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Rivers erode because they possess energy. Their total energy depends on: the weight of the water, the height of the river above its base levels, the steepness of the channel.

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River processes

  • Abrasion – is the scraping, scouring and rubbing action of materials carried along by a river (the load). Rivers carry rock fragments in the flow of the water or drag them along the bed, and in doing so wear away the banks and bed of the river channel. During times when river levels are low, the load consists of small particles, such as sand grains, and these tend to smooth the surface of the river channel.
  • Hydraulic Action – is caused by the sheer power of moving water. It is the movement of loose unconsolidated material due to the frictional drag of the moving water on sediment lying on the channel bed. As velocity increase, turbulent flow lifts a large number of grains, particularly sand-sized particles from the floor of the channel. Hydraulic action is particularly effective at removing loose material in the banks of meanders, which can lead to under-cutting and collapse.
  • Corrosion – the most active on rocks that contain carbonates, such as limestone and chalk. The minerals in the rock are dissolved by weak acids in the river water and carried away in solution.
  • Attrition – the reduction in the size of fragments and particles within a river due to the processes described above. The fragments strike one another as well as the river bed. They therefore become smoother, smaller and more rounded as they move along the river channel. Consequently larger more angular fragments tend to be found upstream while smaller, more rounded fragments are found downstream.
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Vertical and lateral erosion

In the upper course of the river, where the land lies high above sea level, river erosion is predominantly vertical.

Vertical erosion dominates because the river is attempting to cut down to its base level, which it  usually sea level. In times of spate, when the river level and velocity are high, the river cuts down into its valley mainly by abrasion and hydraulic action. Such rivers often produce steep-sided valleys.

Lateral erosion occurs more frequently in the middle and low stretches of the river, where the valley floor lies closer to sea level. Here the river possesses a great deal of energy, particularly when close to bankfull. However, this energy is used laterally to widen the valley as the river meanders. The strongest current is found on the outside of the bend and hydraulic action causes the bank to be undermined and to collapse.

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  • The capacity of a river is a measure of the amount of material it can carry, that is, the total volume of the load. Research has found that a rivers capacity increase to a third power of its velocity.
  • The competence of a river is the diameter of the largest particle that it can carry for a given velocity. Again, research has show that a rivers competence increases according to the sixth power of its velocity.

Traction large stones and boulders are rolled along the river bed by water moving downstream. This process operates only at time of high discharge. Saltation small stones bounce or leap frog along the channel bed. This process is associated with relatively high energy conditions. Suspension very small particles of sand and silt are carried along by the flow of the river. Such material is not only carried but picked up mainly through turbulence that exists within the water. Suspension normally contributes to the largest proportion of sediment to the load of the river. Solution dissolved minerals are transported within the mass of the moving water.

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A river deposits when there is a decrease in its level of energy, it is no longer competent to transport its load. Deposition usually occurs when:

  • There is a reduction in the gradient of the river, e.g. when it enters a lake.
  • The discharge is reduced, such as during and after a dry spell.
  • There is shallow water, e.g. on the inside of a meander.
  • There is an increase in the calibre (size) of the load. This may be due to a tributary bringing in larger particles, increased erosion along the rivers course or a landslide into the river.
  • The river floods and overtops its banks, resulting in a reduced velocity on the floodplain outside the main channel.
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Hjulstroms curve

  • The Hjulstrom graph shows the relationship between the velocity of a river and the size of particles that can be eroded, transported or deposited.
  • Velocity increases as discharge rises and generally this enables a river to pick up larger particles from the bed or banks of the channel.

However, Hjulstrom's research showed three interesting relationships;

  • Sand particles are moved by lower entrainment velocities than smaller silts and clays or larger gravels. The small clay and silt particles are difficult to entrain because they tend to stick together. They lie on the river bed and offer less resistance to water flow than larger particles – so much more powerful flows of water are required to lift them into the water.
  • Once entrained particles can be carried at lower velocities than those required to pick them up. However, for larger particles there is only a small difference between the entrainment velocity (critical erosion) and the settling velocity.
  • The smallest particles, clays and silts are only deposited at very low velocities. This explains why such deposits occur in river estuaries. Here the fresh water of the river meets the salt water of the sea, causing chemical settling of the clays and silts to occur and creating extensive areas of mudflats. This process is called flocculation – this clustering of the clay and silt particles causes them to sink more rapidly.
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River load

Spatial load variations are due to:

Size of the drainage basin – large drainage basins with many tributaries have a greater potential for transporting sediment, particularly in their lower courses, than do small drainage basins.

