As rivers

Rivers revision

• The hydrological cycle
• River discharge
• River processes
• Long profile and channel characteristics
• River landforms
• Causes and impacts of flooding
• case studies
• Flood management strategies
• case studies
• Pink= keywords

Hydrological cycle

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• Bildunterschrift: The water cycle
• Water cycle animation-click me
• Water is continuously cycled between the oceans and the atmosphere it returns back to the ocean when it falls as precipitation.
• This is known as the global hydrological cycle which is a closed system meaning there are no inputs or outputs.
• However there are local open systems known as drainage basins these are most common and contain inputs and outputs.
• A drainage basin is a area surrounding the river where the rain falling on the land flows into that river, it is also known as a catchment.
• The boundary of a drainage system is a watershed.

Continued

• 
• Bildunterschrift: Water balance graph UK (AQA)
• A water balance is the balance between inputs and outputs, it is affected by how much water is in a drainage basin
• Wet seasons-precipitation exceeds evapotranspiration, this creates a water surplus, this is when river levels rise due to the ground stores filling so there is more surface runoff.
• Dry seasons- Precipitation is low and evapotranspiration is generally high this creates a deficit the water in the ground is depleted so therefore river levels are low however the ground store are recharged by the next wet season.

River discharge

• River discharge- the volume of water flowing in a river's- measured in cumecs
• River discharge is affected by:
• precipitation-more-high discharge, less-low discharge
• hot weather- higher the temp, the lower the discharge because evaporation is higher
• removal of water from river-reduces discharge
• 

Hydrographs

• Hydrographs show river discharge over time
• Peak discharge- this is the highest point on the graph this is when the discharge is at its highest.
• Lag time- this is the delay between peak rainfall and peak discharge, this happens because it takes time for the water to flow into the river
• Rising limb- the part of the graph up to peak discharge, the discharge increases as rainwater flows into the river
• Falling limb- part of the graph after peak discharge, a shallow falling limb shows water is flowing in from stores even after it stops raining.
• The hydrograph varies with each river drainage basin and the storm event.
• A basin with rapid runoff and a lack of storage capacity gives a hydrograph with a short lag time and high peak discharge it is called a flashy hydrograph, it has steep symmetrical limbs.
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Hydrographs-physical factors

• Larger drainage basins- catch more precipitation, higher peak discharge,
• Smaller basins-short lag times, water has less distance to travel, quicker.
• Steep sided- short lag times, quicker, increase peak discharge.
• Circular basins- most common for flashy hydrographs, increasing peak discharge due to the equal distance between watersheds.
• Basins with lots of streams have high drainage density, quicker, short lag times.
• The amount of water already present affects lag time (antecedent moisture), if the ground is already waterlogged, soil wont absorb any water, infiltration is reduced and surface runoff increases- it is faster than through flow or baseflow so water reaches the river quickly and reduces lag time.

Hydrographs-geology

• Rock type affects lag time and peak discharge.
• Impermeable rocks don't store water or let water through them, this reduces infiltration, increases surface runoff, reducing lag time. Peak discharge also increases as more water reaches the river in a shorter period.
• Sandy soils allow high infiltration, but clay soils have low infiltration rates- this increases surface runoff, reducing lag time and increasing peak discharge.
• Vegetation intercepts precipitation and slows its movement to the river, increase lag time, interception is highest when there's a lot of vegetation. More vegetation there is the more water is lost through evapotranspiration- this reduces peak discharge.

Hydrographs-other factors

• Precipitation affects peak discharge
• Intense storms generate more precipitation, increase peak discharge.
• Type of precipitation-affects lag time
• Temperature affects lag time and peak discharge
• Hot and cold conditions result in hard ground, reduces infiltration, increases surface runoff, reducing lag time and increasing peak discharge
• Hot temperatures can increase evapotranspiration this reduces peak discharge.
• Human activity
• Man made impermeable materials eg concrete.
• Water cant infiltrate into the soil, this increases surface runoff, lag time is short and peak discharge increases.

