- Created by: Beez123
- Created on: 24-04-19 21:05
Plate Margins/ Boundaries
- when two plates move towards each other
- When an oceanic plate meets a continental plate, the denser oceanic plate is forced down into the mantle and destroyed in the process of subduction.
- THis leads to volcanoes and ocean trenches and earthquakes
- When two plates move away from each other
- Two continental plates diverge forming a mid-oceanic ridge. Magma rises from the mantle to fill the gap and cools to form new crust.
- This leads to volcanoes and fold mountains and earthquakes.
- Two plates sliding past each other.
- Two plates slide past each other. Pressure builds up leading to earthquakes.
Haiti 2010 (Earthquake)
12th January 2010- magnitude 7 on the Richter Scale
- 300,000 people were injured
- roads were blocked by fallen buildings and smashed vehicles
- the main shipping port was badly damaged and part of it collapsed.
- 8 hospitals or healthcare centers in Port-au-Prince were badly damaged or collapsed.
- over 2 million Haitians were left without food or water
- looting became a serious problem
- The Haitian tourist industry declined
- 1.3 million Haitians were displaced
- The dead bodies in the streets and under rubble created a health hazard in the heat- had to be buried in mass graves
- By November 2010 there were outbreaks of cholera.
Christchurch 2011 (Earthquake)
22nd February 2011- magnitude 6.3 on the Richter Scale - Ring of Fire
- 185 people killed
- roads and bridges severly damaged
- Cathedral lost its spire
- landslides occured
- over 1/2 deaths happened in a 6 story CTV building which collapsed and later caught fire.
- water pipes damaged
Responses to the Earthquake
27,000 toilets flown in due to sewage works damage
- bottled water provided
- rescue teams from around the world
- police brought in to enforce cordons, traffic control and prevent looting
- buildings replaced with new designs
Soufriére Hills erupted on 25th June 1997- destructive plate boundary
- Areas covered 12 metres deep in volcanic ash
- 23 deaths
- Port and airport destroyed
- Evacuees forced to live in makeshift shelters with poor sanitation
- 50% population left Montserrat
- Tourist industry declined
- Businesses and industries affected
- silicosis due to quartz content of ash
- volcanic ash improved soil fertility
- other countries learned lessons from eruptions
- tourism eventually increased due to the desire to see the volcano
- Residents evacuated to the north of the island
- charities set up temporary schools and sent emergency food for farm animals
- UK provided £17 million of emergency aid. Used for temporary infrastructure and water purification.
Long term Responses
- Exclusion zone set up
- Many people left the island completely. By Nov 1997, population fell from 12000 to 3500.
- UK donates £41 million in aid. Mostly used to develop the north of the island.
- population structure changed- many younger people left and didn't return.
- Montserrat Volcano Observatory (MVO) set up to predict future eruptions.
Low latitiudes have warm air because they are near the equator where the sun's rays are more intense.
High latitudes have cool air because they are further from the equator
The Tricellular model
The lowest pressure cell is the Hadley cell which is located in the low latitudes near the equator. This cell moves anticlockwise.
The middle cell is the Ferrel cell which is located in the middle latitudes. The air in this cell moves clockwise.
The highest pressure cell is the Polar cell found in the high latitudes near the Arctic circle. This cell moves anticlockwise
Haiyan 2013 (Typhoon)
The Philippines- 8th November 2013
- Blocked roads from debris
- water pipes smashed
- 95% of trees blown down
- dead bodies in the street
- An oil tanker ran aground, killing mangroes and stopping fishing
- Blocked roads block businesses and aid
- disease causing death
- mosquitoes in stagnant water
- 1600 evacuation centres set up
- A curfew imposed to prevent looting
- The UN raised £190 million for aid, Red Cross and Save the Children
Long Term Responses
- Mangroves have been replanted which help to absorb some of the impact of the storms
- The weather bureau is rapidly improving its ability to predict storms.
- Local Governments are improving their typhoon preparation including a warning system
- 'no build' zone set up
Tropical storms develop when:
- sea temperature is 27 degrees or higher
- the wind shear between higher and lower parts of the atmosphere is low.
Tropical storms occur 23.5 degrees above and below the equator between the Tropics of Cancer and Capricorn.
They spin anticlockwise in the northern hemisphere, and clockwise in the southern hemisphere due to the Coriolis Effect.
