The flooding of rivers is a natural and essential part of river basin processes and is only a hazard because they can affect people’s lives.
Flooding occur when a large volume of water enters a river system quickly and cannot be contained within the river channel.
By analysing a river basin carefully we can learn and predict how likely it is to flood. This information can be used to reduce the damage caused by such events, or perhaps even to modify future events themselves.
Engineering structures impose the most response and cause to flood hazards.
The term ‘flood protection’ is a misleading term, as protection is only as good as the design of the protection scheme itself. Once a flood protection scheme is breached the damage is likely to be greater, causing damage to buildings and surroundings, which can be very expensive and harmful.
River management schemes concentrate widely on reducing flood losses, rather than preventing flooding altogether.
Embankments, Levees, and floodwalls.
A levee is a barrier that holds water away from the surrounding buildings and land. It can protect your home from flooding as well as from the force of water from rivers.
A properly built levee takes a lot of space. Its sheer mass keeps it from moving. Long, gentle side slopes increase stability and provide for easier maintenance. Levees must be built to withstand the forces of standing water and flowing water.
The main function of a levee is to restrict flooding to defined limits and to allow easier control of flooding in certain areas. Levees are relatively cheap to build, however take a lot of planning and time to construct.
The construction of levees may however sometimes be said to interfere with the course of the river, because it means that the river channels narrows down and there is more pressure within the river channel if there is a high discharge of water within the river channel.
Channel improvements are structured to confine floodwaters to the river channel in one of two main ways:
* Channel roughness can be reduced by clearing vegetation and other obstructions in the course of the river channel. This creates a lining of the channel with a smooth surface such as concrete, which reduces friction and allows discharge to pass as a more easier and quicker rate. This type of scheme can help the flow of the discharge however it may intervene with the channel discharge downstream.
* The channel can be widened or deepened by dredging. This increases the capacity of the channel.
The disadvantages of these schemes are that maintenance for them is often ended as the river tries to revert back to its natural course.
Channels can be shortened by cutting meander loops and increasing the gradient to discharge water away from the area at risk, much quicker.
However a faster discharge may increase the flooding of the river downstream.
Flood Storage Reservoirs
These store excess water in storage reservoirs, which may appeal to some as lakes. Because of the high costs that are involved in constructing reservoirs, they have not been designed in the UK for sole purpose of flooding alleviation, though some have contributed to flood control schemes.
Flood interception schemes
These involve changing the course of the river channel, this can be done by changing the route of the river channel, which helps by-pass settlements under threat. New channels may also be created which can help store water along with the existing river channels.
Flood embankments help to contain floodwater well away from places under threat.
The Keswick area has undergone many improvements and has been exposed to many flood schemes, which are trying to reduce the risks of floods through the river channel.
Engineers considered the town of Thirlmere and is was found to give some protection against floods. However it would not be possible to rely upon these schemes entirely because it only controls one tributary in the Great basin.
It was analysed that increasing floodplain storage capacity upstream from Keswick would reduce downstream flood flows. This could be achieved by constructing a flood control weir that would raise water levels upstream during flood conditions, or by constructing embankments around the edge of the floodplain, which would create a storage reservoir to hold water for a longer period of time.
The following five schemes were investigated:
1. The construction of floodwalls and embankments at Crosthwaite, and in vicinity of Great Bridge.
2. Removal of the weir at Great Bridge and local re-grading.
3. Re-grading the channel for approximately 400m upstream.
4. Widening of the channel for approximately 350m upstream.
5. Both re-grading and widening of the channel.
The scheme was used as a base with which to compare other schemes. It is probably unrealistic as it is likely to result in peak river levels upstream of great bridge up to 450mm higher than those during the flood of December 1985.
Floodwalls and embankments would be built out of stone with reinforced sheet pilling at certain locations. They would be designed to contain floods to a level predicted to occur only once in 100 years.