River landscapes in the UK - River management Flashcards
Flood risk factors
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.
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.
Relief -
a steep valley is more likely to flood than a flatter valley because the rainfall will run off into the river more quickly.
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.
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.
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.
A hydrograph
shows how a river responds to a period of rainfall.
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
Dams and reservoirs
Hard engineering
A man-made reservoir
The dam traps water, which builds up behind it, forming a reservoir. Water can be released in a controlled way.
Advantages
Can be used to produce electricity by passing the water through a turbine within the dam.
Reservoirs can attract tourists.
Disadvantages
Very expensive.
Dams trap sediment which means the reservoir can hold less water.
Habitats are flooded often leading to rotting vegetation. This releases methane which is a greenhouse gas.
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.
River straightening and dredging
Hard engineering
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.
Advantages
More water can be held in the channel.
It can be used to reduce flood risk in built-up areas.
Disadvantages
Dredging needs to be done frequently.
Speeding up the river increases flood risk downstream.
Embankments
Hard engineering
Raising the banks of a river means that it can hold more water.
Advantages
Cheap with a one-off cost
Allows for flood water to be contained within the river.
Disadvantages
Looks unnatural.
Water speeds up and can increase flood risk downstream.
Flood relief channels
Hard engineering
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.
Advantages
Removes excess water from the river channel to reduce flooding.
Disadvantages
Expensive to build.
If water levels continue to rise, the relief channel may also flood.
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.
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.
The severe flood warning icon is a red house flooded by 3 layers of water.
Advantages
People have time to protect their properties, eg with sandbags.
Many possessions can be saved, resulting in fewer insurance claims.
Disadvantages
Some people may not be able to access the warnings.
Flash floods may happen too quickly for a warning to be effective.
They do not stop land from flooding - they just warn people that a flood is likely.
Floodplain zoning
Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings.
Advantages
More expensive buildings and land uses are further away from the river, so have a reduced flood risk.
Less damage is caused, leading to fewer insurance claims.
Disadvantages
Not always possible to change existing land uses.
Planners have to decide what type of flood to plan for.
