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  • Created on: 18-05-13 14:26

River processes

Transportation -

- Traction - large rocks are rolled along the river bed

- Saltation - Smaller rocks are bounced along the river floor

- Suspension - fine material (sand) carried on the water

- Solution - dissolved in the water

  Erosion -

- Attrition - rocks smash into each other (washing machine effect)

- Abrasion - rocks rub against the bank and its worn away ( sand papering effect)

-Corrosion - rocks dissolved by acids in the water

- Hydraulic action - weight of the water against the bank

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Meanders and Ox-bow lakes

Meanders are bends in the river, they usually occur on floodplains where the river has room to develop bends.

The inside of a meander bend has shallower water and material is deposited there forming a slip-off slope. The slowest flow is on the inside.

On the outside of the meander bend, erosion occurs. The water is deep and the fastest flo is there. A river cliff forms when the bank is undercut by erosion.

Erosion on the outside and deposition on the inside cause a meander bend to change over time. The fastest flow will now be straight down the river and deposition cuts the meander off from the river forming an ox-bow lake (the river cuts through its neck).

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drainage basin

Drainage basin - the area of land drained by a river and its tributaries.

Water shed - boundary separating two river basins

Source - The point at which a river starts

Tributary - a small river that joins a bigger one

Confluence - place where a tributary joins the main river

Mouth - the end of the river

Channel - the course that the river flows.

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3 Courses of the river

upper course - steep-sided, v-shaped valley, interlocking spurs, tributaries, vertical erosion

Middle course - lateral erosion, meanders eroding the sides of the valley

Lower course - large floodplains, lateral erosion, raised river bed, alluvial deposits, levees.

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Waterfalls and gorges

Waterfalls form when there is a difference in underlying rock. the softer rock is worn away more quickly and the harder rock is undercut. The overhanging harder rock will become unsupported and collapse. The rock will fall into the irver and be swirled around forming a plunge pool. Spray action erodes the softer rock and the process repeats many times. Each time the waterfall retreats upstream creating a gorge.

A gorge is a valley created by the waterfall retreating upstream.

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Boscastle flood 16th August 2004

Why it flooded - River valency - short only 10km, high stream density, small,steep sided drainage basin, impermeable rock (slate), recieved 200mm of rain in 4 hours.

effects- carried 80 cars to sea, 37 buildings badly damaged, 5 buildings completely collapsed, huge amounts of sediment deposited throughout the town.

stopping future flooding at Boscastle  - widening and deepening river channel, straighten river, bridges have wider spans, raise carparks above river level.

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long term and short term effects of floods & Preve

Short term - sediment deposited, cars and buildings destroyed, transport interrupted,

Long term - insurance prices increase

Soft engineering - gravel placed on river bed in upper course to make it flood before it gets to town, afforestation to increase interception, trash screens, pre-clearance of trees, demountable flood barriers

Hard engineering - building dams, straightening meanders, concrete enbankments, widening and deepening river channel,

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Flood Hydrographs

Predict risk of floods.

It shows discharge over a period of time. It shows the baseflow and the surface runoff.

Lag time is the time from peak rainfall until peak discharge.

The shape of the hydrograph is affected by steepness, size, rock or soil type, vegetation and stream density.

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