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rivers (River management (soft engineering (floodplain zoning
Allowing…
rivers
River management
Flood risk
causes of flooding
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.
hard engineering
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
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soft engineering
floodplain zoning
Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings.
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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.
types of erosion
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attrition
When rocks that the river is carrying knock against each other. They break apart to become smaller and more rounded.
hydraulic action
This is the sheer power of the water as it smashes against the river banks. Air becomes trapped in the cracks of the river bank and bed, and causes the rock to break apart.
soloution
When the water dissolves certain types of rocks, eg limestone.
River processes
drainage basin
A drainage basin is the area of land around the river that is drained by the river and its tributaries.
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Transportation
Saltation - pebbles are bounced along the river bed, most commonly near the source.
Suspension - lighter sediment is suspended (carried) within the water, most commonly near the mouth of the rive
Traction - large, heavy pebbles are rolled along the river bed. This is most common near the source of a river, as here the load is larger.
Solution - the transport of dissolved chemicals. This varies along the river depending on the presence of soluble rocks.
deposition
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at the end of the river's journey, at the river's mouth
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Landforms
The process of erosion can create different land forms. The erosional features are often found in the upper course of the river.
lower course
estuary
Estuaries
An estuary is where the river meets the sea. The river here is tidal and when the sea retreats the volume of the water in the estuary is less reduced. When there is less water, the river deposits silt to form mudflats which are an important habitat for wildlife.
middle course
meanders
As the river makes its way to the middle course, it gains more water and therefore more energy. Lateral erosion starts to widen the river. When the river flows over flatter land they develop large bends called meanders.
As a river goes around a bend, most of the water is pushed
towards the outside. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion).
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Water on the inner bend is slower, causing the water to slow down and deposit the eroded material,
creating a gentle slope of sand and shingle.
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flood plains
A floodplain is an area of land which is covered in water when a river bursts its banks.
Floodplains form due to both erosion and deposition. Erosion removes any interlocking spurs, creating a wide, flat area on either side of the river.
levees
Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.
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When the river floods, the sediment spreads out across the floodplain.
When a flood occurs, the river loses energy.
The largest material is deposited first on the sides of the river banks and smaller material further away.
After many floods, the sediment builds up to increase the height of the river banks
, meaning that the channel can carry more water (a greater discharge) and flooding is less likely to occur in the future.
Upper course
waterfalls
The soft rock is eroded quicker than the hard rock and this creates a step.
As erosion continues, the hard rock is undercut forming an overhang
Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses.
This process continues and the waterfall retreats upstream.
A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of hard rock positioned over exposed, soft rock.
gorges
The cause of a waterfall retreating upstream and causing a gorge.
A steep-sided valley is left where the waterfall once was. This is called a gorge.
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