River processes

• Erosion can affect the length, depth and width of a river
• Types or erosion-
• HYDRAULIC ACTION-the pressure of the water breaks rock particles away from the bed and banks. Its strongest in rapids and waterfalls,
• ABRASION-eroded pieces of rock in the water scrape and rub against the bed and banks removing material, most common.
• ATTRITION-eroded rocks smash into each other and break into smaller fragments, their edges get rounded off as they rub together.
• CORROSION-dissolving of rocks by chemical processes
• https://www.youtube.com/watch?v=dTAyD3U9Qsc (Einbetten)

Erosion

• HEADWARD EROSION
• This makes the river longer. It happens near a river's source as throughflow and surface runoff causes erosion at the point the water enters the river channel (the valley head)
• VERTICAL EROSION
• This deepens river channels, it occurs on the upper stage of the river.
• LATERAL EROSION
• This makes the river wider, it happens in the middle and lower stages of the river.
• 

Transportation

• Transportation is the process of eroded material being carried in a river. The velocity of the river provides the energy, and the eroded material is called a load.
• Solution- substances that can dissolve are carried along in the water
• Suspension- very fine material, silt and clay particles is whipped up by turbulence and carried along in the water, most common.
• Saltation- larger particles like pebbles are too heavy to be carried in suspension, the force of the water causes them to bounce along the river bed.
• Traction- very large particles, boulders are pushed along the river bed by the force of the water.
• Material transported by traction or saltation is called the rivers bedload.
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Deposition

• Deposition is the process of dropping eroded material, it happens when the river loses energy.
• Reduced rainfall causes lower discharge which means the river slows down and has less energy.
• Increased evaporation or abstraction causes lower discharge
• Friction in shallow areas or close to the banks reduces speed, which reduces the energy.
• When the river is forced to slow down eg near a shallow or narrow part
• Energy is lost when the river meets the sea.
• The capacity of a river is the total amount of material it can carry, the competence describes the maximum particle size that a river is capable of transporting at a given point.

Hjulstrom curve

• The hjulstrom curve shows the link between river velocity and competence.
• The competence is affected by the energy and velocity of the river.
• The critical erosion velocity curve shows minimum velocity needed for the river to pick up and transport particles of different sizes. It takes a higher velocity to erode material than it does to transport it.
• The mean settling velocity curve shows the velocities at which particles of different sizes are deposited.
• Sand particles can be eroded at lower velocities than finer particles. Silt and clay particles stick together which means its harder to erode them so more energy is required.
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Long profile and channel characteristics

• The long profile shows the river's gradient from its source to the sea. It shows the height of the river bed above the base layer.
• The base layer is the lowest point that the river can erode to usually sea level.
• The total amount of erosion and deposition along the course are balanced however the rates change.
• A graded profile is where over time the long profile will change from being uneven to a smooth curve.
• A rivers course is split into 3 stages: the upper, middle and lower.
• 

The upper course

• The gradient is steep and the river is high above sea level which gives a lot of potential energy (energy that can be converted into other forms)
• EROSION
• Mainly vertical, abrasion and hydraulic action. It occurs when there are high energy conditions, this causes turbulence and large angular bedload is dragged along the river bed causing intense vertical erosion.
• TRANSPORTATION
• Mainly large particles eg boulders, carried by traction/saltation during high energy conditions
• DEPOSITION
• Little deposition, mainly large particles deposited in river bed as energy levels drop
• The cross profile is a steep v shape. vertical erosion creates narrow valley's and steeply sloped sides.
• 

