Rivers, floods and management

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  • Created on: 08-01-13 19:42

Valley profiles.

Long profile: shows changes in a rivers altitude of the course, normaly smoothly concave gradient steeper towards upper course, irregularities occur as rapids, waterfalls or lakes.
Cross profile: view of a valley from one side to another. e.g  upland area has v shaped valley.
Graded profile: An even and and progressive decrease i ngradient down the vally creating the smooth concase shave.when a balance is achieved between erosion and desposition.
Potential and Kinetic energy: Potential (stored) energy is fixed by the altitude of the source of the stream in relation to base level.
Kinetic energy: energy due to movement, generated by flow of the river covernting potential energy into kinetic energy, determined by discharge, channel gradient, and average volocity.

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Changing channel characteristics.

Cross Profiles: view of a river bed and banks form one side to the other, morphing along the way.
Upper course: channel is narrow and uneven, due to deepisition of boldiers. features a large wetted perimeter
Middle course: channe; becomes asymmetrical on bends but mainly smooth and symentrical.
Lower course: river deepens and widens, banks of deposition and eyots (islands of desposition) disrupt shape of the channel leading to a braided channel. and sometimes Levees.
Shape effects velocity of river.
Wetted perimeter: total length of the river bed and banks in cross section that are in contact with the water in the channel.
Hydraulic radis=Cross sectional area of the channel/wetter perimeter.

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Landforms of fluvial erosion and deposition

Potholes: holes in river bed created by pebbels in turbulent high-velocity water. pebbels are trapped in hollows, vertical eddies allow sediment to grind a hole into the rock by abrasion (corrasion). pebbles in the hole are rounded and smoothed by attrition.

Waterfalls and rapids: occur at a sudden change in gradient, waterfalls result of resistant bad of rock across te course of a river, edge of a plateau, rejuvenatiof of the area (more erosion power). Weak rock (eg limestone) excavated quicker than resistant rock, hydrualic action and abraison deepen plunge pool undercutting the resistant rock creating a cap which eventualy collapses as the waterfall retreats upstream forming a gorge.

Braided Channels: river splits into channels seperated by islands of deposition. (e.g river Nile) feature of rivers supplied with lots of sand and gravel. banks formed of sand are unstable and erode easily, (e.g Bangladesh) can also occur in glacial streams and semi-arid areas of low relief in the path of mountain rivers

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Landforms of fluvial erosion and deposition

Meanders: bends in the river, shallow sections called riffles deeper sections called pools. undercutting occurs on outside bank and deposition on the inside convex bank. inner bank is called a point bar gently sloping deposit of sand and gravel.
helicoidal flow describes corckscrew motion of water from one side of bend to other, fastest point just under surface in middle.

Oxbow lakes: feature of both erosion and deposition, horseshoe shaped lake of stagnant water gradually silting up over time to become a meander scar. erosion greatest on outer bank, meander neck becomes progressively narrower, cutting through during times of high discharge.

Levees: middle and lower course, created overtime naturally by deposition during high velocity (flood) competence of the river reduces quickly causing load to be dropped forming raised banks and channel sediment build up (higher level than floodplain). artificially strenghtened sometimes e.g Mississippi river strenghening began 1699 and was 3,200 km long by 1990.

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Landforms of fluvial erosion and deposition.

Floodplains: Created by erosion and deposition, mostly deposition, large flat areas of land on valley floor either ide of river, composed of alluvium and width determined by meander migration and lateral erosion.

Deltas: feature of deposition, located at mouth of river. occurs as velocity and capacity drop as a river enters the sea, dumping bedload and suspended material.
"when the rate of deposition exceeds the rate of sediment removal."
deltas only form when:
- Sediment of a river is very large (e.g Mississippi and Nile.)
- coastal area has small tidal range and weak currents( little transportation of deposition away from river mouth by tides. (Gulf of Mexico and Mediterranean Sea.)
Most common type of Delta is
- arcuate delta (Nile delta), has a curving horeline and a dendritic pattern of drainage (braided)
- Bird's foot delta (Mississippi delta) fingers of deposition build out into the sea along channels.
- Cuspate delta A pointed triangle shape like a cup or tooth, shaped by sea currents.