Rock type – in drainage basins where the underlying geology consists of relatively soft (and easily eroded) sandstones and clays, the sediment transported consists mainly of sand or clay particles. Where the rock is limestone, more material will be transported  as dissolved load because this rock type is soluble. Moving water does not easily erode resistant igneous rocks, such as granite and basalt. Therefore, the total sediment yield in river basins of igneous rocks may be low whereas in drainage basins where the underlying rock is softer, sediment yield may be high.

Relief – in drainage basins with low relief, where there is a small difference in altitude between the source and base level, the energy available for erosion and transport is limited.

Precipitation – low loads are generally found in drainage basins with low rates of precipitation.

Human Activity – this can both increase and decrease sediment yield. In areas of the of the world where deforestation is occurring rapidly, there have been marked increases in the load carried by rivers. This is mainly caused by increased soil erosion, which occurs because the vegetation that protected the soil from the actions of moving water on its surface has been removed.

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Effects on channel loads

The effects of channel load on landforms;

A fast flowing river, has the competence to carry a large load. The particles erode the river beds and banks by abrasion, creating distinctive features such as potholes, waterfalls and gorges. If the volume of water in the river falls quickly, the load is deposited because of a fall in competence. When this occurs, depositional features such as levees, floodplains and deltas are created. In some sections of the river, both erosion and deposition occur. This is particularly noticeable on a meander bend, where suspended load carried by the river erodes the outside edge of the bend by abrasion and load in deposited on the inside of the bend to form a point bar.

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Valley profiles- Long profile


The long profile of a river illustrates the changes in altitude of the course of the river from its source, along the entire length of its channel, to the river mouth. In general, the long profile is smoothly concave, with the gradient being steeper in the upper course and becoming progressively gentler towards the mouth. Irregularities in the gradient frequently occur and may be represented by rapids, waterfalls or lakes. There may also be marked breaks of changes in slope, known as knick points – which are generally the product of rejuvenation. Rejuvenation occurs either when the sea level falls or when the land surface rises – either situation allows the river to revive its erosion activity in a vertical direction.

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Valley profiles- Cross profile


The valley cross profile is the view of the valley from one side to another. For example, the valley cross profile of a river in an upland area typically has a V-shape, with steep sides and a narrow bottom. In the upper course – a narrow steep sided valley where the river occupies all of the valley floor. This is a result of dominant vertical erosion by the river. In the middle course – a wider valley with distinct valley bluffs, and a flat floodplain. This is the result of lateral erosion, which widens the valley floor. In the lower course – a very wide, flat floodplain in which the valley sides are difficult to locate. Here there is a lack of erosion and reduced competence of the river, which results in large scale deposition.

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Valley profiles- Graded profile

Over a long period of time a river may display an even and progressive decrease in gradient down the valley, creating the typical smooth concave shape which has adjusted to the discharge and the load of the river.

The idea of grade was originally put forward by W.M Davies, who argued that irregularities in the long profile which would reflect changes in underlying geology are eventually worn away by river erosion to give a smooth graded profile. This may also be referred to as the profile of dynamic equilibrium, where a balance has been achieved between the processes of erosion and deposition. More recently, it has been accepted that the channel may still be graded if it exhibits some irregularities in its long profile. Some geographers define the graded river as that which has been attained when the river uses up all of its energy in the movement of water and sediment so that no free energy is left to undertake further erosion. In this situation the gradient at each point along the river is sufficient to discharge the water and load but there is little excess energy available for further erosion. Such a balance between energy and work cannot occur at a particular moment in time but is suggested as an average position over a long time.

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Potential and kinetic energy

In relation to rivers, potential or stored energy if fixed by the altitude of the source of the stream in relation to base level.

