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Rivers: Flood management strategies

What techniques are used to help prevent flooding? Hard engineering, soft engineering - discover more about flood management here.

Introduction

The best methods to prevent and reduce the risk and impact of flooding are widely debated. The two approaches are to work against nature – hard engineering, or to work with nature – soft engineering. Soft engineering tends to focus on lessening the effects of a flood and hard engineering is very much concerned with prevention. A successfully managed drainage basin might use both hard and soft engineering.

Hard engineering techniques

Dams – these are built across the river channel to stop the flow of the water. A lake or reservoir will form behind the dam and water can be let out in a controlled manner to prevent flooding. One of the disadvantages of a dam is that a large area of land has to be flooded and this can destroy natural habitat or even mean humans have to be relocated. Dams are expensive to build and maintain although they can be used to produce HEP. Dams do provide a high level of control to reduce the chances of flooding and are very effective in reducing the risk in this way however, they stop sediment from flowing downstream and this can lead to greater erosion in a similar way that holding back material on the coast by using groynes reduces protection further along the cliff. A good example of a dual purpose dam is the Kariba on the Zambezi in Mozambique .

Levees (reinforcing or man-made) – a levee is a naturally occurring feature on the bank of a river in the middle and sometimes lower stages of a river, on the flood plain. When the river floods, any load it is carrying in suspension is dropped and the heavier material is dropped first, just on the river bank. Over time this will build up and has the effect of increasing the capacity of the river as the banks are higher. These levees may be enforced in some way by humans. Planting vegetation on them helps to protect them. Adding even more height to them is also effective. An artificial levee can be built from scratch and this has the same effect as a natural one. Levees are very common in Holland where much of the country (more than 25%) is below sea level and is at risk from flooding. The Dutch have a complex network of wing dykes (see below) and levees to protect the land.

Channel straightening – getting the water out of an area at risk of flooding as quickly as possible is a way to reduce likelihood of problems during times of peak discharge. Meanders may be cut through and the channel is literally straightened so that water can move very quickly. This takes the water away from built up area for example where water can cause havoc to houses and businesses. Straightening has been one of many management techniques used along the Mississippi in the USA. This of course means that the water reaches further downstream more quickly too. Straightening often just diverts the problem elsewhere rather than providing a solution.

Wing Dyke – these work in a similar way to groynes on a beach in that they trap sediment moving through the river channel. They are usually placed in pairs either side of the channel and once sediment has built up behind them water is forced between them more quickly. Some good examples are on the Missouri river in the USA. As with channel straightening, they mean that water reaches downstream more quickly so careful planning is needed when they are installed to lessen the impact of increased discharge further along the river’s path.

Diversion Spillway – these divert water away from the main channel and either route it to another river or the same river lower down. They may also route water to a lake which is what the Bonnet Carre Spillway does by diverting water from the Mississippi to Lake Ponchartrain. These are effective at preventing flooding in the immediate area but the place where water re-enters the channel is at risk because of increased discharge.

Soft engineering techniques

Floodplain Zoning – common sense would dictate that a sensible approach to stop flooding causing damage is to build away from the river and off the floodplains. Whilst this may be too late for some areas that have been safe from flooding for years in seemingly low risk areas (increased urbanisation has brought these areas into the risk zone) it is possible to control the development of flood plains. This does two things. Firstly there may be no buildings or infrastructure at risk from flooding if it is simply not there and secondly, by modelling the effect of increased impermeable areas on the land, the impact of proposed development can be assessed and plans made to minimise risk and impact. Following the Lynmouth floods of 1952 (Devon) floodplain zoning was part of a series of measures to protect the area and it has proved successful. Restrictions were placed on the building and development in areas close to the river channel.

Wetland Restoration – this method can be summed up as “returning an area to nature” although this is a little simplistic. In real terms this is a lengthy process that tries to work in harmony with the environment and restore the natural habitat of an area. By increasing the vegetation cover, soil depth and reducing the percentage of impermeable surface an area can cope with more water. This requires a long term approach and is difficult to manage because of this. This works most effectively when the outcome of the restoration is linked to other things in addition to reducing flood risk such as nature conservation. For example, in Cornwall there is a joint project between Natural England, the Environment Agency and Cornwall County Council to restore the Camel Valley Wetland, an important area for wetland birds and an area at risk from flooding.

River Restoration – this is similar in principle to wetland restoration in that the main principle is to restore an area back to its natural state which can benefit the local environment but also help with flood risk. In Gelderland, the Netherlands, the Groenlose Singe is an area that has had a lot of hard engineering in place. This has caused so many problems further downstream that the authorities are employing river restoration techniques.

Afforestation – This may be part of wetland and river restoration when vegetation may be planted to return an area to its original form. Large scale afforestation can not only lower flood risk by intercepting and storing water but it can reduce the erosion of soil which ends up in the river channel. Material in the river channel effectively decreases its depth and the river level is higher, increasing flood risk during times of high discharge. Afforestation is widely used in Australia to achieve several things which includes helping to manage water flow in a catchment area (drainage basin).