Bangladesh (LEDC Flooding)
River Thames and the Thames Barrier
The components of the water (hydrological ) cycle
Flooding in LEDCs and MEDCs (causes and consequences)
BoscaStle (MEDC River Flooding)
Blakeney Point (spit formation)
River landforms - Meanders, - ox-bow lakes - and waterfalls
Holderness Coast (coastal erosion and protection)
River processes (erosion, transportation and deposition)
GEOGRAPHY DEPARTMENT
Pevensey Bay (sustainable coastal management)
Coastal conflicts and protection
Coastal landforms - Spits, caves, arches , stacks and wave-cut platforms
Coastal processes (erosion, transportation and deposition)
TOPIC OVERVIEW
Source = the beginning of a river Watershed = it is in an invisible boundary around an area of land drained by a river Tributary = a smaller channel that flows into the main channel
GEOGRAPHY DEPARTMENT
Confluence = where two channels meet Mouth = where the river meets the sea
INPUTS: PRECIPITATION
FLOWS: SURFACE RUNOFF THROUGH FLOW INFILTRATION PERCOLATION
STORES VEGETATION (PLANTS) SOIL RIVER CHANNEL ATMOSPHERE Precipitation = any moisture that hits the earths surface e.g. rain, sleet, snow etc Evaporation = the process by which liquid water transforms to gaseous water vapour
OUTPUTS EVAPORATION EVAPOTRANSPIRATION CHANNEL FLOW
Transpiration = the movement of water from the soil store back to the atmosphere as a result of biological processes in plants Evapotranspiration = the combined effect of evaporation and transpiration in moving water from the soil store back to the atmosphere Interception = when precipitation is caught by vegetation Infiltration = the process by which water moves from the soil surface into the soil store Percolation = the vertical movement of water through the soil layers
Throughflow = the horizontal movement of water through the soil layers to the river channel Surface runoff = when water cannot infiltrate into the soil it travels across the soil surface to the river channel Channel flow = water being carried by river body
GEOGRAPHY DEPARTMENT
What factors influence the main components of the local water cycle? EVAPOTRANSPIRATION
Temperature High temperature = more evapotranspiration
Vegetation
The more vegetation the greater the surface area for water to be lost from so the greater the evapotranspiration .
9
Wind
Greater wind prevents the air from becoming saturated and allows for greater evaporation to
Sunshine More hours of sunshine = more evapotranspiration
occur
THROUGH FLOW
99
999993
The more porous the soil the greater the amount and rate of throughflow
99 3
If humans alter the structure of the soil e.g. through ploughing it or machinery going over it then pores are closed off and no throughflow/baseflow will occur
INFILTRATION
93
3333
Soil thickness- The thinner the soil layer the less water that can be infiltrated as less pores to fill
Rock type – is composed of an impermeable material such as granite then no infiltration will occur. If however it is composed of a permeable material then more infiltration will occur
Land use- if soil is compacted by heavy machinery then pores will be closed off and no infiltration will occur.
SURFACE RUNOFF
99 9999
GEOGRAPHY DEPARTMENT
99
SOIL TYPE if soil is porous then surface runoff will only occur once all its pores are full. If however the soil is non-porous e.g. granite or composed of man-made material such as tarmac then infiltration will not occur and surface runoff will occur instantly and rapidly.
Relief- the steeper the relief the less chance it has to infiltrate and the greater the surface runoff
Soil thicknessthe thinner the soil the smaller its infiltration capacity so it will become saturated quickly resulting in surface runoff
Amount of vegetationthe more vegetation the more interception and the less water reaching the ground for surface runoff to occur.
INTERCEPTION
Seasonalityin winter deciduous trees loose their leaves
Vegetation Type: Deciduous trees have larger leaves so intercept more precipitation then coniferous trees which have smaller leaves. Trees will intercept more then grass as have
Age- the older the tree the taller and the greater the mosaic of branches and leaves it has. This results in more interception.
bigger surface area.
