Flashcards in Rivers, floods and management Deck (194):
What is the difference between a local and global hydrological cycle?
The global hydrological cycle is continuously cycled between the oceans and atmosphere, returning to the oceans when it falls as precipitation. It is a closed system as there are no inputs and outputs. A local (drainage basin) hydrological cycle is an open system: there are inputs and outputs.
What is a river's drainage basin?
The area surrounding the river where the rain falling on the land flows into that river. It is also called the river's catchment.
What is the watershed?
The boundary of the drainage basin. Any precipitation falling beyond this point enters a different drainage basin.
What are the inputs of a drainage basin?
What is precipitation?
Mainly rain, but also snow, hail, dew and frost.
What are the 5 ways water can be stored in a drainage basin?
Interception, vegetation storage, surface storage, groundwater storage and channel storage.
What is interception?
When some precipitation lands on vegetation or other structures before it reaches the soil.
What is vegetation storage?
Water that's been taken up by plants.
What is surface storage?
Water in puddles, ponds and lakes.
What is groundwater storage?
Water stored in the ground, either on the soil or in rocks.
What is channel storage?
The water held in a river or stream channel.
What are the 10 flows and processes?
Surface runoff, throughfall, stemflow, throughflow, infiltration, percolation, groundwater flow, baseflow, interflow and channel flow.
What is surface runoff?
Water flowing over the land.
What is throughfall?
Water dripping from one leaf (or other plant part) to another.
What is stemflow?
Water running down a plant stem or tree trunk.
What is throughflow?
Water moving downhill through the soil.
What is infiltration?
Water soaking into the soil.
What is percolation?
Water seeping down through soil into the water table.
What is groundwater flow?
Water flowing below the water table through permeable rock.
What is baseflow?
Groundwater flow that feeds into rivers through river banks and beds.
What is interflow?
Water flowing downhill through permeable rock above the water table.
What is channel flow?
Water flowing in the river or stream. Also called the river's discharge.
What are the outputs of a drainage basin?
Evaporation, transpiration, evapotranspiration and river discharge.
What is evaporation?
Water turning into water vapour (from a liquid to a gas).
What is transpiration?
Evaporation from plant leaves.
What is evapotranspiration?
Evaporation and transpiration together.
What is potential evapotranspiration?
The amount of water that could be lost by evapotranspiration.
What is the water balance?
The water balance shows the balance between inputs and outputs.
What is the water balance like in wet seasons?
Precipitation exceeds evapotranspiration, creating a water surplus.
What is the water balance like in dry seasons?
Precipitation is lower than evapotranspiration, creating a water deficit.
What is river discharge?
The volume of water that flows in a river per second (measured in cumecs).
What is river discharge affected by and how?
Precipitation (the more precipitation, the higher the discharge), hot weather (the higher the temperature, the lower the discharge because of the increased rate of evaporation), removal of water from the river/abstraction (reduces the discharge.
What do hydrographs show?
River discharge over time.
What do storm hydrographs show?
River discharge around the time of a storm event.
What is the peak discharge on a hydrograph?
The highest point on the graph, where the river discharge is at its greatest.
What is the lag time on a hydrograph?
The delay between the peak rainfall and peak discharge.
What is the rising limb on a hydrograph?
The part of the graph up to peak discharge.
What is the falling limb on a hydrograph?
The part of the graph after peak discharge.
What is a flashy hydrograph?
A hydrograph that has steep, roughly symmetrical rising and falling limbs.
How do drainage basin characteristics affect lag time and peak discharge?
Larger drainage basins can catch more precipitation, so they have a higher peak discharge compared to smaller basins. Smaller basins generally have shorter lag times because precipitation has less distance to travel, so reaches the main channel quicker. Steep-sided drainage basins have shorter lag times than shallower basins - water flows more quickly downhill into the river on steep slopes. This can also increase peak discharge. Circular basins are more likely to have a flashy hydrograph than long, narrow basins. This is because all points on the watershed are roughly the same distance from the point of discharge measurement. This means lots of water will reach the measuring point at the same time, increasing peak discharge. Basins with lots of streams (high drainage density) drain quickly, so have shorter lag times.
How does the amount of water already present in the drainage basin (antecedent moisture) affect lag time?
If the ground's already waterlogged (the soil can't absorb any more water) then infiltration is reduced and surface runoff increases. Surface runoff is much faster than throughflow or baseflow, so rainwater reaches the river more quickly, reducing lag time.