The centre of the storm is called the eye- there's low pressure, light winds, no clouds, no rain and a high temperature in the eye.
It is surrounded by the eyewall- where there's spiralling rising air, very strong winds (around 160 km per hour), storm cloud, torrential rain and a low temperature.
UK Weather Hazards
Rain- Too much rain in a short amount of time can cause flooding.
Wind- Strong gales can damage properties and cause disruption to transport
Snow and Ice- Snow and ice can cause injuries due to slipping and deaths due to the cold. It can damage crops and other plants.
Thunderstorms- Heavy rain, lightening and strong winds occur in thunderstorms. Fires that damage property or the environment.
Hailstorms- Hailstones make driving very dangerous and can damage property and destroy crops
Heat Waves- heat exhaustion and pollution can build.
Drought- lack of precipitation, water supplies can run dry
Extreme UK Weather
December 2010 was the coldest for over 100 years
April 2011 was the warmest on record
Storm Desmond 2015
Storm Desmond caused damage in Cumbria and Northern Ireland in December 2015
- River Eden, Greta and Kent and their tributaries flooded.
- Erosion caused landslides which tore trees up and damaged ecosystems and habitats.
- Man-made climate change was partly responsible as 341 mm of rain fell within 24 hours.
More than 5000 homes were affected by flooding
Thousands of properties were without electricity- 60,000 houses, hospitals and schools involved.
there were between £250m- £300m in insurance claims.
up to 25,000 businesses in Cumbria were affected by the floods.
`Global Warming is the term used to describe the sharp rise in global temperatures over the last century. I'ts a type of climate change.
Ice and Sediment Cores- Scientists analyse the gases trapped in different layers of ices to tell what temperatures were each year.
The remains of organisms found in cores taken from ocean sediments can also be analysed.
Tree Rings- Scientists take cores and count the rings to find the age of a tree. The thickness of each ring shows what the climate was like.
Pollen Analysis- Pollen from plants gets preserved in sediment e.g at the bottom of lakes or bogs.
Temperature Records / Historical records
Climate Change Causes
- Orbital Changes- The path of the Eart'sorbit around the Sun changs from an almost perfect circle to an ellipse and back again. These changes affect the amount of solar radiation the earth receives. ORbital changes may have caused the glacial and interglacial cyles of the Quaternary period.
- Volcanic Activity- Some of the partticles released in a volcanic eruption reflect the Sun's rays back out to space (cooling). Volcanic eruptions release CO2 and SO2 causing short-term changes in climate.
- Burning Fossil Fuels
- Farming- livestock produce methane + Rice Paddies contribute to global warming when flooded fields emit methane.
- Deforestation- Plants remove CO2 from the atmosphere during photosynthesis. When trees and plants are cut down they stop absorbing CO2.
Managing Climate Change
Mitigation- (to reduce climate change)
- Carbon Capture- Carbond Capture and Storage is new technology designed to reduce climate change by reducing emissions from fossil fuel burning power stations. Involves capturing C)2 and transporting in to places where it can be stored safely (undergrounds)
- Planting Trees- increse absorbtion of CO2
- Alternative Energy Production-Renewable instead of non-renewable like fossil fuels.
- International Agreements- From 1997 most countries agreed to monitor and cut greenhouse gas emissions by signing the Kyoto Protocol. UK agreed to cut emissions by 12.5% by 2012.
Adaptation- (responding to climate change)
- Changing Agricultural Systems- it may be necessary to plant new crop types that are more suited to the new climate conditions. biotechnology is being ued to created new crop varieties which are more resistant to extreme weather events.
- Managing Water Supply- Water meters can be installed to discourage high water usage. Rainwater can be collected and waste water recycled.
- Sea levels- Physical defences like flood barriers can be built. Better flood warning systems are being initiated. In LICs people are building houses on earth embankmnets/ building flood shelters.
A producer is an organism that uses sunlight energy to produce food.
A consumer is an organism that eats other organisms.
- A decomposer is an organism that gets its energy by breaking down dead materials.
- When dead material is decomposed, nutrients are released into the soil.
- The nutrients are then taken up from the soil by plants.
- The plants may be eaten by consumers.
- When the plants or consumers die, the nutrients are returned to the soil.
- This transfer of nutrients is called nutrient cycling.
