Realignment/Diversion. This is where a meandering river has a new straighter channel cut, to provide a shorter and alternative route, often this new channel is concrete lined to aid flow and reduce flooding. Realignment also steepens the gradient of the river and aids navigation for boats and water-borne craft.
Levees. Used to hold and contain water, so that the floodplains can, be developed.
(A side view of a levee – Picture obtained from http://www.publicaffairs.water.ca.gov/newsreleases/2006/03-22-06levee.jpg)
Weirs. These are built to slow the velocity of water down in order to stop erosion of the river banks. They can also be used as a bridge in order to cross the river itself.
The Weir pictured below is on the River Cole.
Soft engineering Techniques
Revetments. Where the riverbank needs strengthening, revetments made from willow bed mattresses and other natural materials
Lowering the floodplains. This is done to allow for storage should the river flood. It creates space into which the river can overflow, without damaging any infrastructure.
Removing diversions/realignment. Where a river has been, diverted to a straighter course, it often causes flooding downstream and/or damages habitat. To reverse this, the old meander is re-opened or a new one is constructed, to allow the river to flow back on its original route. By causing a river to meander, it slows down the velocity and speed of the river.
Forestation.Trees are planted near to the river. By doing this, it means improved interception of rainwater and lower river discharge. This is a relatively low cost option, which enhances the environmental quality of the drainage basin.
Planning. Local authorities and the national government introduce policies to control urban development close to or on the floodplain. This reduces the chance of flooding and the risk of damage to property.
There can be resistance to development restrictions in areas where there is a shortage of housing. Enforcing planning regulations and controls may be harder in LEDCs.
Different interest groups have different views about flood management techniques:
- Governments and developers often favour large hard engineering options, such as dam building. Building a dam and a reservoir can generate income. Profits can be made from generating electric or leisure revenue.
- Environmental groups and local residents often prefer softer options, such as planting trees. This causes little damage to the environment and does not involve the resettlement of communities.
An example of river restoration is the River Skerne
The River Skerne is in Darlington in the north – east of the UK. It flows through urban parkland surrounded by housing estates as well as industrial estates. Over the past 200 years, it has undergone straightening and channelisation procedures to combat flood control and improve drainage. Much of the rivers floodplain had been, raised high above the river – this was due to industrial waste tipping.
Before the river was changed, it was a slow meandering river with huge floodplains, with a diverse habitat. However as industry started to move, in on the river, its course was changed, to prevent it from flooding, and its floodplains were raised.
In an effort to return the river to its natural course and state of habitat, the Environment Agency teamed up with the River Restoration Centre, to return the river back to its original state.
Many parts of the river had been channelised – this was to quickly and efficiently carry silt produced by the near by industrial estate away from the immediate area. The RRC (River Restoration Centre) where possible, re-instated the river’s meanders, and where this was not possible due to space constraints, they altered the bed of the river by inserting a riffle or in-stream flow deflector, this is a shale or shingle, or rock mound that is put onto the river bed to deflect the water around it, creating a small meander within a straight channel.
Sewers and gas mains which used to run parallel to the river in full view were re-routed below the river bed, and where they had to cross the river fish friendly pipes were used so that migrating fish could still pass over them.
Backwaters were also engineered into the restored river. These are important for a diverse habitat, and in times of high water provide a calm pool of water for young fish and insect larvae.
Along the meanders, willow cushions were used to stop the river from eroding the sides of the bank. Where this was not possible, trees were planted in close proximity to the bank, so that their roots would hold the bank together, not only were they used for this purpose but to catch surface rainfall and run-off, therefore slowing down the time it takes for water to enter the river, and reducing the risk of the river flooding.
The wider benefits of river restoration using soft engineering to restore the river back to its original state are:
- Better management of floods, droughts and water quality
- Increasing the public awareness of past river management and encouraging the use of softer techniques
- A huge increase in the benefit to the environment.
