Types of Erosion
Gravity- river attempting to cut down to the base level (slumping)
In times of spute (high water flow) there is abrasion and hydraulic action
Middle - Lower Course
River lies closer to base level. Lots of energy when bankfall (just before flooding)
Traction: Larger stones/ boulders rolled along valley bottom.
only really at times of high Q. Bocastle 2004- dislodged boulders upstream causes additonal issues downstream.
Saltation: Smaller sediment can be bounced along the river bed.
Relatively high energy conditions needed. Generally one landing particle can help dislodge the next.
Suspension: Fine particles are carried by the flow of the river.
Carried by turbulence within the water. Normally the largest proportion of the load. Causes the dark brown colours of many rivers.
Solution: Dissolving oof soluble rock.
- Steep V shape downward erosion.
- Features: Rapids and Waterfalls
- Predominantly Erosion
- What is formed?: V shaped Valley, Waterfalls and Interlocking Spurs
- U shaped lateral erosion
- Features: Meanders
- What is formed?: River Cliffs, Meander, River Beach, (Slip Off Slope)
- Open U shaped (wide) lateral erosion and depositon
- Features:Oxbow Lakes, Floodplains, Levees, Deltas
- Lots of depostion
- What is formed?: Estuaries, Oxbow Lakes, Levees, Flood Plain
V- Shaped Valley
- The river erodes vertically at a greater rate than it does horizontally deepening the valley.
- The river continues to erode vertically, the river banks become less stable and after a period of heavy rain, due to gravity they collapse.
- This creates a V-Shaped valley.
- The process is repeated, deepening the valley further.
- As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs).
- As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of hight land creating interlocking spurs.
- Many waterfalls form when river meet a band of softer rock after flowing over a relatively hard rock. the softer rock is worn away more quickly, and the hard rock undercut.
- The overhead harder rock forms an overhang which will eventually collapse for form a deep plunge pool.
- This process is repeated causing the waterfall to retreat creating a gorge in its wake.
- Within sections of the river channel, the fow tends to wind from side to side throught a pattern of deep pools and shallower riffles. Riffles are formed by bed load deposits.
- Ameander forms when the river channel bends, most of the water is directed to the outside of the bend. This reduces friction and increases the speed of the river at this point. The river therefore has more energy to transport through suspension, which will lead to erosion of the outside bank by abrasion. The bank will be undercut, collapse and retreat to leave a small river cliff.
- Meanwhile, there is less water on the inside of the bend, there is an increase in frition and a decrease in velocity. As the river loses energy, it deposits some of its load to form a slip off slope.
- Following on from the development of a meander.
- Continued erosion (abrasion) on the outside of the bend results in the neck of the meander getting narower until, usually at a time of flood, the river cuts across.
- The fastet current will now be flowing in the centre of the channel and depostion is now next to the banks.
- The original meanders is blocked off to leave a crescent shaped oxbow lake.
Flood Plains and Levees
Natural Levee~ a deposit of sand or mud built up along, and sloping away from, either the side of the flood plain of a river or stream.
- In the lower course, the river widens its valley throgh lateral erosion. At times of high discharge the river river has considerable amounts of energy, which it uses to transport material through suspension.
- When the river overflows its banks, it will spread out over the surrounding area which is flat. This sudden increase in friction will reduce the velocity of the river causing it to deposit its load. (silt). Each time the river floods, another layer of silt is added creating a flat floodplain.
- The coarsest material will be dropped first, and this can form a natural embankment, called a levee nexxt to the river. Levees can help the prevent further flooding.
Deltas are found at the mouth of large rivers- for example, The Mississippi. A delta is formed when a river deposits its material fater than the sea can remove it.
Deltas only form under certain conditions:
- the river must be tranposting a large amount of sediment.
- the sea must have a small tidal range and weak currents.
- the sea must be shallow at the river mouth.
Causes of Flooding
Intense Rainfall- reduces the effectiveness of infiltraion
A Long Period of Rain- causes the ground to become saturated
Snow Melt- during spring time in ice-capped regions melts and adds water to rivers
Impermable Rock- water cannot pass through rock and therefore moves as surface run-off quickly into the channels
Deforestation- removal of trees causes less interception and water is therefore added to the drainage basin system
Urbanisation- man made surfaces and materials are often impermeable
Global Climate Cange- increasing global temperatures will encourage polar and high altitude ice to melt
The Effects of Flooding
On the people:
- loss of belongings
- damage to property
- disruption to tranposrt
- diseases and illnesses
- insurance claims
- comtamination of water supply
- crops and animals lost
On the environment:
- soil contampination by sewage
- vegetation destroyed
- animals drowned
- loss of wildlife habitats
- soil erosion
Effects of Flooding- Mini Examples
Northen England and The MIdlands
2007, 13 killed, £2b damage
Lincolnshire 40% pea crop damaged
N. Wales 30 tonnes debris blocked the roads
Feb 2007 - 121,000 homeless
12,000 livestock and 16,000 chicken lost
Diarrhoea killed dozens
Prediction and Prevention of Flooding