Found in a broad band along the equator and between the tropics of cancer and capricorn (e.g. the Amazon in Brazil)
Climate- between 20-28 degrees all year round. Rainfall is high at around 2000mm per year.
Soil- not fertile as heavy rain washes nutrients away.
Plants- evergreen to take advantage of the continual growing season.
50% of world's species.
- Buttress roots- wide roots above ground allows the tree to absorb nutrients directly from the leaf litter.
- Epiphytes- Parasitic plants that live directly in the boughs of trees.
- Drip Tips- leaves have spiky drip tips to allow water to run off.
- Toucan- Huge beaks for cracking nuts
- Chameleon- changes colour to blend in with surroundings.
Emergents- The tops of the tallest trees. Get the most sunlight.
Continuous Canopy- Upper parts of the tree found here. Home to insects, mammals and birds.
Under Canopy- dark/gloomy. Little vegetation.
Shrub Layer- Sunlight is limited. saplings and seedlings wait for larger trees to die.
Reasons for Deforestation
- Population Pressure- as a population increases, trees are cleared to make space for new settlement.
- Mineral Extraction- minerals like iron are mined for profit.
- Commercial Logging- Trees are felled for money (road building for logging also requires tree clearance)
- Commercial Farming- forest space is cleared for cattle grazing or palm oil or soya plantations. (responsible for 80% of clear felling)
- Subsistence Farming- forest is cleared so farmers can grow food for families.
TRF Deforestation Impacts
- Soil Erosion- with no roots to hold soil together, heavy rain can wash away soil leading to landslides and flooding.
- Reduction in Soil Fertility- Without a tree canopy to intercept rainfall and roots to absorb it, nutrients in the soil are washed away.
- Global Warming- Trees remove CO2 from the atmosphere so burning vegetation releases it and adds to the greenhouse effect. Deforestation is responsible for at least 15% of global CO2 emissions each year- more than all of the world's annual transport emissions combined.
- Loss of Biodiversity- animals lose habitats so die out leading to a loss in biodiversity.
- Employment- Logging, farming and mining create jobs.
- Profit- money is made from selling timber, mining and commercial farming.
- Fewer Tourists- Deforestation can make areas of the TRF less attractive for tourists.
- Livelihoods Destroyed- local people who rely on the TRF for certain animals and plants lose business due to lack of goods.
TRF Deforestation Impacts 2
- Decline of indigenous tribes
- Loss of potential medicines
TRF Sustainable Management
Reducing debts of TRF countries- They wouldn't have to sell wood to pay off debts.
Afforestation- the recreation of supposedly lost forests. Absorbs CO2 in atmosphere. Increases biodiversity. Improves soil fertility.
Selective Logging- only cutitng down trees of a certain height- younger trees have a chance to become emergent; habitats aren't lost.
Ecotourism- Forests are preserved as tourists come to see and experience it. Increases economy and conservation projects.
Agroforestry- where trees are planted alongside crops like coffee. Helps bind the soil together preventing soil erosion.
Found in two broad bands along the Tropics of Cancer and Capricorn (e.g. the Sahara in Africa)
Climate- little rainfall at less than 250mm a year. Temperatures are extreme, 45 degrees in day to 5 degrees at night.
Soil- low organic content, barely any leaf litter, little moisture to accelerate decomposition. Little biomass.
Hot Desert Adaptations
Plant roots are either extremely long to reach deep water or spread out wide near the surface to catch rainfall.
Saguaro Cacti- wide shallow roots and store water in their stems.
Acacia Trees- fire resistant and have long tap roots to seek water underground.
Nocturnal- animals can stay cool in burrows during the day and come out in cool night.
High temperatures- lizards and snakes are able to tolerate high body temperatures.
Fennec Foxes- have thick fur on their feet to withstand the heat of the sand and huge ears to radiate heat.
The Western (Hot Desert)
- The Western Desert extends over 200,000 square kilometres and is located in south west of the USA.
- Contains the Sonoran Desert, the Mojave Desert and the Chihuahuan Desert.
- Extends across several states including California, New Mexico and Arizona.
- low population density with the majority of its inhabitants living in cities like Las Vegas and Phoenix, Arizona.
- Farming- High temp and sunlight. Aquifers and drip irrigation needed.
- Mining- Copper mined in Arizona- jobs and profits- destroys the landscaps and habitats, river pollution.