Kinetic energy, or energy due to movement, is generated by the flow of the river which converts  potential energy into moving energy. The amount of kinetic energy is determined by the volume of flowing water (discharge), the slope or channel gradient down which it is flowing and its average velocity. An increase in velocity and/or discharge results in an increase in kinetic energy. All channel process are dependant on the amount of energy available – this is a delicate balance. If there is excess energy after transportation of load the river will erode, but if energy is insufficient to move the load, deposition will occur. The river channel adjusts in shape and size to accommodate changes in the volume of water and sediment.

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Changing channel characteristics

In the upper course, the channel is narrow and uneven, because of the presence of deposited boulders, Where both banks are being eroded channels tend to be broadly rectangular in shape. As the river enters its middle course it starts to meander, the channel becomes asymmetrical on the river bends by mainly smooth and symmetrical on the straight stretches.

In the lower course, the river widens and deepens further, but banks of depositions and eyots (islands of deposition) can disrupt the shape of the channel cross-section, leading to a braided channel. Sometimes embankments called levees can be seen on either side of the channel. The shape of the channel influences the velocity of the river In the upper course there is a large wetted perimeter. The wetted perimeter is the total length of the river bed and banks in cross section that are in contact with the water. When there is a large wetted perimeter in relation to the amount of water in the river, there is more friction. The wetted perimeter is proportionately smaller than the volume of water flowing in the channel – therefore, there is less friction to reduce velocity. Although the turbulent flow of mountain streams might appear faster than that of the gently meandering downstream channel , average velocity is actually slower.

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Hydraulic radius

Hydraulic radius = cross-sectional area of the channel/

                                            wetted perimeter

The wetted perimeter is the length of banks and bed added together. The cross-sectional area is the length of the bed x the length of one bank. A high hydraulic radius means that the river is efficient – this is because moving water loses proportionately less energy in overcoming friction than when the ratio between the cross-sectional area and the wetted perimeter is low. Larger channels tend to be more efficient; area increases to a greater degree than wetted perimeter.

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Landforms of fluvial erosion


Waterfalls and rapids occur when there is a sudden change in the gradient of the river as it flows downstream. Waterfalls are more dramatic features than rapids and may be the result of;

-A resistant band of rock occurring across the course of the river. -The edge of a plateau. The rejuvenation of the area, the river renewed erosional power as sea level falls.

Rapids are found where there is a sudden increase in the slope of the channel or where the river flows over a series of gently dipping harder banks of rocks. As the water becomes more turbulent its erosive power increases.

•The river falls over a rock edge into a deep plunge pool at the foot of the fall, where the layers of weak rock are excavated more quickly than the overlying resistant rock. The force of the swirling water around the rocks and boulders enlarges and deepens the plunge pool by hydraulic action and abrasion. This undercuts the resistant (cap) rock above.

An example is the Upper Teesdale, High force. it is off of the igneous rock, the Whin Sill.

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Landforms of fluvial erosion 2


In the upper course of rivers the characteristically large-calibre sediment load is usually only transported when the discharge has risen as a result of heavy rain or snowmelt. At such times the bouncing and rolling of boulders and cobbles may cause intensive vertical erosion, which in turn produces a relatively steep-sided V-shaped valley profile. The exact shape depends on three factors;

1.Climate – sufficient water is required for the high discharge levels needed to instigate vertical erosion in the channel and to aid mass movement on the valley sides above the eroding channel.

2.Geology – the type of rock and its structure may tend towards very steep sides (limestone) or gently sloping valley sides (clays and shales)

3.Vegetation – more vegetated slopes tend to bind the soil better and may lead to more stable valley sides.

Interlocking spurs are also characteristic of the upper courses of rivers. These form when the river winds around protrusions, hills or ridges of land (spurs) which appear to interlock when viewed looking up or down a valley.

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Landforms of fluvial erosion 3


Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high-velocity water loaded with pebbles. The pebbles become trapped in slight hollows and vertical eddies in the water are strong enough to allow the sediment to grind a hole into the rock by abrasion. Attrition rounds and smooth's the pebbles caught in the hole and helps to reduce the size of the bedload. Potholes can vary in width from a few centimetres to several metres. They are generally found in the upper or early-middle course of river – this is where the valley lies well above base level, giving more potential for downcutting, and where the river bed is more likely to be rocky in nature.