How does the rock type affect lag time and peak discharge?
Impermeable rocks don't store water or let water flow through them. This reduces infiltration and increases surface runoff, reducing lag time. Peak discharge also increases as more water reaches the river in a shorter period.
How does the soil type affect lag time and peak discharge?
Sandy soils allow a lot of infiltration, but clay soils have very low infiltration rates. Low infiltration rates increase surface runoff, reducing lag time and increasing peak discharge.
How does vegetation affect lag time and peak discharge?
Vegetation intercepts precipitation and slows its movement to the river channel, increasing lag time. The more vegetation there is in a basin, the more water lost (through transpiration and evaporation directly from the vegetation) before it reaches the river channel, reducing peak discharge.
How does precipitation affect peak discharge?
Intense storms will generate more precipitation and so greater peak discharges than light rain showers. The type of precipitation also affects lag time - e.g. snow that's fallen in a winter storm can melt and flow into the river in spring, giving it a very long lag time.
How does temperature affect lag time and peak discharge?
Hot, dry conditions and cold, freezing conditions both result in hard ground. This reduces infiltration and increases surface runoff, reducing lag time and increasing peak discharge. High temperatures can increase evapotranspiration, so less water reaches the river channel, reducing peak discharge.
How can human activity affect the hydrograph?
In urban areas, much of the soil is covered with man-made impermeable materials like concrete. Water can't infiltrate into the soil, which increases surface runoff, so water flows more quickly into the river. This makes the lag time short and increases peak discharge. Man-made drainage systems affect the hydrograph in a similar way. Water flows down drains into the river before it can evaporation or infiltrate into the soil, causing a shorter lag time and increased peak discharge.
What does headward erosion do and where does it happen?
Headward erosion makes a 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).
What does vertical erosion do and where does it happen?
Vertical erosion deepens the river and happens in the upper stages.
What does lateral erosion do and where does it happen?
Lateral erosion makes the river wider and it happens in the middle and lower stages of a river.
What are the 5 ways in which river erosion happens?
Hydraulic action, abrasion (corrasion), attrition, cavitation and corrosion (solution).
What is hydraulic action?
The pressure of the water breaks rock particles away from the bed and banks.
What is abrasion (corrasion)?
Eroded pieces of rock in the water scrape and rub against the bed and banks, removing material.
What is attrition?
Eroded rocks smash into each other and break into smaller fragments, their edges also get rounded off as they rub together. Attrition doesn't erode the bed and banks, just makes the particles of rock in the river smaller and more rounded.
What is cavitation?
Air bubbles in turbulent stretches of water implode causing shockwaves that break pieces of rock off the bed and banks.
What is corrosion (solution)?
The dissolving of rock by chemical processes. Carbon Dioxide dissolves in water to form a weak acid, which reacts with rocks like limestone and chalk, breaking them down.
What are the 4 ways that eroded material can be transported in a channel?
Solution, suspension, saltation and traction.
What is solution?
Substances that can dissolve are carried along in the water.
What is suspension?
Very fine material is whipped up by turbulence and carried along in the water.
What is saltation?
Larger particles are too heavy to be carried in suspension. Instead, the force of the water causes them to bounce along the river bed.
What is traction?
Very large particles are pushed along the river bed by the force of the water.
What is the river's bedload?
Material transported by traction or saltation.
How can the speed and energy of a river can be reduced?
Reduced rainfall causes lower discharge, which means the river slows down and has less energy. Increased evaporation or abstraction also causes lower discharge Friction reduces the speed of the river, reducing its energy. When the river is forced to slow down due to a narrow section of the channel, it loses energy. A lot of energy is lost when the river meets the sea (the sea absorbs the energy).
What is the capacity of a river?
The total load that a river can transport at a given point.
What is the competence of a river?
The maximum particle size that a river is capable of transporting at a given point.
What does the Hjulstrom Curve show?
The relationship between river velocity and competence. It also shows how the process of erosion, deposition and transportation vary with river velocity.
What does the critical erosion velocity curve show?
The minimum velocity needed for the river to pick up (erode) and transport particles of different sizes in suspension or as bedload. It takes a higher velocity to erode material than it does just to transport it.
What does the mean settling velocity curve show?
The velocities at which particles of different sizes are deposited.
