UNIT 1C - RIVER MANAGEMENT (Flood risk - Flooding occurs when a river…
UNIT 1C - RIVER MANAGEMENT
Flood management case study: Boscastle, Cornwall
- In August 2004, the village of Boscastle saw a month's worth of rain fall in two hours. The drainage basin of Boscastle is steep and impermeable rock. Boscastle is also located on a confluence of three rivers. These factors led to a flash flood which caused over one thousand homes, cars and businesses to be swept away and damaged.
Tree management - dead trees were removed to prevent them being swept away, causing blockages under bridges. Land owners were encouraged to maintain vegetation and plant new trees.
Raising the car park and using a permeable surface - this allowed cars to be much higher and so they were less likely to be swept away.
Removing low bridges and replacing them with wider bridges - this meant large amounts of water could flow freely underneath the bridge and the bridge wouldn't act like a dam (in the 2004 flood, vegetation and debris became blocked, creating the effect of a dam).
Widening and deepening the river channel - this allowed the river to carry more water.
To prevent this type of flood happening again, the environmental agency invested £10 million into several flood defences, such as:
The flood defence scheme
- Soft engineering does not involve building artificial structures, but takes a more sustainable and natural approach to managing the potential for river flooding. Each approach has its advantages and disadvantages.
- Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings.
Planners have to decide what type of flood to plan for.
Not always possible to change existing land uses.
Less damage is caused, leading to fewer insurance claims.
More expensive buildings and land uses are further away from the river, so have a reduced flood risk.
Flood warnings and preparation
- The environmental agency monitors rivers and issues warnings via newspapers, TV, radio and the internet when they are likely to flood so people can prepare.
They do not stop land from flooding - they just warn people that a flood is likely.
Flash floods may happen too quickly for a warning to be effective.
Some people may not be able to access the warnings.
Many possessions can be saved, resulting in fewer insurance claims.
People have time to protect their properties, eg with sandbags.
- Flooding can cause damage to homes, businesses, infrastructure and communications.Hard engineering involves building artificial structures which try to control rivers. They tend to be more expensive. Each hard engineering strategy has its advantages and disadvantages.
Flood relief channels
- The floodwater flows into the relief channel and is taken either to an area where it can be absorbed, or re-enters the river further down its course.
If water levels continue to rise, the relief channel may also flood.
Expensive to build.
Removes excess water from the river channel to reduce flooding.
- Raising the banks of a river means that it can hold more water.
Water speeds up and can increase flood risk downstream.
Allows for flood water to be contained within the river.
Cheap with a one-off cost
River straightening and dredging
- Straightening the river speeds up the water so high volumes of water can pass through an area quickly. Dredging makes the river deeper so it can hold more water.
Speeding up the river increases flood risk downstream.
Dredging needs to be done frequently.
It can be used to reduce flood risk in built-up areas.
More water can be held in the channel.
Dams and reservoirs
- The dam traps water, which builds up behind it, forming a reservoir. Water can be released in a controlled way.
Settlements are lost leading to the displacement of people. In developing countries locals are not always consulted and have little say in where they are relocated.
Habitats are flooded often leading to rotting vegetation. This releases methane which is a greenhouse gas.
Dams trap sediment which means the reservoir can hold less water.
Reservoirs can attract tourists.
Can be used to produce electricity by passing the water through a turbine within the dam.
- Flooding occurs when a river bursts its banks and overflows onto the surrounding land. There are many factors which can cause a flood - often the natural landscape can influence flooding and also human interactions can increase the risk.
Urban land use - when an area surrounding a river is built on, there is an increase in the amount of tarmac and concrete, which are impermeable surfaces. Drains and sewers take water directly to the river which increases flood risk.
Vegetation - trees and plants absorb water, this is known as interception. Lots of vegetation reduces flood risk. Sometimes people cut down trees (deforestation). This will increase the flood risk, as the water will not be intercepted and flow into the river.
Geology - permeable rocks allow water to pass through pores and cracks, whereas impermeable rocks do not. If a valley is made up of impermeable rocks, there is a higher chance of flooding as there is an increase in surface run-off.
Relief - a steep valley is more likely to flood than a flatter valley because the rainfall will run off into the river more quickly.
Heavy rainfall - if there is heavy rainfall there is less chance of it being soaked up by the soil (infiltration) so it runs off into the river. The faster the water reaches the river, the more likely it will flood.
Prolonged rainfall - if it rains for a long time, the land around a river can become saturated (it's holding as much water or moisture as can be absorbed). If there is more rainfall it cannot be soaked up, so it runs along the surface - this is known as surface run-off.
Causes of flooding