- Energy Production- HEP, Solar, Oil. jobs and profits. Pollution and habitat destruction.
- Tourism- Culture (Joshua Tree and Las Vegas)- jobs and profits. Destruction of habitats and water scarsity.
- Settlement- Snowbirds (retired migrants) have increased population.
Western Desert Water Crisis
The Hoover Dam
- 1.4 mil acres of irrigated land produces about 15% of the USA's crops. It makes $1.5bn a year.
- Water is brought to great cities such as Las Vegas and Los Angeles.
- The Colorado River now flows at the same rate throughout the year.
- Salt and sand build up means the water is colder, changing the habitat for many species.
- Sandbank habitats are now smaller leading to less biodiversity.
- Lake Mead is in danger of running dry due to overuse meaning millions lack water security.
Desertification in the Sahel
Fuelwood- trees are cut down as they are a cheap source of fuel. Leading to soil erosion (during periods of strong winds and rainfall, trees cannot form cannopies over)
Overgrazing- Soil is exposed as animals graze on the top layer of grass.
Overpopulation- people move to marginal land- land on the edge of the desrt that is close to useless.
Overcultivation- monocultures are grown & encouraged by Gov as they can be exported and sold easily. leads to a deterioration in soil quality.
Afforestation- planting trees - The Great Green Wall
Terraces- steps cut into hillsides- cheap
Bunds- low walls built along hillsides- cheap
Gabions- wire cubes filled with stones- cheaps
- (Freeze-thaw weathering)
- When water gets into cracks of rocks, and the temperature drops to/ below 0, it freezes and expands, pressuring the rock.
- When the water thaws, it contracts, releasing the pressure on the rock.
- Repeated freezing and thawing widens the crack and causes the rock to break up.
- Rainwater has carbon dioxide dissolved in it, which makes it a weak carbonic acid.
- Carbonic acid reacts with rock that contains calcium carbonate e.g limestone, so the rocks are dissolved by the rainwater.
Mass Movement (coasts)
- Mass movement is the shifting of rocks and loose material down a slope e.g. a cliff.
- Mass movements cause coasts to retreat rapidly.
- They're more likely to happen when the material is full of water- heavier.
Slides- Material shifts in a straight line
Slumps- Material shifts with a rotation
Rockfalls- Material breaks up and falls down slopes
Hydraulic power- waves crash against rock and compress the air in the cracks. This puts pressure on the rock. Repeated compression widens the cracks and causes rock to break off
Abrasion- eroded particles scrape and rub against rock, removing small pieces.
Attrition- eroded particles in the water smash into each other and break into smaller fragments. Their edges also get rounded off as they rub together.
- High and steep (tall breaker)
- Powerful backwash
- Weak swash
- material is removed from the coast
- Low wave (in proportion to its length)
- Weak backwash
- Strong swash
- Deposits sediment
Wave-cut Platforms (coasts)
- Waves cause most erosion at the foot of a cliff.
- This forms a wave-cut notch, which is enlarged as erosion continues.
- The rock above the notch becomes unstable and eventually collapses.
- The collapsed material is washed away and a new wave-cut notch starts to form
- Repeated collapsing results in the cliff retreating.
- A wave-cut platform is the platform that's left behind as the cliff retreat.
Example of a Wave-cut Platform: Kimmeridge on the Dorset Coastline
- The prevailing wind blows waves carrying sediment into the beach at an angle,
- the waves break on the shore and as the water runs back into the sea it carries the sediment back down the beach,
- perpendicular to the angle of the shoreline under the influence of gravity.
- This results in a zigzag motion as sediment is transported along the coastline.
Headlands and Bays (coasts)
A discordant headland is one of alternating bands of hard (resistant) and soft (less resistant) rock.
The less resistant rock is eroded quickly and forms a bay (a low-lying inlet of land)
The more resistant rock is eroded more slowly and results in a headland (A high area of land that extends out into the sea)
Swanage Bay is an example of a headland and bay
Crack, Cave, Arch, Stack, Stump
- Cracks are widened in the headland through the erosional processes of hydraulic action and abrasion.
- As the waves continue to grind away at the crack, it begins to open up to form a cave.
- The cave becomes larger and eventually breaks through the headland to form an arch.