Why does the graph show that it requires a higher velocity to erode silt and clay than it does sand if sand is larger?
Silt and clay particles stick together more and are harder to dislodge, so more energy (velocity) is required to erode them.
What is the long profile?
A long profile shows you how the gradient of the river changes from source to mouth by showing the height of the river bed above the base level.
What is the base level?
The lowest point a river can erode to (usually sea level).
What is a graded profile and why would it happen?
Because the total amount of erosion and deposition is balanced, a graded profile is the idea that over time the long profile will change from being uneven to a smooth curve (but it hardly ever happens).
What is the energy like in the upper stage of a river?
Because the river is high above sea level, there is lots of potential energy.
What is the energy like in the middle stage of a river?
Potential energy is converted into kinetic energy and the river guns velocity.
What is the energy like in the lower stage of a river?
There's very little potential energy, but lots of kinetic energy, and so it flows faster.
How do the velocity and discharge change as you go downstream?
They both increase.
What affects river velocity?
Gradient, discharge and channel characteristics (shape and roughness).
What is a river's kinetic energy used for?
Most of it is used to overcome friction, the rest causes erosion.
What is an efficient river and what are the properties of one?
The more energy available for erosion, the more efficient a river is. The properties of an efficient river are high velocity, high discharge and little friction.
How do you measure the efficiency of a river?
By calculating the hydraulic radius. The larger the hydraulic radius the more efficient a river is. Hydraulic radius = cross section area/length of wetted perimeter.
What is the hydraulic radius?
The total length of the river banks and bed that are in contact with the water.
How does channel roughness affect the efficiency of a river?
Protruding banks and large, angular boulders on the river bed increase the wetted perimeter and cause more friction. This reduces efficiency, velocity and discharge. Channel roughness causes turbulence, which is more effective at picking up particles from a river bed, causing greater erosion.
What is erosion like in the upper stage of a river?
The erosion is mainly vertical and by abrasion (and a bit of hydraulic action). The rough channel causes turbulence and the large, angular bedlam being dragged along the river bed causes intense vertical erosion.
What is erosion like in the middle stage of a river?
The erosion is mainly lateral and by abrasion. Attrition of larger particles in this stage means that sediment particle size decreases from source to mouth.
What is erosion like in the lower stage of a river?
Although velocity and discharge are highest in this stage, there's less erosion because turbulence is lower and sediment particle size is reduced. Some lateral erosion occurs.
What is transportation like in the upper stage of a river?
Mainly large particles carried by traction or saltation.
What is transportation like in the middle stage of a river?
More material carried in suspension as particle size decreases.
What is transportation like in the lower stage of a river?
Mainly smaller particles carried by suspension or substances carried in solution.
What is deposition like in the upper stage of a river?
There's little deposition, mainly larger particles deposited as energy levels drop.
What is deposition like in the middle stage of a river?
Sand and gravel are deposited across the flood plain as the river floods and friction reduced the river's energy.
What is deposition like in the lower stage of a river?
Smaller particles are deposited on the flood plain when the river floods and in the river mouth as the sea absorbs river energy.
What is the cross profile?
The cross profile shows you what a cross-section of the river channel or river valley looks like.
What is the cross profile of the upper stage of a river like?
A steep V-shape.
What is the cross profile of the middle stage of a river like?
A wider valley with a flood plain from deposition.
What is the cross profile of the lower stage of a river like?
Wide with gently sloping sides, with a much wider flood plain from deposition.
How are waterfalls formed?
Waterfalls form where a band of hard rock meets softer rock. The 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 lack of friction as it drops over the step. This increase in speed gives water greater erosive power, causing further erosion of the soft rock and undercutting of the hard rock. As the harder rock is undercut, it can collapse. A deep plunge pool is carved out by abrasion at the foot of the waterfall as the bits of collapsed rock are swirled round by turbulence. Over time, more undercutting causes more collapse. The waterfall will retreat, leaving behind a steep-sided gorge.
How are potholes formed?
Potholes are small, circular hollows in the river bed. They're formed by abrasion as turbulence swirls a river's bedload round in a circular motion, causing it to rub and scrape out holes.
How are rapids formed?
Rapids are relatively steep section of river with turbulent flow where there are several sections of hard rock. They're a bit like mini waterfalls.
How are meanders formed?
