is going to flood and warn
the people so that they can decrease the effects of the flooding.
Flood walls are designed to increase the height of the channel to stop water spilling out
onto the floodplain. The restrict access to the riverside and offer little in the way of floodwater
storage capacity. Embankments are often made of earth with rubble rill and are more common
outside the town centre where the land is cheaper. Levées can be produced artificially as well
as naturally. They are introduced to raise the level of the river banks so that flood water
cannot overflow the river. These three methods seem faultless however they just speed up the
water downstream to create problems else where.
Channel improvements are another way to manage the affects of flooding. They
attempt to restrict floods either by creating a smoother channel for faster flow to get the water
out of the area as quickly as possible or by deepening/widening the channel. By creating a
smoother channel it's achieved by lining the channel with concrete so that the river is given a
new route to flow. Widening/deepening the channel is done by dredging to increase the
capacity. However, these two techniques need to be watched carefully as the channel with
naturally revert back to its old route due to erosion and deposition.
Another way to reduce the influence of flooding is by using relief channels. These are
channels which are constructed to redirect excess water upstream of a settlement via and
alternative route. Water is able to reenter the main channel further downstream, thereby
reducing the affects of flooding for that particular area. The bypass can only be activated at
times of high discharge so the new route might stay dry until a flood occurs.
A flood storage reservoir aims to store excess water in the upper reaches of the
catchment but they're very expensive and require a large amount of land to construct hence
why they are often found away from settlements, on cheaper land.
Flood abatements measures aim to reduce the possibility of flooding by managing the
land use upstream. This can include different techniques for example, afforestation and
reducing the amount of bare earth to avoid excess runoff problems. Afforestation increases
interception storage and evapotranspiration and help to reduce runoff as well as holding the
soil together to reduce silting up of river channels. The stand out problem is that trees
obviously take many years to grow. Furthermore initial soil distrurbance during planting
generates more run off at first. Therefore predictions and warnings must be used so that
afforestation can occur when no floods are forecasted.
This follows onto my next point of flood prediction and warning. In the UK, the
Environment Agency are constantly monitoring the rivers. Records of river discharge and past
floods are kept to help predict future flood events. Weather radars are used to collect data to
aid flood forecasting, they identify likely and actual rainfall in weather systems across the UK.
However, this alone is not sufficient and so must be aided along side a river gauge. These
measure the amount of rain that has actually fallen at specific locations, which record the
rising level of the rivers at different locations. So, when a river has risen to the top of the
channel, a weather radar will be used to predict future rainfall and then a flood prediction
software will be used to model the likely outcomes. Warnings can then be issued out to areas
which are likely to be affected.
In Carlisle flood defences have been put in place to reduce the effects of flooding.
They had three main ideas which they considered. Doing a minimum this compromises only
maintenance work to ensure no further deterioration in existing structure, thereby sustaining
the present level of flood defence. The main advantage to this is that it clearly costs nothing
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and it assumes that increased flooding would have a large impact on surrounding houses,
buildings and all types of land use if built on the flood plain. Secondly they thought about
upstream storage they would have created reservoirs and a dam. However, if built a dam it
would require the M6 motorway to have a diversion during the time of its construction.…read more