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Landforms of fluvial erosion 4


Braiding occurs when the river is forced to split into several channel separated by islands. It is a feature of rivers that are supplied with large loads of sand and gravel and it most likely to occur when a river has variable discharges. The banks formed from sand and gravel are generally unstable and easily eroded – as a consequence the river becomes very wide. The river can become choked, with several sandbars and channels that are constantly changing their locations.

Braiding also occurs in environment in which there are rapidly fluctuating discharges;

-semi-arid areas of low relief that receive rivers from mountainous areas. -Glacial streams with variable annual discharge. In spring, meltwater causes river discharge and competence to increase, therefore the river can transport more particles. As the temperature drops and the river level falls, the load is deposited as islands of deposition in the channel.

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Landforms of fluvial erosion 5

MEANDERS; Meanders are sinuous bends in a river.

  • At such times of low flow the hydraulic radius of that part of the channel is reduced
  • Once created, the riffle serves to reduce the hydraulic radius for that area.
  • Water needs to find a way around this areas of higher frictional contact, so it flows around them.
  • Between these shallow riffle areas deeper sections are eroded (pools).
  • So a series of pools and riffles develop over time.
  • At times of higher flow the water swinging around one side of a riffle will be propelled by centripetal forces towards one of the banks, eroding it by undercutting.
  • An outer concave bank is created while slower flow on the inside bend leads to deposition on the inside bend.
  • This helicoidal flow allows material eroded from the outer bank to be deposited in part on the inner bank of the next meander
  • Even where meanders are well developed, riffles may occur in the straighter sections of the channel between the meanders.
  • Because the water in the meander bend is travelling at a higher velocity (due to deeper water and higher hydraulic radius) it erodes more effectively and carries the eroded sediments with it.
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Landforms of fluvial erosion 6


Oxbow lakes are features of both erosion and deposition. An oxbow lake is a horseshoe-shaped lake separated from an adjacent river. The water is stagnant, and in time the lake gradually silts up, becoming a crescent-shaped stretch of marsh called a meander scar. An oxbow lake is formed by the increasing sinuosity of a meander. Erosion is greatest on the outer bank, and with deposition on the inner bank, the neck of the meander becomes progressively narrower. During times of high discharge, such as floods, the river cuts through this neck, and the new cut eventually becomes the main channel. The former channel is sealed off by deposition.

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Landforms of fluvial erosion 6


Oxbow lakes are features of both erosion and deposition. An oxbow lake is a horseshoe-shaped lake separated from an adjacent river. The water is stagnant, and in time the lake gradually silts up, becoming a crescent-shaped stretch of marsh called a meander scar. An oxbow lake is formed by the increasing sinuosity of a meander. Erosion is greatest on the outer bank, and with deposition on the inner bank, the neck of the meander becomes progressively narrower. During times of high discharge, such as floods, the river cuts through this neck, and the new cut eventually becomes the main channel. The former channel is sealed off by deposition.

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Landforms of fluvial erosion 7


In its middle and lower courses, the river is at risk from flooding during times of high discharge. If it floods, the velocity of the water falls as it overflows the banks. This results in deposition, because the competence of the river is suddenly reduced it is usually the coarsest material to be deposited first – forming small raised banks (Levees) along the sides to the channel. Subsequent floods increase the size of these banks and further deposition on the bed of the river also occurs. This means that the river, with channel sediment build-up, now flows at a higher level than the floodplain. On the Mississippi River, for example levee strengthening began in 1699. By the 1990’s the length of engineered levees was 3,200km

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Landforms of fluvial erosion 8

DELTAS; A delta is a feature of deposition, located at the mouth of a river as it enters a sea or lake.

Deposition occurs as the velocity and sediment-carrying capacity of the river decrease on entering the lake and bedload and suspended material are dumped. Flocculation occurs as fresh water mixes with seas water and clay particles coagulate due to chemical reaction.