Meanders form where alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. The distance between pools is 5-6 times the width of a river bed. Because the river channel is deeper in pools it's more efficient, so it has greater energy and more erosive power. Energy is lost as the river flows over a riffle due to friction. The spacing and 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. Turbulence increases in and around pools as the water speeds up, so the flow of the water begins to twist and coil. This causes corkscrew-like currents in the river called helicoidal flow, which spiral from bank to bank between pools. The helicoidal flow causes me erosion and deepening of the pools. It also causes eroded material to be deposited on the inside of the next bend, where the river loses energy. The combination of erosion and deposition exaggerates the bends until large meanders are formed. The combined processes also create the meanders' distinctive asymmetrical cross-section. Oxbow lakes are formed when the neck of the loop of a meander is broken through, often during flooding. Deposition dams off the loop, leaving an oxbow lake.
How is braiding formed?
Braiding occurs when rivers are carrying a vast amount of eroded sediment. If the river's velocity drops, or the sediment load becomes too much for the river to carry, sediment is deposited in the channel. This causes the river to divide into many small, winding channels that eventually rejoin to form a single channel.
How are flood plains formed?
When a river overflows its banks and floods the flat land either side of the river (the flood plain), there's an increase in the wetted perimeter and reduction in the hydraulic radius. This increases friction, reducing the velocity of the river and causing fine silt and sand to be deposited across the flood plain.
How are levees formed?
Levees are natural, raised embankments formed as a river overflows its banks. During a flood, material is deposited across the whole flood plain as the river loses velocity and energy due to increased friction. The heaviest material is dropped first, closest to the river channel. Over time, this material builds up on the river bank, creating a levee.
How are deltas formed?
When a river reaches the sea, the energy of the river is absorbed by the slower moving water of the sea. This causes the river to deposit its load. These deposits build up on the sea bed, until the alluvium (deposited sediment) rises above sea level, partially blocking the mouth of the river. The river has to braid into several distributaries in order to reach the sea, forming a delta.
What is rejuvenation and how can it be caused?
A river is said to be rejuvenated if its base level is lowered. This can be caused either by the ground level rising (crustal uplift) or by a drop in sea level.
How does rejuvenation change the long profile?
The drop in base level gives the river greater potential energy, increasing its vertical erosion potential. The long profile of a river is extended and a knickpoint (a sharp change in gradient, often a waterfall) will form and mark the junction between the original long profile and the new one.
What 2 landforms are associated with rejuvenation?
River terraces and incised meanders.
How are river terraces formed?
River terraces are former flood plains which have been left above the level of present-day flooding following increased vertical erosion.
How are incised meanders formed?
Incised meanders are formed when a river keeps its meandering course as vertical erosion increases. The result is a deep, winding valley with steep sides. The river is left far below the level of the former flood plain.
What are the main reasons for flooding?
Heavy and prolonged rainfall.
What are the 5 physical factors that increase the risk of flooding?
Sparse vegetation/deciduous trees, impermeable ground, circular drainage basin, high drainage density and steep slopes.
How does sparse vegetation or deciduous trees increase the risk of flooding?
Sparse vegetation the drainage basin means little rainfall is intercepted, so more rain reaches the ground, this increases the volume of water reaching the river, which increases discharge. Deciduous trees have no leaves in winter, which has the same affect as sparse vegetation - little rainfall is intercepted.
How does impermeable ground increase the risk of flooding?
Impermeable ground doesn't allow infiltration of surface water. This increases surface runoff, which increases discharge. If the ground has been baked hard by the heat of the summer or its frozen, the same thing happens.
How does a circular drainage basin increase the risk of flooding?
Water draining into the main river channel will all arrive in a short space of time because all points in the basin area similar distance from the river. This increases discharge.
How does a high drainage density increase the risk of flooding?
Drainage basins with a high drainage density (lots of streams) drain quickly, so have short lag times. Lots of water flows from the streams into the main river in a short space of time, increasing discharge.
How do steep slopes increase the risk of flooding?
Water will reach the river channel much faster because water flows more quickly on steeper slopes. This increases discharge.
What are the 5 human factors that increase the risk of flooding?
Urbanisation, deforestation, flood management strategies, agriculture and climate change.
How does urbanisation increase the risk of flooding?
Urban areas have large areas of impermeable tarmac and concrete, so when it rains surface runoff is very rapid. Gutters and drains quickly take runoff to rivers. Both of these things reduce lag time and so increase discharge.