THINK!!!
Why is understanding the water cycle important?
Amount of precipitation - intense rainfall has a battering effect on the ground which inhibits infiltration and results in surface runoff. Prolonged rainfall results in the soil reaching its infiltration capacity and subsequently surface runoff. If precipitation is in the form of snow then no infiltration can occur and instead there will be surface runoff.
AbundanceThe more trees the greater the amount of interception
Discharge: is the volume of water transported by a river in a certain amount of time. Measured in cumecs. A storm hydrograph/flood hydrograph: shows the river’s response to a single rainfall event
GEOGRAPHY DEPARTMENT
Peak discharge= the highest point on the hydrograph when there is the greatest amount of water in the river Falling (recession) limb= shows the amount of discharge decreasing and thus reflecting how rapidly the rivers discharge responds to normal. Lag time = period of time between peak rainfall and peak discharge Approaching segment = the normal water level of the river
Rising limb= shows how quickly the rivers discharge increases after a rainfall event
Peak rainfall = the greatest amount of rainfall during event
Hydrograph A could represent a drainage basin with the following features: Urban area Area of impermeable rock Saturated ground Steep relief After snow melt Deciduous woodland in winter Heavy or prolonged rainfall Hydrograph B could represent a drainage basin with the following features: Permeable rock Rural area with lots of plants Gentle slopes Non-saturated soil Light rainfall and warm temperatures.
THINK!!!
Why are hydrographs important in managing rivers?
EROSION – IS THE WEARING AWAY OF ROCK.
GEOGRAPHY DEPARTMENT
TYPES OF EROSION: HYDRAULIC ACTION -Crack in river banks sealed off by passing river. This traps air within the crack and pressure increases until a mini-explosion occurs and rock crumbles. SOLUTION - Chemicals in water dissolve rock ABRASION - Material hurled at river banks or bed causes it to wear away ATTRITION - Material hits one another and gets smaller and smaller making river bank, cliff more vulnerable to other types of erosion
TRANSPORTATION – IS THE MOVEMENT OF MATERIAL TYPES OF TRANSPORTATION: TRACTION- Large material cannot be picked up so it is rolled along the river bed
SALTATION- Material that is fairly large is only picked up when the river has enough energy. When the river does not it is dropped. This occurs repeatedly and the material appears to be bounced along the bottom. SUSPENSION- When material is light enough to be carried with river current. SOLUTION- When material is dissolved within the river and carried by the current
DEPOSITION – THE LAYING DOWN OF MATEIRAL
Deposition occurs when a river looses energy When a river erodes sideways it is classified as lateral erosion. This is common in meanders. When a river erodes downwards it is classified as vertical erosion
Erosion Transportation
Deposition
Rock type is key. The more resistant the rock (e.g. granite) the slower it will erode. The amount of load being carried depends on: The volume of water- the greater the volume the more load it can carry. The more tributaries the greater the volume of water. The velocity- a fast flowing river has more energy to transport and can move larger particles The rock types- some rocks e.g. shales, are more easily eroded than others e.g. granite Deposition is encouraged by The amount of material carried by the river, a reduction in velocity, an obstruction and a fall in the volume of river water
The river can be divided into three sections: Upper course Middle course Lower course
GEOGRAPHY DEPARTMENT
UPPER COURSE
River features include: steep sided V-shape valleys, interlocking spurs, waterfalls and gorges River characteristics: narrow and shallow Relief: steep gradient Velocity: slow except at waterfalls Bedload: large and angular as not been subjected too much erosion. MIDDLE COURSE
River features include: U-shape valleys and meanders River characteristics: fairly wide and fairly deep Relief: becoming less steep Velocity: increasing Bedload: becoming smaller and rounder as experiencing more erosion.
LOWER COURSE
River features include: floodplains, meanders, levees and ox-bow lakes River characteristics: wide and deep Relief: gentle Velocity: fast Bedload: small and round as been subjected to lots of erosion.