The middle course

• As the gradient decreases towards the middle stage potential energy is converted to kinetic energy and the river gains velocity.
• EROSION
• Mainly lateral, abrasion. Attrition of larger particles in this stage means that sediment particle size decreases from source to mouth.
• TRANSPORTATION
• More material carried in suspension, particle size decreases, larger particles moved by saltation.
• DEPOSITION
• Sand and gravel deposited across the flood plain as the river floods and friction reduces rivers energy
• The cross profile shows a wider v shape caused by lateral erosion, deposition creates a flood plain
• 

The lower course

• In the lower stage the river has little potential energy but lots of kinetic energy- it flows faster.
• EROSION
• Velocity and discharge highest at this stage, low erosion because turbulence is lower and sediment particle size is reduced. lateral erosion occurs during the formation of meanders.
• TRANSPORTATION
• Mainly smaller particles eg silt and clay are carried by suspension and solution.
• DEPOSITION
• Smaller particles eg silt and clay deposited on flood plain when the river floods and in the river mouth as the sea absorbs
• 

River landforms

• Waterfalls, rapids and potholes are caused by fluvial erosion.
• WATERFALLS
• They form when a band of hard rock meets softer rock, this soft rock is eroded more than the harder rock causing a step in the river bed.
• The water flowing over the step speeds up due to the lack of friction, this increase in speed gives the water more erosive power, this causes further erosion of the soft rock and undercutting of the hard rock.
• As the hard rock is undercut it collapses. A plunge pool is carved out by abrasion at the foot of the waterfall as bits of collapsed rock is swirled around by turbulence.
• More undercutting causes more collapses, the waterfall retreats back up the channel leaving a steep sided gorge
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Potholes and rapids

• Potholes are small circular hollows in the river bed. They're formed by abrasion as turbulence swirl's river's bed load round in a circular motion, causing it to rub and scrape out holes.
• Rapids are relatively steep sections of river with turbulent flow where there are several sections of hard rock (mini waterfalls).
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Meanders

• Meanders are formed by erosion and deposition.
• They form where alternating pools (deep water) and riffles (shallow water) develop at equal space intervals along a stretch of river. Distance between pools is 5-6 times the width of the river bed.
• The river channel is deeper in pools its more efficient so it has greater energy and more erosive power. Energy is lost over a riffle due to friction
• The distance between riffles and pools causes the river's flow to become uneven and maximum flow to be concentrated on one side of the river.
• 

Continued

• 4. Turbulence increases in and around pools as water speeds up, it begins to twist and curl.
• 5. This causes a corkscrew like current which is called helicoidal flow.
• 6. The helicoidal flow causes more erosion and deepening of the pools, it causes eroded material to be deposited on the inside of the bank, where the river loses energy.
• 7.The combination of erosion and deposition exaggerates the bends to make large meanders, this also creates the asymmetric cross section.
• 8. Oxbow lakes are formed when the neck of a loop is broken through usually during flooding. Deposition dams off the loop leaving an oxbow lake.
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River landforms

• Braiding, flood plains, levees and deltas are caused by fluvial deposition.
• Braiding occurs when rivers carry a vast amount of eroded material. If the velocity drops or sediment becomes too heavy it is deposited in the channel. The river divides into small winding channels that eventually rejoin.
• A floodplain is when a river overflows its banks and floods the flat land either side, there's an increase in wetted perimeter and reduction in a hydraulic radius. This increases friction, reducing velocity and causes silt and sand to be deposited .
• levees are natural raised embankments formed when a river overflows, during a flood material is deposited across the flood plain, the river loses velocity and energy due to friction, heaviest material is dropped first and over time this material builds up to form a levee.
• Deltas are when a river reaches a sea the energy of the river is absorbed, this causes the river to deposit its load, this builds up on the sea bed until the alluvium rises which partially blocks the mouth of a river, the river braids which forms a delta
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Rejuvenation