- The base of the arch continually becomes wider through further erosion, until its roof becomes too heavy and collapses into the sea. This leaves a stack (an isolated column of rock).
- The stack is undercut at the base until it collapses to form a stump.
Example of an arch: Durdle Door- Dorset Coastline
Example of a stack: Old Harry Rock- Dorset Coastline
- Form where there is a good supply of sand and an onshore wind
- sand is trapped by marram grass, which gets buried and regrows through the dune
Beaches are found on coasts between the high water mark and the low water mark
formed by constructive waves
- flat and wide
- long gentle slope
- shingle beaches are steep and narrow
- steep slope
Spits, Bars, Tombolos
- Spits occur at sharp bends in the coastline e.g. at a river mouth
- Longshore drift transports sand and shingle past the bend and deposits it in the sea.
- Strong winds and waves can curve the end of the spit (forming a recurved end)
- The sheltered area behind the spit is protected from waves- lots of material accumulates here meaning plants can grow.
- A bar forms when a spit joins two headlands together
- a lagoon can form behind the bar.
A tombolo is a spit extending out to an island.
Example of a spit/tombolo- Chesil Beach on the Dorset Coastline
- A wall made out of a hard material like concrete to reflect waves back to the sea
- It prevents erosion of the coast.
- It also acts as a barrier to prevent flooding
- It creates a strong backwash which erodes the wall
- Expensive to build and maintain
- A wall of wire cages filled with rocks usually built at the foot of cliffs.
- The gabions absorb wave energy and reduce erosion
- cheap and easy to build
- wire can corrode over time
Hard Engineering (further)
- Wooden/ stone fences built at right angles to the coast
- They trao materials transported by longshore drift
- They create wider beaches which slow the waves- this gives greater protection from flooding and erosion
- Fairly cheap
- starve beach further down the coast, making them narrower
- Narrow beaches dont protect coast well from greater erosion and floods.
- Sand and shingle from elsewhere, added to beach
- create wider beaches which slow the waves- greater protection from erosion and flooding
- Taking material from seabed can kill organisms like coral
- Very expensive
- has to be replenished
- Creating or restoring sand dunes by nourishment, planting vegetation in stabilisation
- Provides a barrier between the land and the sea- wave energy is absorbed to prevent flooding and erosion
- stabilisation is cheap
- protection limited to small area
- nourishment is expensive
Also known as coastal realignment
- This is a cheap option compared to paying for sea defences.
- Creates a salt marsh which can provide habitats for wildlife and a natural defence against erosion and flooding.
- Salt marshes are diverse ecosystems supporting many species.
- Land is lost as it is reclaimed by the sea.
- Affects landowners/ livelihoods of farmers
- Landowners need to be compensated - this can cost between £5,000 - £10,000 per hectare.
Coastal Management Scheme Case Study
Lyme Regis, Dorset
Town is built on unstable cliff, with many properties destroyed or damaged
- Lyme Regis Environmental Improvement Scheme- early 1990s-2015
- £1.4 mil to nail together cliffs
- £22 mil for new sea wall + creation of beach
- £20 mil for wall and nailing to east of town
New beach attracts tourists
defences have withstood stormy winters
increased tourism causes congestion and litter
spoilt natural landscape and disrupts processes
The River Valley
- A long profile is a line representing the river from its source (where it starts) to its mouth (where it meets the sea). It shows how the gradient changes over different courses.
- A cross profile shows a cross-section of a river’s channel and valley at a certain point along the river’s course.
Upper Course- steep gradient- v shaped valley, narrow, shallow channel
Middle Course- medium gradient- gently sloping valley sides, wider, deeper channel
Lower Course- gentle gradient- very wide valley, almost flat, wide,deep channel
- this deepens the valley (and channel), making it V-shaped.
- dominant in the upper course
- High turbulence causes the rough, angular particles to be scraped along the river bed causing intense downward erosion.
- this widens the river valley (and channel) during the formation of meanders.
- it's dominant in the middle and lower courses.
- Hydraulic action- the sheer force of the waves break rock particles away from the river channel
- Abrasion- eroded rocks picked up by the river scrape and rub against the channel, wearing it away.
- Attrition- eroded rocks picked up by the river smash into each other and break into smaller fragments. They become more rounded. Causes particle size to decrease.