THINK!!!
Can you draw a diagram of the three stages of a river from memory and add annotations to highlight each stages key features?
GEOGRAPHY DEPARTMENT
V-shaped valleys and interlocking spurs Vertical erosion creates V-shaped valleys which are steep sided and narrow. The river winds its way around interlocking spurs of hard rock which should not be confused for meanders. No valley floor and gradient is very steep
WATERFALLS A waterfall is a steep drop in the course of a river. A band of hard rock lies over softer, less resistant rock As water flows over the cliff it splashes back causing erosion (abrasion, hydraulic action and scouring) Softer rock eroded more quickly, causing undercutting of the hard rock The harder rock forms an overhang When harder rock can no longer support its weight it collapses Large boulders are deposited at the base of waterfall
The greater power of the water falling to the base moves the material around (attrition), eroding the base into a deep plunge pool The deposited material temporarily offers the base of cliff protection from splash back The fallen rocks become smaller due to attrition and are eventually carried away leaving the cliff exposed once again. The cycle then begins again and the waterfall begins to retreat backwards forming a gorge of recession
Meanders are bends in rivers Meanders form in order for rivers to use up its surplus energy On the outside of a meander the water is deeper and the current flows faster. The force of the water erodes and undercuts the outside bend by corrasion, scouring and corrosion, forming a steep bank called a river cliff On the inside bend the current is less strong which encourages deposition forming a slip-off slope.
An underwater current spirals down the river, carrying the eroded material from the river cliff to the slip-off slope. Lateral erosion on the outside bend of the meander widens the valley floor and erodes away the ends of the interlocking spurs.
GEOGRAPHY DEPARTMENT
*
As the river flows downstream it becomes deeper and wider. This is due to more water reaching the channel thus increasing its velocity and inducing erosion. The meanders become larger and wider and they migrate downstream The erosion (scouring and abrasion) on the outside bend of the meander causes the meander to move laterally thus widening the valley floor At the same time deposition occurs in the inside bend aiding the gradual migration of meanders downstream. This creates a line of river cliffs along the edge of the valley floor
MIGRATING MEANDERS
OX BOW LAKES Erosion occurs on the outside bend as the velocity is greatest there. Deposition occurs in the inside bend where the velocity is slowest. The area between the beginning and end of the meander is referred to as the ‘neck’ The neck of meander narrows as erosion (corrosion, scouring etc) continues on the outside bends Eventually the neck is broken through, creating a straight channel This often happens during a flood when the river is particularly powerful Less water enters the meander as it opts to take the quickest route i.e. through straight channel Low flow in the bend results in the deposition of alluvium (sediment) which seals off the old meander bend and forms an ox-bow lake. Over time the ox-bow lake dries up forming a meander scar.
THINK!!!
Can you (a) identify where you can find a waterfall or a ox bow lake (b) explain what they look like (c) draw a diagram to explain how they have formed (d) write a description of how they have formed using key terms.
GEOGRAPHY DEPARTMENT
FLOOD PLAINS
The flood plain is the wide, flat area of land either side of the river in its lower course The flood plain formed by both erosion and deposition Lateral erosion is caused by meanders and the slow migration downstream to widen the flood plain The deposition on the inside bend of rivers provides sediment to build up the valley floor. When rivers flood further sediment is deposited onto the surrounding land. This occurs because the floodwater present is shallow and velocity is low and this in conjunction with greater friction induces deposition Over thousands of years these deposits build up into great thicknesses of alluvium.
LEVEES Levees are natural embankments of silt along the banks of a river, often several metres higher than the floodplain Levees are found along the lower stretches of the River Mississippi, USA. In many cases the height of the levees are heightened to increase the capacity of river and subsequently prevent flooding. Levees are formed in the following way: THINK!!! Why do you think Levees are a popular form of river management? 1. River in flood- as the water flow slows, energy is lost. Coarser, heavier material is
deposited on the bank and finer material further away 2. River at low flow – during a dry spell the river’s velocity slows down and the
volume falls. This causes the deposits of material on the bed 3. After repeated floods – after many floods the river banks form levees and the bed
may be raised so much that the river rises above the flood plain. This can lead to more flooding.