• Rejuvenation allows a river to resume vertical erosion.
• A river is rejuvenated if its base level is lowered this can be caused by crustal uplift or a drop in sea level.
• The drop in base level gives the river more potential energy increasing its vertical erosion potential.
• The long profile of the river is extended and a knick point ( a sharp change in gradient) will form and mark the junction between the original and the new profile.
• River terraces are former flood plains which have bee left above the level of present day flooding following increased vertical erosion.
• Incised meanders are formed when a river keeps its meandering course as vertical erosion increases, it is a deep winding valley with steep sides. The river is below the former flood plain.
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Causes and impacts of flooding

• Prolonged rainfall and heavy rainfall are the main causes.
• Flooding occurs when the discharge of a river is so high that the river spills its banks.
• Prolonged rainfall- after a long period of rain, the ground becomes saturated, any further rainfall cant infiltrate which increases surface runoff.
• Heavy rainfall- can lead to rapid surface runoff if the rainfall is too intense for infiltration to occur, this can lead to a sharp rise in river discharge called a flash flood.
• Melting snow and ice also lead to a huge increase in a river's discharge
• Physical factors
• Sparse vegetation-little rainfall is intercepted, increases discharge. Deciduous trees have no leaves in winter.
• Impermeable ground- clay soils and rock types don't allow any infiltration, increases surface runoff and discharge. If the ground is hard water can't infiltrate.
• Circular drainage basins- water draining into the main channel will arrive in a short space of time due to equal distances between the points, this increase discharge.
• High drainage density- drainage basins with lots of streams drain quickly, short lag times, increases discharge.
• Steep slopes-water reaches the river channel quicker, increases discharge.

Human factors

• URBANISATION-urban areas have large areas of impermeable tarmac and concrete, surface runoff is very rapid, gutters and drains take runoff quickly to rivers, reduce lag times but increases discharge.
• FLOOD MANAGEMENT STRATEGIES- e.g if dams fail a lot of water is released all at once increasing discharge.
• DEFORESTATION-clearing trees reduces interception, increases discharge. Soil is not bound to roots so it is eroded which raises the river bed which increases flooding.
• AGRICULTURE- overgrazing leaves areas with less vegetation, it can increase soil erosion.
• CLIMATE CHANGE- Can increase rainfall and storms which would increase flooding.
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• Bildunterschrift: Flooding, Yorkshire 2015

Causes and impacts of flooding

• SOCIAL IMPACTS- people and animals can be killed, floodwater is often contaminated with sewage so lack of clean water. Contaminated water can also put people at risk of diseases. Possessions are damaged by floodwater.
• People are made homeless as their properties are destroyed.
• ECONOMIC IMPACTS- Businesses often shut down, power supplies are affected, rescue work and repairs are costly, insurance premiums go up. Unemployment levels rise. Public transport is destroyed and crops are destroyed, which can lead to a rise in the price of food.
• ENVIRONMENTAL IMPACTS- floodwater contaminated can pollute rivers, river banks are eroded.
• Positive impacts include- river sediment is deposited on flood plains making the ground fertile, wetlands can be created which create new habitats for species.
• Flood frequency can be calculated.
• Floods on a very large scale aren't as frequent whereas smaller floods are frequent
• People can predict how often a flood of a certain magnitude may occur this is called the flood return interval.

Flooding case studies- South Asia

• -South Asia has a monsoon climate- 80% of the rain falls in this period.
• -It is low lying land
• -Melting snow and ice from the Himalayas increase the river discharge
• PHYSICAL FACTORS- Monsoon season came suddenly after a long hot summer, heavy rainfall, saturated ground, increasing surface runoff and discharge.
• HUMAN FACTORS- Deforestation in Nepal meant less rainfall was intercepted, increased discharge. Urban areas, increased surface runoff
• Bangladesh is a poor country, not many flood defences. Low incomes limited people's ability to recover. Corrupt official diverted aid money away from the people who needed it.
• SOCIAL IMPACTS- over 2000 people died, wells became polluted with sewage, over 100 000 people caught water-borne diseases. 25 million people were left homeless. 112000 houses destroyed. 4000 schools affected and 44 schools destroyed.
• ECONOMIC IMPACTS- The cost of the flood was US$ 1 billion, factories were closed, loss of livestock. 550 000 hectares of land couldn't be planted on. 10000km of roads were destroyed.
• ENVIRONMENTAL IMPACTS- The flood deposited fertile sands on flood plain. Rivers were polluted.