Deltas form only when the rate of deposition exceeds the rate of sediment removal. In order for a delta to form these conditions are likely to be met:

-The sediment load of the river is very large, as in the Mississippi and Nile river. -The coastal area into which the river empties its load has a small tidal range and weak currents. This means there is limited wave action, therefore little transportation of sediment after deposition.

Deltas are usually composed of three types of deposit:

-The large and heavier particles are the first to be deposited as the river loses its energy – these form topset beds. -Medium graded particles travel a further before they are deposited as steep-angled wedges of sediment – forming foreset beds. -The very finest particles travel furthest into the lake before deposition and form the bottomset beds.

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Rejuvenation occurs when there is either a fall in sea level relative to the level of the land or a rise of the land relative to the sea. This enables a river to renew its capacity to erode as its potential energy is increased. The river adjusts to its new base level, at first in its lower reaches and then progressively inland.

Erosional features:


A knick point is a sudden break or irregularity in the gradient along the long profile of a river. When a river is rejuvenated, adjustment to the new base level starts at the sea and gradually works its way up the rivers course. The river gains renewed cutting power (in the form of vertical erosion) – which encourages it to adjust its long profile. The knick point recedes upstream at a rate which is dependent on the resistance of the rocks, and may linger at a relatively hard outcrop.

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Rejuvination 2


A river terrace is a remnant of a former floodplain – which has been left at a higher level after rejuvenation of the river. Where a river renews its downcutting, it sinks its new channel into the former floodplain, leaving the old floodplain above the level of the present river. The terraces are cut back as the new valley is widened by lateral erosion.


If a rejuvenated river occupies a valley with well-developed meander, renewed energy results in them being incised or deepened. With rapid incision, where downcutting or vertical erosion dominates, the valley is more symmetrical, with steep sides and a gorge-like appearance. This are described as entrenched meanders.

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Causes of flooding- Physical

Excessive levels of precipitation occurring over a prolonged period of time. This eventually leads to the saturation of the soil. When the water table reaches the ground surface, there is increased overland flow or runoff.

Intensive precipitation over a short period of time, particularly when the ground surface is baked hard after a long period without rainfall. In such circumstances the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough. The melting of snow, particularly when the subsoil is still frozen, so the infiltration capacity is reduced. Climatic hazards such as cyclones in Bangladesh.


By midday on 16th August 2004, heavy thundery downpours had developed across Southwest England. As a result of an intensive low-pressure weather system that encouraged the uplift of warm, moist arm. Some 200mm of rain fell in 24 hours – most it between 12 and 5pm. Around 70-80 cars were swept away in Boscastle and 100 people were airlifted to safety by the emergencys.

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Causes of flooding- Human


Due to an increasing demand for space, people build on floodplains because their flat land is suitable to build on. Concrete and tarmac are used in urban areas for roads and pavement – these are impermeable. Surface water is channelled directly into drains and sewers in an urban area – so precipitation reaches the river quickly. Natural river channels may become constricted by bridges which can slow down discharge and reduce the carrying capacity of the river.


Once trees have been removed there is a greater risk of soil erosion and sediment finds its way into river, obstructing them and adding to the flood risk.

Trees intercept water and take it up through there roots, so in deforested areas more water reaches the channel as runoff. climate change

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MEDC flood case study- Southern britain


The jet stream which influences the path taken by low-pressure weather systems in the North Atlantic, had followed an abnormally southerly track. Rainfall totals for May to July 2007 were the highest on record for England and Wales since 1766, with many areas registering more than twice the long-term average. Including 145mm of rain. Flood risk during the summer is normally reduced by dry soil conditions – but in this case the record early summer rainfall meant that soils were already close to saturation. Groundwater levels were also much higher than normal – so there was little infiltration or percolation capacity. Three people died as a direct result of the floods in Gloucestershire. At one point 45,000 households were without power. Some 350,000 homes in the county had no running water after a water treatment works was flooding.

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LEDC flood case study- Bangladesh 2004

Bangladesh is a low-lying country most of which lies on the delta land of three major rivers, the Ganges, Brahmaputra and Meghna. The sources of these rivers are in the Himalayas. Snow melt affects the discharge.

This part of South Asia has a monsoon climate and experiences a wet season between May and September. Human factors have played an increasing role in the severity of the floods – urbanisation has occurred. • Deforestation in the Himalayas has had a negative effect on the rates of interception and evapotranspiration.