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Rejuvenation: occuinr with a fall in sea level relative to the land or rise in of land relative to the sea. causing the river to attempt to grade its profile. river re-adjusts from lower course.

Knick points: sudden break of irregulairty of a graident on the long profile of a river. Some sharply defined e.g waterfalls (waterfalls not normally asscociated with rejuvenation) vertical power gained as river rejuvenates as it adjusts to new base level.knick point joins old and new long profiles.

River terraces: left over from old floodplain left on a higher level than new floodplain due to rejuvenation. new downcutting sinks new channel into former floodplain, cutting back terraces as new valley is widend by ;atteral erosion. Can occur multiple times.
River Thames has several river terraces in its lower stages. used as useful shelter from floods (higher ground), roads and railways, chaning the relief to create natural routewas in London and Oxford.

Incised meanders:  well developed meanders, renewed energy makes them become incised  (deepened, e.g British Isles). dependent on rate of vertical erosion. when incision is slow and lateral erosion is occuring, ingrown meander is produced. asymetrical shape, steep cliffs on outer bend and gentle slip off slopes. rapid incision downcutting dominates, valley is symmetrical with steep sides and gorge like appearence, known as entrenched meanders

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Physical cause of flooding

Occurs when discharge exceeds the capacity of a rivers channel to carry the discharge, causing a river to overflow its banks.

  • High levels of precipitation over a prolonged period of time. leading to saturation of the soil,
  • a large amount of precipitation over a short time period, especially after a dry season where ground is baked hard, little infiltration capacity.
  • snow melt, especially when subsoil still frozen, little infiltration capacity
  • climatic hazards e.g cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep-low pressure weather systems in mid latitudes bringing unnartualy large levels of precipitation

Also the nature of the drainage basin can influence the rate of flooding, relief, vegetation, soil type, and geology all effect the chance of flooding. Areas of dense vegetation/forest have high interception rates and thus reduce risk of flooding.

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Human impact of flooding.

Urbanisation: largest cause due to tarmac and drains making infiltration difficult, concrete and tarmac create fast channels and water passes through drains into rivers much more quickly creating a reduced lag time between peak rainfall and peak discharge.
Roads and tarmac impermeable, less intake of trees due to high density of housing (interception low), natural river channels constricted and straightened increasing the velocity of the river but reduce capacit, depoisited material can pile up on bridge supports.

Deforestation: mainly LEDCs (E.G Brazil and the amazon rainforest), rapid deforestation means less interception from trees thus more water reaches the rivers and a poorer quality of soil creating soil erosion.
Damage greatest near mouth of a river, wide flat floodplains more suceptible to damage,
flooding most damaging near mouth of river due to tributaries adding to volumn of water.

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Bangladesh 2004 case study

Low-lying country lying on delta land between the rivers Ganges, Brahamputra and Meghna. Snow melt from the Himalayas feeds mountains. Monsoon climeate and wet season from may-september, low pressure builds ad womds from south west brng heavy rain to coastal regions. causing cyclones which also cause storm surges.
Humans also increased risk of flooding, urbanisation occure in dhaka plus rapid deforestation in the Himalayas, one of the poorest countries in teh world, can't use hard engeering.

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Three gorges dam china

Largest dam in the world and project ever taken, fully completed in 2012. begun 1994
Located: Yangtze River in China.
Capacity: 22 000 MW
1.2 million people re-settled25% of farmland underwater.
Increase national output by 10%
River has history of flooding. Dam benefits 200 million people downstream
Dam generates electricity for central and eastern China. as well as making river more naviagable.
Cost 20 billion dollars.

rRver sediment transported fell by 50%, causing increased erosion rate downstrea. money must be spent of afforestation on slopes downstream. negative environmental and ecological effects. local animal habitats disrupted

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Soft engineering

River Quaggy, southeast London.
artificially managed in 1960s

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