How does deforestation increase the risk of flooding?
Clearing trees and plants reduces interception and evapotranspiration. This increases the volume of water that reaches the channel, which increases the discharge. Deforestation leaves the soil loose. The soil is eroded by rainwater and carried to the river, which raises the river bed. This reduces the channel capacity, so it takes less water for the river bed to flood.
How do flood management strategies increase the risk of flooding?
Flood management strategies can actually end up making flooding worse. For example, if dams fail they release a huge volume of water all at once - giving a huge increase in discharge.
How does agriculture increase the risk of flooding?
Overgrazing leaves areas with less vegetation, it reduces interception and evapotranspiration. This increases the volume of water that reaches the channel, which increases the discharge. Overgrazing and ploughing also increase soil erosion - the soil is eroded by rainwater and carried to the river, which raises the river bed. This reduces the channel capacity, so it takes less water for the river bed to flood.
How does climate change increase the risk of flooding?
Climate change could cause an increase in rainfall and more storms in some areas, which could increase flooding.
What are the social impacts of flooding?
People and animals can be killed. Floodwater is often contaminated with sewage, which can lead to a lack of clean drinking water. Contaminated water can also put people at risk of diseases (e.g. diarrhoea). Possessions can be damaged by floodwater or lost. People can be made homeless as their properties are inundated or damaged.
What are the economic impacts of flooding?
Businesses often have to shut down as premises are inundated and power supplies are affected. Rescue work and repairs are usually costly. Insurance premiums go up after floods. Unemployment levels often rise as businesses shut down because they can't recover from the flooding. Public transport, roads and bridges can be destroyed. Crops can be destroyed, this can lead to a rise in the price of food.
What are the environmental impacts of flooding?
Floodwater contaminated with sewage and rubbish can pollute rivers. River banks are eroded. River sediment is deposited on the flood plain, this makes the land more fertile. Wetlands can be created e.g. marshes and ponds , which are habitats for many species.
Are social impacts of flooding usually worse in poorer or richer countries?
The social impacts are usually higher in poorer countries because flood defences are poorer, people are less able to evacuate, sanitation systems aren't as good and buildings are of a poorer quality.
Are economic impacts of flooding usually worse in poorer or richer countries?
The absolute economic impact is usually higher in richer countries as they have more high value buildings and infrastructure. However, the relative economic impact is usually higher in poorer countries - the buildings and crops that are damaged are worth less money, but this affects the economy more because they have less money to recover from it.
What is the flood return interval?
By keeping records over many years, people can predict how often a flood of a certain magnitude may occur.
What is the flooding case study for a poorer country?
South Asia, July & August 2007.
Why does South Asia flood most years?
South Asia has a monsoon climate - 80% of rain falls in 4 months. Much of South Asia is low-lying land, particularly Bangladesh where 90% of land is less than 10m above sea level. Melting snow and ice from the Himalayas in the late summer months increase the Brahmaputra River discharge.
What physical factors contributed to the 2007 South Asia floods?
The monsoon came suddenly after a very dry, early summer. There was heavy rainfall - Assam had a record 1.69.5mm in 24 hours in July, and 900mm in total for July. The long duration of heavy rainfall completely saturated the soil, increasing surface runoff and increasing discharge. The peak discharges of the River Ganges and Brahmaputra coincided, which increased the river discharge downstream.
What human activities worsened the 2007 South Asia floods?
Deforestation in Nepal and the Himalayas meant less rainfall was intercepted, which increased discharge. The growth of urban areas, due to migration, also increased surface runoff. Collapse of old earth dams in Madhya Pradesh, India, caused further flooding.
What were the social impacts of the 2007 South Asia floods?
Over 2000 people died, due to people being reluctant to evacuate (as they'd have to leave their land and livestock unattended) and many children drowned because they couldn't swim. Poor transport links meant evacuation was slow. As wells became polluted with sewage, there was a lack of clean drinking water, and over 100,000 caught water-borne diseases. An estimated 25 million people were made homeless. 112,000 houses were destroyed in India, as porous mud bricks became saturated by floodwater. Dhaka (Bangladesh's capital) was inundated, especially the poorer districts and shanty towns near the river. Children lost out on education as 4000 schools were affected and 44 schools were totally destroyed.
What were the economic impacts of the 2007 South Asia floods?