During July and August 2004, approximately 38% of the total land area was flooding. Nationwide, 36 million people (from a total population of 125mil) were made homeless. By mid-September the death toll had risen to 800 – many people had died as a result of disease. The flood also caused serious damage to infrastructure – including roads, bridges, embankments, railway lines. Boats were afloat on the main runway at Sylhet airport and all domestic and internal flights had to be suspended. The floods also caused four environmental impacts; river-bank erosion, soil erosion, water logging in urban areas.

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LEDC flood case study- Bangladesh 2004 2

Short-term responses;

Th government, working with non-governmental organisations provided emergency relief in the form of rice, clothing, medicines, blankets and towels. In July, the UN activated a disaster management team to coordinate the activities of the various UN agencies. They supplied critical emergency supplies and conducted a ‘damage and needs assessment’ in the affected areas. Bilateral aid from individual countries was directed to the UN team. People in Bangladesh are resilient, and self-help schemes, in which local people work together to rebuild there properties.

Long-term Responses;

For a developing country like Bangladesh long-term responses to major floods are largely dependent on foreign aid.Food shelters and early-warning systems have been successfully put in place. Following the 2004 floods, additional financial aid was granted for a period of five years – to pay for repairs to infrastructure, water resource management and education.Disaster-preparedness is a key priority for the future – including flood management and improved water resources.

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Flood management strategies- Hard engineering

Hard Engineering methods;

The banks and or channel can be modified to enable the river to carry a larger volume of water. The removal of large boulders from the bed of the river reduced roughness, therefore increasing the velocity of the flow. Dams and weirs can be built to regulate the rate at which water passes down the river. Diversion channels can be constructed to divert rivers away from areas vulnerable to flooding. Dredging can be used to create a deeper channel so that greater volumes of water can pass through. The height of the floodplain can be increased by dumping material on it. Retention basins and balancing lakes can be constructed into which water is diverted at times of high discharge.

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Case study: Hard engineering- Three Gorges Dam, Ch

It is located on the Yangtze River in China. The dam will be used to generate electricity from hydroelectric power for central and eastern China. The dam will also reduce the flood risk for 15 million people and improve navigation along the river.

A huge reservoir, some 660km long and 1km wide, has been created behind the dam. The dam itself, completed in 2006 is 2.3km long and more than 100m high. According to official figures it cost some where in the region of £25 billion.


The creation of the reservoir has forced the resettlement of 1.2 million people from several cities. Between 1998 and 2004 the amount of sediment transported by the river below the dam fell by over 50%, resulting in increased rates of erosion downstream – sediment will accumulate behind the dam and this will require dredging Afforestation is needed on the slopes in the drainage basin to reduce the amount of sediment washed into the river and the reservoir behind the dam. There is an increased risk of landslides in the immediate area around the reservoir and species, such as white flag dolphin, will have their habitats disrupted.

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Case study: Soft engineering- River Quaggy restora

The River Quaggy runs through southeast London. Since the 1960’s it has been heavily managed and artificial channels and culverts were built to divert it beneath the ground surface as it passed through Greenwich.

As a result of increased flood risk due to continued urban developed in Lewisham and Greenwich. A more sustainable approach of soft engineering was chose. The plan was to bring the river back above ground once against, cutting a new channel for it through Sutcliffe Park and creating a multifunctional open space. In this way flood management and the quality of the park would be improved. Although the culvert remained to take some water underground during flood conditions, a new lake was created to take over. The park itself was lowered  and shaped to create a floodplain where water could collect naturally instead of rushing downstream through artificial channels to flood Lewisham town centre. The parks flood storage capacity of 85,000m³ has reduced the risk of flooding for 600 homes and businesses in Greenwich and Lewisham and has created a diverse environment for wildlife. By redirecting the river to a more natural course and including a flood storage area, the scheme has created a wetland environment with reed beds, wild flower meadows and trees. The scheme won  the natural environment category in the 2007 waterways renaissance awards and the living wetlands award, run jointly by the RSPB and the Chartered Institution of Water and Environmental Management,

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