The River Thames is 215 miles long and is the second longest river in the United Kingdom. Its source is at Trewsbury Mead, Gloucestershire and its mouth, the Thames Estuary, flows into the North Sea. It flows through central London as well as other towns including Oxford, Reading, Henley-on-Thames, Windsor, Kingston Upon Thames and Richmond. SOURCE
GEOGRAPHY DEPARTMENT
MOUTH
UPPER
MIDDLE
LOWER
WATERFALLS Size of Thames drainage basin c.5,250 sq mi (13,600 sq km) MEANDERS
Here are some facts about the River Thames for you...
UPPER COURSE The upper course of the River Thames is that from its source at Trewsbury Mead to Oxford It doesn’t start in large mountains, just hills - the upper valley of the Thames is made of clay soil, through which the river winds and turns constantly in all directions. At Goring Gap the valley narrows, separating the Chiltern Hills from the Berkshire Downs.
MIDDLE COURSE The middle course varies in width considerably from 18 to 80 metres Flows through countryside and villages, winding its way through the historical towns of Oxford, Henley before passing Windsor Castle. Although it generally flows east, it meanders in various directions with the formation of oxbow lakes. It is non-tidal as the Thames is only tidal from West London.
LOWER COURSE The Thames only becomes tidal from London eastwards (non-tidal for 147 miles of its 215 miles – its mouth is at Southend. The lower course of the River Thames is flat. There is a lot of marshland along it. There is also industry on the northern bank. Here the river flows in a deep, wide channel. London is protected by the Thames Barrier, as well as concrete levees to protect its economy.
GEOGRAPHY DEPARTMENT
HARD ENGINEERING: techniques are structures that are built/imposed on the river. They try to control the river.
SOFT ENGINEERING: methods work with the river and use natural processes.
DAMS -trap and store water, then let it out in a controlled way. They’re very expensive, but very effective. FLOOD WALLS - built around settlements. They’re quite expensive and don’t look very natural. But they’re pretty effective.
WARNING SYSTEMS - Warnings are issued by the Environment Agency so that local people can put sandbags by their homes, take furniture upstairs or even evacuate the area.
EMBANKMENTS - raised banks along the river. They effectively make the river deeper so it can hold more water when it floods. They’re expensive and don’t look natural. RIVER CHANNELISATION Straightening and deepening the river. It makes the water move through that part of the river faster, but it’s expensive.
WASHLANDS - Parts of the floodplain that are allowed to flood. They can’t be built on. AFFORESTATION - Re-planting trees in the catchment area. FLOOD PLAIN ZONING - Land has different building controls depending on how far away from the river it is. Land next to the river isn’t allowed to be built on, the next land use zone is for low risk buildings and the last zone for high risk buildings such as hospitals and residential homes.
Why is the Thames Barrier needed? To protect 750,000 Londoners at risk from a major storm surge could disrupt London’s future as international centre for trade and commerce and cost £20 billion if there was a flood contamination of water supplies from sewage, wildlife habitats polluted with extremely toxic substances London has had a history of flooding and a severe storm combined with a high tide would flood the city (floods in 1953 killed 300 people) What is the Thames Barrier? A method of hard engineering that cost £530 million when built in 1982, and opened in 1984 It is located in East London at Woolwich It uses four largest rising sector gates that span across 61.5 metres, and weigh 3,300 tons each These gates are open most of the time, and closed when there is an extraordinary high tide Closure can be completed in 15 minutes, but it usually takes up to 3 hours. The ideal situation is for the gates to be closed at low tide.