Flooding case study- Carlisle

• The river Eden runs through North Cumbria and reaches the sea near Carlisle.
• The drainage basin at the Eden is large so it catches a large volume of rainfall leading to a high river discharge, it has steep sides, there are many streams which reduces the lag time. the flood return interval was 200 years it was that big.
• PHYSICAL FACTORS- Heavy rainfall, rain fell on saturated ground, high peak discharge.
• HUMAN FACTORS- Carlisle is a built up area, little infiltration and interception, high surface runoff and discharge, drains and sewage systems overflowed.
• SOCIAL IMPACTS- 3 people died, 3000 people homeless, travel disrupted, network problems, schools were closed, increase in stress related illnesses.
• ECONOMIC IMPACTS- The damage cost £100 million, 350 businesses shut, unemployment rose, transport links and building flooded with no power.
• ENVIRONMENTAL IMPACTS- The flooding increased river bank erosion, rivers were polluted with sewage.

Flood management strategies

• HARD ENGINEERING
• -Man made structures
• -Large structures and important buildings are more likely to be protected
• - Flood defences are expensive to build and maintain
• - Floods happen less however if they do it can be made worse
• -Natural processes are disrupted
• 

flood management strategies

• SOFT ENGINEERING
• -structures that work with nature
• -Relatively cheap
• -Flooding is more predictable
• -Asthetic reason
• -Recreational use
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Flood management China

• Hard engineering is used on the Yangtze.
• -Yangtze river flows through China at 6380 km its the third longest river in the world.
• -China has a wet season where there is a huge increase in discharge brought on by the amount of rain.
• -The 1954 flood killed 33169 people.
• -There are 46 dams under construction one of them is the three gorges dam.
• -A reservoir is building up behind the dam, it also has the largest hydroelectric station in the world
• POSITIVE EFFECTS OF THE DAM- Reduced major flooding from every 10 years to 100 years. The turbines in the dam produce 22.5 gigawatts of electricity which supplies around 3% of China's population.
• NEGATIVE EFFECTS OF THE DAM- People have had to relocate due the reservoir flooding the land behind the dam, 13 cities and 1352 villages will be destroyed. The reservoir will also destroy farmland. A huge amount of sediment would be trapped behind the dam which could lead it to fail. The dam might destroy habitats and endanger species, it could also lead to flooding in tributaries which will flood other areas.
• LEVEES- Along the Yangtze river there was over 3600 km of levees in 1998 they broke causing devastating floods.

Flood management Abingdon

• Soft engineering is Abingdon.
• -Its a town in south-east england built on River Thames and River oak
• -Abingdon have regular floods.
• -In 2007 the River Thames broke its bank and flooded 660 properties.
• Gravel soakaways have been built along the A34 road, low value land is allowed to flood. There are planning restrictions on new housing developments. The local flood warning plan warns specific areas at risk.
• Improvements have been made to riparian buffers, local flood wardens, detailed advice on the internet about reducing flood damage.
• Soft engineering reduces damage but floods still happen.
• It's difficult to measure the severity of flooding.

Revision guidance

• Some helpful websites-
• www.coolgeography.co.uk
• www.s-cool.co.uk/a-level/geography
• www.acegeography.com/aqa-a-level.html
• https://geographyas.info
• http://www.bbc.co.uk/schools/gcsebitesize/geography/
• http://www.topmarks.co.uk/search.aspx?subject=12&AgeGroup=6
• http://www.geographynewsroom.co.uk/
• http://milliethegeographer.blogspot.com/
• The student room
• Youtube
• Prezi
• Vimeo
•