The cost of the flood was estimated at $1 billion, including damage to crops and property. Factories were closed around Dhaka, due to flood damage and loss of raw materials, many of the poorest workers became unemployed. There was a widespread loss of livestock, and since 80% of Bangladeshis rely on agriculture, many lost their livelihoods. 550,000 hectares of land couldn't be planted with rice at peak time, because of flooded fields. A lower rice crop meant the world price of basmati rice rose by 10%. 10,000km of roads were destroyed. Debt increased, both individually (e.g. farmers borrowed money for food and seeds) and nationally (e.g. governments imported food and medicine).
What were the environmental impacts of the 2007 South Asia floods?
The flood deposited fertile silt on the flood plain. Rivers were polluted with sewage.
How did human factors make the impacts worse?
Bangladesh is a poor country so there aren't many flood defences or flood warning systems in place. Low incomes, few savings and little insurance limited people's ability to recover after the flood. Corrupt government officials diverted aid money away from people in the most need.
What is the flooding case study for a richer country?
Carlisle, 8th January 2005.
Why is the River Eden prone to flooding?
It has a very large drainage basin, so it catches a lot of rainfall, leading to a high river discharge. Some parts of the basin have steep sides, so water runs quickly down into the river. There are many streams that drain quickly into the river, making the lag time short.
What is the flood return interval for the 2005 Carlisle flood?
What physical factors contributed to the 2005 Carlisle flood?
There was heavy rainfall on the 6th of January for 36 hours. 200mm of rainfall was recorded, which was the equivalent of four months rain. Rain fell on saturated ground so the water ran straight off into the river. This causes a very high peak discharge (over 1500 cumecs, compared to an average discharge of 52 cumecs).
What human activities worsened the 2005 Carlisle flood?
Carlisle is a large built-up area, with impermeable concrete and tarmac surfaces, and little soil or vegetation. This meant there was little infiltration of rainfall and high surface runoff, which increased discharge. Drains and sewerage systems overflowed in some areas - becoming a source of flooding themselves. 25% of the flooding problems were associated with overflowing drains.
What were the social impacts of the 2005 Carlisle flood?
Three people died. Over 3000 people were made homeless for up to a year and thousands of personal possessions were damaged. Living in temporary accommodation disrupted lives in many ways, e.g. travel arrangements were disrupted, people were separated from community networks and friends, and they had problems receiving post. Children lost out on education as four schools were severely flooded. There was an increase in stress-related illnesses following the floods.
What were the economic impacts of the 2005 Carlisle flood?
It took about a year to repair the damage to homes and repairs cost about £100 million. 350 businesses had to shut down as there was no electricity, telephone service or transport. Trade activities from Carlisle railway station were suspended. United Biscuits, the largest employer in Carlisle, was flooded with over 3m of water that caused over £5 million damage. 33 out of 1100 employees lost their jobs. 70,000 addresses had no power. The sewage works, police station, fire station and council offices were severely flooded. 80 buses were destroyed. Many roads and bridges were damaged.
What were the environmental impacts of the 2005 Carlisle flood?
The flooding increased river bank erosion in some areas. Rivers were polluted with rubbish and sewage.
What is the aim of flood management?
To protect homes, business and the environment from flooding. This is because flooding can have severe social, economic and environmental impacts.
What is cost-benefit analysis used for?
Choosing which places are protected and how is done using cost-benefit analysis, as there isn't enough money to protect everywhere from flooding. Large settlements and important industrial sites are more likely to be protected than small settlements and farmland.
What is hard engineering?
Man-made structures that reduce flooding.
What are the general disadvantages of hard engineering?
They're expensive to build and maintain, and need technical skill. Poorer countries often can't afford these flood defences. Floods happen less often, but they can be more hazardous if they do happen (e.g. if a dam breaks then a huge amount of water will rapidly flood the land. Natural processes are disrupted (e.g. crops don't get fertile silt from river sediment during low level flooding. Some people think they are ugly.
What are the 4 most common types of hard engineering?
Dams, channel straightening, levees and diversion spillways.
What are dams and how do they work?
Dams are huge walls built across rivers. A reservoir is formed behind the dam. Flood water is caught behind the dam, which prevents flooding downstream. The water is released as a steady flow throughout the year.
What are the advantages of dams?
Turbines are often built into the dams, which generate electricity. Steady water release allows irrigation of the land below the dam throughout the year. People can use the reservoir for recreational activities (e.g. sailing).