- Solution- river water dissolves some types of rock e.g. chalk
Traction- large particles like boulders are pushed along the river bed by the force of the water.
Saltation- pebble-sized particles are bounced along the river bed by the force of the water.
Suspension- Small particles like silt and clay are carried along by the water
Solution- Soluble materials dissolve in the water and are carried along.
Waterfalls/Gorges (river erosion)
- A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of resistant rock (hard rock) positioned over exposed, less resistant rock (soft rock).
- The softer rock is eroded (by hydraulic action and abrasion) more than the hard rock creating a 'step' .
- As water goes over the step it erodes more and more of the softer rock
- A steep drop is eventually created, which is called a waterfall
- The hard rock is eventually undercut by erosion. It becomes unsupported and collapses
- The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock through abrasion creating a deep plunge pool.
- Over time, more undercutting causes more collapses, The waterfall will retreat, leaving behind a steep-sided gorge.
- In the upper course of a river most of the erosion is verticallt downwards. This creates steep-sided, v-shaped valleys.
- The rivers aren't powerful enough to erode laterally- they have to wind around the high hillsides that stick out into their paths on either side.
- The hillsides that interlock with each other as the river winds around them are called interlocking spurs.
Meanders and Oxbow Lakes
- The current is faster on the outside of the bend because the river channel is deeper (there's less friction to slow the water down)
- So more erosion takes place on the outside of the bend,causing undercutting to form river cliffs.
- The current is slower on the inside of the bend because the river channel is shallower (there's more friction to slow the water down)
- eroded material is deposited on the inside of the bend, forming slip-off slopes.
- Erosion narrows the neck of the land within the meander and as the process continues, the meanders move closer together.
- When there is a very high discharge (usually during a flood), the river cuts across the neck, taking a new, straighter and shorter route.
- Deposition will occur to cut off the original meander, leaving a horseshoe-shaped oxbow lake.
- The flood plain is the wide valley floor on either side of a river which occasionally gets flooded.
- When a river floods onto the flood plain, the water slows down and deposits the eroded material that it's transporting. This builds up the flood plain.
- Meanders migrate across the flood plain, making it wider.
- Meanders also migrate downstream, flattening out the valley floor.
- The deposition that happens on the slip-off slopes of meanders also builds up the flood plain.
- Levees are natural embankments along the edges of a river channel.
- During a flood, eroded material is deposited over the whole flood plain.
- The heaviest material is deposited closest to the river channel, because it gets dropped first when the river slows down.
- Over time, the deposited material builds up creating levees along the edges of the channel.
River discharge is the volume of water flowing in a river per second
measured in cumecs- cubic metres per second.
Hydrographs show how the discharge at a certain point in a river changes over time in relation to rainfall.
Peak discharge- the highest discharge in the period of time you're looking at.
Lag time- the delay between peak rainfall and peak discharge.
Rising Limb- the increase in river discharge as rainfall flows into the river
Falling limb- the deecrease in river discharge as the river returns to its normal level.
Lag time happens as most rainwater doesn't land direcly in the river channel
River level increases when discharge increases- more warer in the channel.
Prolonged rainfall- the soil is saturated and cannot absorb water
Heavy rainfall- water arrives too rapidly for infiltration so there's a lot of runoff.
Geology- Impermeable soils like granite and shale mean water cannot infiltrate-runoff is increased
Relief (change in the height of the land)- if the river is in a steep-sided valley, water will reach the river channel much faster as water flows quicker on steeper slopes- discharge increases rapidly
Land use- buildings, tarmac is impermeable so increase runoff
Trees- intercept rainwater which is then evaporated, cutting down trees will increases volume of water that reaches the river.
- huge walls/ artificial lake
- Revervoirs store water, reducing risk of flooding
- Can be used to generate HEP and drinking water
- very expensive
- flood existing settlements
- Eroded material in reservoir means lower land is infertile
- The river's course is straightened- artificial channels made
- Water moves out of an area quicker, reducing flooding
- Flooding may still occur downstream
- more erosion downstream as current is faster
HE flooding further
- Raised walls are built along the river banks.
- Can hold more water so floods less frequently
- severe flooding if they break.
Flood Relief channels
- built to divert water if water levels rise
- discharge is reduced
- Gates mean release of water can be controlled
- increased discharge where relief channel rejoins the river causing flooding in that area
- water level can get too high for relief channel