Rivers
- Created by: Hannah
- Created on: 02-04-13 15:07
Erosion
Corrasion- River picks up materail and rubs it against bed and banks. Most effective during times of flood.
Attrition- Boulders collide and break into smaller peices. Over time, rocks become more rounded.
Hydraulic Action- The force of water that hits river banks and pushes its way into cracks. Air becomes comprssed, preasure increases and river bank may collapse.
Solution- The result of chemical composition on the water.
Transportation
Suspention- Very small particles of clay as silt are carried in the water. the higher the velocity, the more paticles it can pick up.
Solution- The acids in the water can dissolve bedrock.
Slaltation- Pepples, sand and gravel are lifted up by the current and bounce along the bed.
Traction- Larger boulders roll along the river bed as they are too heavy to be liften. Happens in times of great flood.
Deposition
Sediment may be deposited because of low rainfall, shallow water, river entering sea/lake, increased load and rivers overflowing its banks, therefore depositing material on the flood plain.
Landforms
V-Shaped Valleys and Interlocking Spurs
- In the upper part of a river, vertical erosion is common creating a steep sided river valley.
- Interlocking spurs form as the river has to take a winding course due to the highland areas that jut out.
Waterfalls
- Waterfalls form as a result of a river that has flowed over hard rock and meets a band of soft rock.
- The velocity increases as the water nears the edge of the fall because of a decrease in friction as the water is no longer in contact with the river bed
- the soft rock is worn away and over time, the hard rock is undercut and may collapse.
- Plunge pools are common at the base of waterfalls.
Landforms
Rapids
- If the gradient of a river increases quickly or flows over gently dipping areas of harder rock rapids will result.
Flood Plains
- This land more likely to experience flooding.
- If a river floods, silt is deposited on the land increasing its fertility and the height of the flood plain increases.
- The flood plain can be made wider by the lateral erosion of meanders.
- The edge of a flood plain is marked by a clear slope called a bluff line.
Braiding
- At some times of the year, due to snowmelt, rivers may carry a very highload in comparison to their velocity and the channel can therefore become full of sediment.
Landforms
Levees
- If a river floods, material is deposited on the banks. The material that is dropped first is the coarsest and creates a natural embankment.
- When the amount of water in the river is low deposition takes place, the bed of the river rises and flooding is likely.
- in some countries, artificial levees are put in place to reduce the flood risk
Deltas
- These are made of fine sediment and occur when a river has a decrease in energy as it enters the lake or the sea.
Pools and Riffles
- These occur due to both erosion and deposition. Pools are areas of deeper water and riffles are areas of shallow water. A pool has more erosion than a riffle and is caused by turbulance.
Landforms
Meanders
- Meanders form where alternating pools and riffles develop at equally spaced intervals.
- The river has more erosive power and greater energy in the deep pools.
- Energy is lost as the river flows over a riffle because of friction.
- Turbulence increases in and around pools as the water speeds up, causing the flow of water to begin to twist and coil.
- This causes corkscrew-like currents in the river called helicoidal flow, which spiral from bank to bank between pools.
- The helicoidal flow causes more erosion and deepening of the pools. It also causes eroded material to be deposited on the inside of the next bend, where the river loses energy.
- The combination of erosion and deposition exaggerates the bends until large meanders are formed.
Landforms
Oxbow Lakes
- Oxbow lakes are formed when the neck of the loop of a meander is broken through, often during flooding.
- Deposition dams off the loop, leaving an oxbow lake.
The Graded River and Base Level
Changes in Base Level
Base Level: The lowest point to which erosion by running water can occur. For rivers, the base level (limit) is usually the sea.
Causes of a Change in Base Level
Climatic Change- Glaciations and changes in rainfall.
Tectonic- Where land is uplifted after plate movement or volcanic activity. Changes can be positive (sea level rises in relation to the land) or negative (sea level fall in relation to the land).
Rejuvenation
If land emerges from the sea following a negative change in base level, the potential energy of a river for erosion is rivived and a re-grading of the river can occur.
The Graded River and Base Level
River Terraces
River terraces are the remains of former flood plains that lie far above the extent of present day flooding as a result of vertical erosion caused by rejuvenation.
Incised Meanders
If land is uplifted for a considerable period of time, a river may cut downwards anf form incised meanders. The 2 types of incised meanders are:
Entrenched meanders- Symmetrical cross section, and occur if valley sides are resistant to erosion, or rapid incision by the river.
Ingrown meanders- Less rapid uplift of land, allowing river to shift laterally across floodplain.
River Flooding
Causes of Flooding:
- Basin size
- Precipitation
- Temperature
- Land use
- Geology
- Soil type
- Drainage density
- Tides
- Urbanisation
River Flooding- Case Study
River Ouse Flood, Yorkshire, 1982
Causes:
- Initial snowfall from deep depressions
- Subsequent warmer temperatures, heavy rainfall and rapid snow melt
- Smaller rivers unable to cope with amount of water
- Infiltration capacities exceeded resulted in much overland flow
- Increased overland flow in urbanised areas
Effects:
- Destuction of communications and buildings
- Loss of life and property and destruction of crops
- Drainage problems
- Spread of disease and impacts on the economy
River Flooding- Case Study
Impacts
- 800 properties were flooded
- 19,000 hectares of agricultural land under water
- Military assistance was needed to distribute food and protect property in York, Selby and Boroughbridge
- Commercial properties were flooded with water to a depth of about 1 metre.
Managment
Regulate Land Use
Aim: Promote wise use of land, to reduce flooding and limit building
Advantages: Cheap, immediate effect, long term, provide for most suitable use of land
Disadvantages: Doesn't prevent floods, not always applied well or to existing land uses
River Flooding- Case Study
Dams and Reservoirs
Aim: Protect existing land uses, limit flooding, promote recreation
Advantages: Protects existing land uses, controls water flow, allows recreation
Disadvantages: Costly, lengthy construction, maintenance needed, wildlife destoyed, communities disrupted, sedimentation
Land Treatment
Aim: Promote soil and water conservation, reduce flood levels
Advantages: Cheap, attacks flood problem at source
Disadvantages: Not always relevant, limited effect
River Flooding- Case Study
Acquire Flood Plain Land for Public Use
Aim: Reduce losses from floods
Advantages: Permanent, numerous benefits
Disadvantages: Costly, sites not always suitable for other uses, land shortage may result
Insure Against Floods
Aim: Promote flood regulations
Advantages: Spreads cost of flood losses
Disadvantages: Can be costly
River Flooding- Case Study
Install Warning Systems
Aim: Warn property owners of threats, allow for evacuation
Advantages: Allows people to adjust to the flood threat
Disadvantes: Requires education, systems must be carefully maintained
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