What are the disadvantages of dams?
They're very expensive. Land is flooded when a reservoir is created, this often destroys farmland and forces people to move elsewhere. They affect wildlife (e.g. they can affect salmon migrating upstream to breeding grounds). They trap sediment normally carried in rivers. This can cause the dam to fail, it can also increase erosion downstream, as there's less protective sediment being deposited.
What is channel straightening and how does it work?
Channel straightening is where meanders are removed by building artificial cut-throughs. This makes the water flow faster, which reduces flooding because water drains downstream more quickly and doesn't build up to a point where the river channel can't contain it anymore.
What are the advantages of channel straightening?
It takes less time to navigate the river because it has been made shorter.
What are the disadvantages of channel straightening?
Flooding may happen downstream instead, as flood water is carried there faster. More erosion occurs downstream because the river flows faster. Altering river channels disturbs wildlife habitats.
What are levees and how do they work?
Levees are embankments built along rivers. The river can hold more water without overflowing and so it floods less often.
What are the advantages of levees?
The allow the flood plain to be built upon.
What are the disadvantages of levees?
They're quite expensive. There's a risk of severe flooding if the levees are breached.
What are diversion spillways, how do they work and what are the advantages?
Diversion spillways are channels that take water elsewhere if the water level in the river is too high. Water is normally diverted around an important are or another river. They prevent flooding because river discharge is reduced. The spillways often have gates that can be opened, so the release of water can be controlled.
What are the disadvantages of diversion spillways?
An increase in discharge when the diverted water joins another river (or rejoins the same one) could cause flooding below that point. If spillways are overwhelmed, water will flood areas not used to flooding, which could cause even bigger problems.
What is soft engineering?
Soft engineering defences use knowledge of the whole river basin and processes, to try to work with nature.
What are the general advantages of soft engineering?
They're cheaper to maintain than hard engineering defences. Flooding is more predictable, reducing the risk of an unexpected disaster. They can improve opportunities for recreation, such as fishing. Some people think they're more attractive than hard engineering.
What are the 5 most common types of soft engineering?
Land use management, wetland and river bank conservation, river restoration, alteration of urban surfaces and weather forecasts and flood warnings.
What is land use management and how does it work?
Planning restrictions prevent buildings or roads being constructed on the flood plain. Use of the flood plain is restricted to things like playing fields, allotments or parks. More water can infiltrate so there's less surface runoff, which reduces discharge and flooding.
What are the advantages of land use management?
There are no new buildings or roads on the flood plain to be damaged, so the impact of any flooding is reduced. It provides recreational opportunities, e.g. football fields.
What are the disadvantages of land use management?
It restricts development, this is especially a problem when there's a shortage of housing. It can't be used in areas that are already urbanised.
What is wetland and river bank conservation and how does it work?
Wetlands store floodwater and also slow it down, reducing flooding downstream. So conserving or re-establishing wetlands gives natural protection from flooding. Planting trees and shrubs along the river banks increases interception and lag time, therefore reduces discharge and in turn flooding.
What are the advantages of wetland and river bank conservation?
Vegetation protects the surface soil from erosion. The vegetation provides habitats for wildlife.
What are the disadvantages of wetland and river bank conservation?
Less land is available for farming.
What is river restoration and how does it work?
River restoration involves making the river more natural, e.g. by removing man-made levees. The flood plain can then flood naturally. As the water spreads out over the flood plain the river's discharge is reduced (because less water is in the channel), which reduces flooding downstream.
What are the advantages of river restoration?
Little maintenance is needed, as the river is left in its natural state. The river provides a better habitat for wildlife.
What are the disadvantages of river restoration?
Local flood risk can increase, especially if nothing's done to prevent major flooding.
What is alteration of urban surfaces and how does it work?
Building porous pavements or soakaways increases infiltration, which reduces rapid surface runoff to the river channel. This increases lag time, which reduces discharge and flooding.
What are the advantages of altering urban surfaces?
Any pollutants in the water are filtered out by the soil before the water reaches the channel.
What are the disadvantages of altering urban surfaces?
What are weather forecasts and flood warnings, how do they work and what are the advantages?
The environment agency monitors weather forecasts, rainfall and river discharge. They warn people about possible floods through TV, radio, newspapers and the internet. This means people can evacuate before the flood happens, saving lives. People can also move possessions and use sandbags to help reduce damage if flooding occurs.