Will the Thames Barrier work in the future? In its current design, it will only protect London until 2030 due to predicted sea level rise There is already design work going on by The Environment Agency to make sure that there is a new scheme put in place for the future to update the Thames Barrier.
GEOGRAPHY DEPARTMENT
CAUSES: Tropical cyclone hit the coast bring heavy rain Also caused storm surge which reached 8m in height. RESPONSE Before: Cyclone was predicted by Bangladesh Space Research and Remote sensing Organisation. 21,000 volunteers used megaphones to warn the general public. After: It was difficult to recover after the flood because: There is no insurance The country relies on exports of crops. Farmland takes a long time to recover. The government is not well organised which hinders recovery process Disease means that the workforce is weakened so process is slow Loans are needed from MEDCs. Debt repayments are high.
WHAT MADE THINGS WORSE: High population density (lots of people in one area) Flat low lying land (floodwater spreads easily) Buildings were not strong and stable. Flood embankments were in need of repair EFFECTS: Death toll over 150,000 people Half a million cattle drowned 63,000 hectares of crops lost Roads, bridges and power supplies lost Water supplies were contaminated with raw sewage and dead bodies causing diseases 4 million at risk of starvation and disease Port destroyed. Reducing flood risk: Levees (flood banks) have been proposed but this would require huge loans Dhaka has flood embankments Flood shelters have been built Improvements for forecasting. Leaflets distributed so as to educated people in the event of a flood.
GEOGRAPHY DEPARTMENT
Physical causes: Exceptional amount of precipitation due to storm (20cm of rain in 4 hours) Ground already saturated from rain over previous few days steep valley & narrow river, allowing massive amounts of water to flow into the same place all at once
What and when: Boscastle is on the north coast of Cornwall. Climatic conditions were the main reason for the floods in Boscastle. 200mm fell over Ottersham Moor, to the east of Boscastle, within a 4 hour period. This was combined with coastal winds and a rising tide. The ground was also already saturated from previous rainfall events. Management strategy to reduce flooding: • £4.6 million flood defence scheme completed in 2008 • New pumping station put in place • Widening of the river channel • Buildings to be built slightly set back from the river • New flood defence wall built in the village
Boscastle also lies at the confluence of two rivers – the River Valency and River Jordan – further increasing flood risk
Human causes: Human causes hardly responsible in any way – it was a freak event Global warming increases extreme weather events Lack of any flood control system Sewer and drainage systems were old and partially blocked Boscastle had never been flooded before so the people were not prepared for such an event Effects: Short-term £500 million damage caused to buildings Cars washed into the sea Trees uprooted and roads ripped up Long-term Old buildings and homes permanently destroyed Tourism reduced due to poor reputation Future insurance for homes and businesses higher
GEOGRAPHY DEPARTMENT
MEDC EFFECTS: homes and possessions are able to be insured against flood damage good water and sewage systems providing back up clean water when local supplies become contaminated disease not the problem it is during flooding in LEDCs Very few lives (if any) lost
LEDC EFFECTS: poor quality housing, lots of it near rivers poor infrastructure is easily damaged with roads, bridges and communications destroyed lack of sanitation and clean water supplies resulting in further loss of life through spread of diseases such as cholera, dysentery etc large loss of life
MEDC RESPONSES: good communication networks means it is easier to get aid and helpworkers to affected areas increasing survival through air rescues planning restrictions are usually in place to discourage new building of houses on floodplain areas governments in MEDCs are able to invest more heavily in flood defence systems
LEDC RESPONSES: little co-operation between Bangladesh and neighbouring countries - makes it difficult to reduce flood risks such as deforestation in India and Nepal lack of government money means many flood defences can not be constructed have to rely on foreign aid for help Very few flood warning systems
THINK!!! THINK!!! Why do you think there are differences in flood management strategies between MEDCs and LEDCs?
Which type of country, MEDC or LEDC, is most likely to use hard engineering techniques to manage flooding? Explain your answer.