Evaluate the role of hard and soft engineering strategies in managing rivers
Hard engineering strategies tend to be the more expensive strategies, and can also cause a negative impact on the landscape and environment. Channelization of the river channel removes meanders to reduce the length of the river so the water flows faster downstream. The faster flowing water washes away sediment that has accumulated on the river bed minimising the risk of flooding, as well as the flow of the water deepening the river which means there is an increase in the capacity of the river, also reducing the risk of flooding.
However, this strategy can cause problems further downstream as there is an increase in the speed and discharge, so areas that do not have flood defences are more vulnerable and the risk increases. This is also a very expensive method and a lot of resources and manpower are required. An example of where this particular strategy has been used is the Kissimmee River in Florida, where the river channel was shortened from 103 miles to just 56 miles in length.
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Two thirds of the floodplain was destroyed and due to the river’s increased velocity many species of fish were unable to survive and began to die out, causing problems low down on the food chain which resulted in a negative impact on surrounding ecosystems. Containing some of the discharge of a river via methods such as dams and control structures allows the amount of water flowing downstream to be regulated so that in times of peak flow more water can be held back, preventing the river from bursting its banks.
The river Kissimmee is again an example of where these have been put in place with two water control structures near the beginning and end of the C-38 canal which channels into the Kissimmee River, enabling the amount of water entering the river here to be controlled, preventing floods downstream. However, in order for these to be built huge amounts of land had to be flooded which destroyed many habitats, lessening biodiversity and again causing damage to the ecosystem making then environmentally unsustainable.
As well as this, dams and water control structures are very expensive to build and require a lot of resources and so are not always economically sustainable, which can be seen from the construction of seven dams in Bangladesh, costing approximately $30-40billion and would take forty years to build. They would then need continuous maintain which would also be costly, putting great strain on the government. They can also be seen as socially unsustainable as people can be forced to move off their land if it is to be used to store the water.
Overall, measures such as dams and water control features are effective at holding back the water and regulating flow yet they are expensive and not always sustainable. Embankments are raised banks along the river, enabling it to increase its capacity and deepening the channel, meaning more water can be held in the river before it reaches bank full discharge and floods. 7500 km of flood embankments have been built in Bangladesh since 1947 and have been effective at holding back smaller floods.
However, they are not built to withstand extreme floods such as the floods in Bangladesh in 1998 and so when the embankments collapsed, vast amounts of water flooded into the surrounding area which was exacerbated by the fact that the river had been made to carry more discharge due to deepening of the channel. The embankments then prevented backflow into the river so the water remained trapped and caused more problems. Embankments/levees are easy to build, yet they are very expensive and are usually short term, so cannot always be seen as sustainable.
Soft management strategies can also be used to help prevent flooding. Washlands are areas of the floodplain which are left to flood so as to reduce the chance of flooding downstream. They do not cost anything so are economically sustainable, and they protect the washland ecosystem so are environmentally sustainable. However, they cannot always be seen as socially sustainable as people can be displaced and forced to move off their land in order for it to be used as a washland area.
Land use zoning is another soft strategy which has been used in the Kissimmee river and in the town of Shrewsbury which is built on the inside of a meander of the river Severn. Land has different building controls depending on how far from the river the buildings lie. The land next to the river is unused land and as the distance from the river increases so does the value and land quality, as these are areas less likely to flood.
However, this is not always effective as they can be ignored and do not actually prevent flooding, just help to lessen the impacts. They can also cause problems as people who live in high-risk zones will face a rise in insurance prices. Afforestation is economically sustainable as it is not too expensive, environmentally sustainable as it creates habitats and therefore can help increase biodiversity and socially sustainable as it does not affect anyone.
However, it is not always effective as it takes years for the trees to grow and they only intercept a certain amount of rain so will not prevent extreme floods. Overall, hard engineering strategies such as embankments, water control features, dams and channelization are more effective than soft engineering strategies such as washland areas and land use zoning. However, hard engineering strategies tend to be a lot more expensive and costly to the environment.