What are the disadvantages of weather forecasts and flood warnings?
Some people might not be able to access the communication network. Flash floods can happen too fast for warnings. People may ignore warnings if they were inaccurate in the past.
Why is soft engineering more sustainable than hard engineering?
Because it has a lower economic cost and environmental impact. Hard engineering is often expensive and disrupts natural processes. Soft engineering tends to be cheaper and requires much less time and money to maintain. Soft engineering is designed to work with the natural environment and it creates areas like wetlands, which are important habitats for wildlife.
What is the Yangtze river?
A river that runs through China, it is the third longest river in the world.
Why is flooding common around the Yangtze?
China has a rainy season that lasts from about June until August.
What does flooding of the Yangtze cause issues?
There is lots of farmland and loads of major cities along the river.
When were the last 5 major floods of the Yangtze?
1931, 1935, 1949, 1954 and 1998.
Why was the flood of 1954 so bad (Yangtze river)?
It covered nearly 200,000km^2 of land and killed over 33,000 people. Over 18 million people had to move. The city of Wuhan was covered for over 3 months.
What hard engineering does the Yangtze river have?
There are 3600km of levees along the middle and lower parts of the river. There are 46 dams that are planned or under construction.
What is the Three Gorges Dam?
The biggest dam on the Yangtze river.
What are the features of the Three Gorges Dam?
It is 101m high. A reservoir is building up behind the dam that can store 22km^2 of floodwater. It is also the biggest hydroelectric power station in the world, the flood water turns 26 turbines built into the dam. Locks have been built alongside the dam so ships can get past it.
What problems have levees caused?
In the 1998 floods many levees broke, which contributed to devastating flooding. After this, many levees were reinforced, they were effective at reducing flooding in the 2002 floods.
What are the positive affects of the Three Gorges Dam?
It's thought that the dam has reduced major flooding from once every 10 years, to once every 100 years. The turbines in the dam produce a lot of electricity - enough to supply about 3% of China's demand. The reduction in flooding has made it much safer to navigate up the Yangtze. River shipping has also increased as bigger ships can now travel up the river because the reservoir is deeper than the old river.
What are the negative effects of the Three Gorges Dam?
People have had to relocate as the water level in the reservoir has risen. It's thought that up to 2 million people will have to relocate by the time it's full - submerging 13 cities and 1352 villages. The reservoir will also flood farmland, 657 factories and 1300 sites of cultural and historic interest - the Temple of Zhang Fei will be submerged. A huge amount of sediment is usually carried down the Yangtze river, the dam will trap the sediment, which could lead to failure of the dam and cause catastrophic flooding. The dam could destroy habitats and endanger species - fewer than 100 baiji dolphins are left in the Yangtze and the dam could reduce their food supply. Rising water levels in the reservoir will increase flooding along the tributaries that lead into it, the increased water levels in the tributaries will also increase erosion of riverbanks, causing collapses and landslides.
Why is Abingdon in the UK at risk of flooding?
It was built on the flood plains of the River Thames and Ock.
When were the 7 most recent floods?
1947, 1968, 1977, 1979, 1992, 2000 and 2007.
Why was the flood of 2007 so bad (River Thames and Ock)?
The flash floods caused the River Thames and Ock to burst their banks, flooding 660 properties in Abingdon.
Why have hard engineering defences been rejected for parts of the River Thames and Ock that run through Abingdon?
Diversion spillways to transport Ock floodwater south of Abingdon was too expensive, and flood barriers to protect properties along the Ock would increase flood risk downstream.
What soft engineering has taken place in Abingdon?
Gravel soakaways have been built along the A34 road. Low value land is allowed to flood (e.g. Tilsley Park sports ground), There are planning restrictions on new housing developments built on the Ock flood plain, stating they must have improved drainage systems. Tesco were forced to revise recent extension plans - they had to add drainage improvements such as soakaways and permeable tarmac. The Environment Agency's Local Flood Warning Plan warns specific areas at risk and provides a 24 hour Floodline. There are restrictions on land use, e.g. planning permission was refused for buildings on the Thames flood plain. There's detailed advice on the internet about reducing flood damage. Local voluntary flood wardens communicate advice and warnings. Improvements have been made to riparian buffers along smaller rivers. Planting trees reduces the volume of water reaching the Thames and Ock rivers where the flood problem is greater.