Geography- Rivers

water enters and continually cycles around the earth through the global hydrological cycle, it is a closed system with no inputs or outputs

there are also local hydrological cycles e.g. drainage basin hydrological cycles

• a rivers drainage basin is the area surrounding the river where the rain falling on the land flows into that river- also called the rivers catchment
• the boundary of the drainage basin is the watershed- any precipitation falling beyond the watershed enters a different basin
• drainage basins are open systems with inputs and outputs
• water come into the system as precipitation and leaves via evaporation, transpiration and river discharge

Inputs- water coming into the system

• precipitation includes all the ways moisture comes out of the atmosphere
• precipitation is mainly rain, but there are other times like snow, hail, dew etc

Storage- water stored in the system

• Interception- some precipitation lands on vegetation and structures like buildings, concrete and tarmac before it reaches the soil. Interception creates a significant store of water in wooded areas, it is only temporary
• vegetation storage is water that has been taken up by plants
• surface water includes puddles, ponds and lakes
• groundwater storage is water in soil or in rocks, porous rocks that hold water are called aquifers
• channel storage is the water held in a river or stream

• surface runoff is water flowing over the land, is common in arid areas where torrential rain falls on hard baked land
• throughfall is water dripping from one leaf to another
• stemflow is water running down a plant stem or a tree trunk
• throughflow is ware moving downhill through the soil
• infiltration is water soaking into the soil, it is influenced by soil types, soil structures and how much water is already in the soil
• percolation is water seeping down through soil into the water table
• groundwater flow is water flowing slowly below the water table through permeable rock
• baseflow is groundwater flow that feeds into rivers through river banks and river beds
• interflow is water flowing downhill through permeable rock above the water table
• channel flow is the water flowing in the river or stream itself

• evaporation is water turning into water vapour
• transpiration is evaporation from plant leaves
• evapotranspiration is the process of evaporation and transpiration together
• river discharge or river flow is another output

water balance is worked out from inputs and outputs and affects how much water is stored in the basin

the general water balance in the UK shows seasonal patterns:

• in wet seasons, precipitation exceeds evapotranspiration creating water surplus. the ground stores fill with water so there's more surface runoff and higher discharge- river levels rise
• in drier seasons, precipitation is lower than evapotranspiration. ground stores are depleted as some ware is used and flows into the river channel but isn't replaced by precipitation
• at the end of a dry season there's a deficit of water in the ground. the ground stores are recharged in the next wet season.

= the volume of water in a river per second

river discharge is affected by:

• precipitation- more precipitation, higher the discharge
• hot weather- higher temperature, lower the discharge because evaporation is higher
• removal of water from the river- reduces discharge

show how the volume of water flowing at a certain point changes over time, storm hydrographs show river discharge around the time of a storm event

• peak discharge- highest point on the graph, where discharge is greatest
• lag time- is the delay between peak rainfall and peak discharge- delay happens because it takes time for the rainwater to flow into the river
• rising limb- part of the graph up to the peak discharge- increases as rainwater flows into the river
• falling limb- part of the graph after the peak- discharge is decreasing because less water is flowing into the river

Drainage basin characteristics:

• larger drainage basins can catch more precipitation so have a larger peak discharge, smaller basins generally have shorter lag times
• steep-sided drainage basins have shorter lag times because water flows more quickly downhill
• circular basins- more likely to have flashy hydrograph because all points on the watershed are roughly the same distance from the point of measurement means a lot of water will reach the measuring point at the same time
• basins with lots of streams drain quickly so have shorter lag times

the amount of water already present in the drainage basin affects lag time:

• if ground is already waterlogged 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

Rock type- affects lag time and peak discharge

• impermeable rock types don't store water or let water flow through them
• reduces infiltration and increases surface runoff, reducing lag time
• peak discharge also increases as more water reaches the river in a shorter period

Soil type- affects lag time and peak discharge

• sandy soils allow a lot of infiltration but clay soils allow little. low infiltration rates increases surface runoff, reducing lag time, increasing peak discharge

Vegetation- affects lag time and peak discharge

• intercepts precipitation and slows its movement- increasing lag time
• the more vegetation there is, the more water is lost before it reaches the river channel, reducing peak discharge

Precipitation- affects peak discharge

• intense storms will generate more precipitation and so greater peak discharges than light rain showers
• the type of precipitation affects lag time e.g. snow thats fallen in a winter storm can melt and flow into the river in spring, giving a very long lag time

temperature- affects lag time and peak discharge

• hot, dry conditions and cold, freezing conditions both result in hard ground- 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

Human activity affecting the hydrographs:

• 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 making 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 evaporate or infiltrate into the soil, causing a shorter lag time and increased peak discharge

headward erosion- makes the river longer, happens near the rivers source as throughflow and surface runoff causes erosion at the point the water enters the river channel

vertical erosion- deepens river channels, happens in the upper stages of the river

lateral erosion- makes the river wider. it happens in the middle and lower stages of a river

• Hydraulic Action
  • the pressure of the water breaks the rock particles away from the bed and banks. It's strongest rapids and waterfalls and during floods
• Abrasion (or corrasion)
  • eroded pieces of rock in the water scrape and rub against the bed and banks, removing material. most erosion of river bed and banks happens this way
• Attrition
  • eroded rocks smash into each other and break into smaller fragments, their edges get rounded off as they rub together.
  • attrition doesn't erode the bed and banks just the particles in the river
• 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

the velocity of a river provides the energy needed for it to transport eroded material

the eroded material carried in a river= the load, and it can be carried in 4 ways:

• solution- substances that can dissolve are carried along in the water
  • e.g. limestone is dissolved into river water that's slightly acidic
• Suspension- very fine material like silt and clay particles, is whipped up by turbulence and carried along in the water. most eroded material is transported this way
• saltation- larger particles like pebbles or gravel, are too heavy to be carried in suspension so the force of the water causes them to bounce along the river bed
• traction- very large particles e.g. boulders, are pushed along the river bed by the force of the water

material transported by traction or saltation is called the river's bedload

Happens when the river loses energy, when it slows down, it loses energy and drops some of its load

the speed and energy of a river can be reduced in many ways:

• Reduced rainfall causes lower discharge, which means the river slows down and has less energy
• increased evaporation or abstraction also causes slower discharge
• friction e.g. in shallow areas of the river and close to the banks, reduces the speed of the reducing its energy
• when the river is forced to slow down e.g. before a narrow section of the channel, it loses energy
• a lot of energy is lost when the river meets the sea 

=the total amount of material it can carry

• capacity is the total load 
• the load of a river can be divided into different categories according to particle size- particle size varies from fine silt and clay to big boulders
• the competence describes the maximum particle size that a river is capable of transporting at a given point

the competence of a river is affected by the amount of energy is has, which is related to its velocity

The hjulstrom curve shows the relationship between river velocity and competence, also shows how the processes of erosion, deposition and transportation vary with river velocity

• the critical erosion velocity curve on the graph shows the minimum velocity needed for the river to pick up and transport particles of different sizes. it takes a higher velocity to erode material than it does to just transport material
• the mean settling velocity curve shows the velocities at which particles of different sizes are deposited i.e. it shows the competence of the river at different velocities

• shows you how the gradient of the river channel changes from the river's source to its mouth by showing the height of the river bed above the base level for the whole length of the river
• base level= the lowest point that the river can erode to
• total amounts of erosion and deposition along the full course are balanced, but the rates change along the course of the river- results in formation of landforms
• because the total amount of erosion and deposition is balanced, the rate of erosion of landforms like waterfalls is equal to the rate of deposition elsewhere along the river- means that over time the long profile will change from being uneven to a smooth profile

• the velocity and discharge of a river increase as you go downstream
• discharge increases as tributaries and more surface runoff join
• river velocity is influenced by gradient, discharge and channel characteristics

• most of river's kinetic energy is used to overcome friction- rest causes erosion. the more energy a river has available for erosion and transportation, the more efficient it is. An efficient river will have a high velocity, high discharge and little friction
• efficiency is measured by hydraulic radius, larger the radius the more efficient a river is

hydraulic radius:

• hydraulic radius is the channel's cross-section area divided by the length of its wetted perimeter
• contact between the water and the wetted perimeter creates friction which increases energy loss and slows the river down
• a larger hydraulic radius means that a smaller proportion of water is in contact with the wetted perimeter, so friction is lower, which reduces energy loss, increasing velocity and discharge
• smooth, narrow, deep channels have a larger hydraulic radius and so are more efficient than shallow, broader ones

channel roughness also affects the efficiency. protruding banks and large, angular boulders on the river bed increase the wetted perimeter and cause more friction reducing efficiency, velocity and discharge

as channel roughness increases, so does turbulence, turbulent flow is more effective at picking up particles from the river bed causing more erosion

• channel roughness is greatest in the upper stages of the river. so although the gradient is steep, the river loses a lot of energy to friction so discharge and velocity are lowest here during normal conditions
• in the lower stages, the banks and bed of the river are smooth, so there's much less friction. this means less energy lost, so discharge and velocity are the highest in this stage.

split into three stages: upper, middle and lower. the energy of a river varies in each stage:

• in the upper stage, the gradient of steep and the river is high above sea level, which gives it lots of potential energy into other forms
• as the gradient decreases towards the middle stage, potential energy is converted to kinetic energy- the river gains velocity
• in the lower stage, the river has little potential energy, but lots of kinetic energy- it flows faster

Erosion

• mainly vertical and by abrasion
• occurs when there is high energy conditions
• rough channel causes turbulence and the large, angular bedload us dragged along the river bed, causing intense downwards erosion

Transportation

• mainly large particles such as boulders carried by traction or saltation during high energy conditions

Deposition

• little deposition- mainly largest particles deposited in the river bed as energy levels drop

Erosion:

• mainly lateral and by abrasion
• attrition of larger particles in this stage means that sediment particle size decreases from source to mouth

Transportation

• more material carried in suspension as particle size decreases
• some larger particles moved by saltation

Deposition

• Sand and gravel are deposited across the flood plain as the river floods and friction reduces the river's energy

Erosion

• 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 during the formation of meanders

Transportation

• mainly smaller particles such as silt and clay carried by suspension or substances carried in solution

Deposition

• smaller particles such as sand, silt and clay are deposited on the flood plain when the river floods and in the river mouth as the sea absorbs river energy

The cross profile of a river shows you what a cross-section of the river channel or the river valley looks like

the valley cross profile changes during different stages of a river's long profile:

• Upper stage valleys are steep V stage. vertical erosion creates narrow valley floors and steeply sloped sides
• Middle stage valleys are wider, caused by lateral erosion. Deposition creates a flood plain on the valley floor
• Lower stage valleys are wide with gently sloping sides. There's a much wider flood plain caused by deposition

Slope processes

• defined as part of the solid land surface where there is an incline
• slope processes transfer materials downslope to the river reducing the angle of the slope
• mass movements= large scale movement of the earth's surface not accompanied by any moving agent like water
• include small movements like soil creep, fast movements like avalanches, dry movements like rock falls and fluid movements like mudflows

Soil creep:

• individual soil particles are pushed or heaved to the surface by wetting, heating or freezing of water
• move at right angles to the surface as it is the zone of least resistance
• fall under the influence of gravity and net movement us downslope
• rates are slow
• they form small scale terracettes such as the Manger in the Vale of the White House, Oxfordshire

Rain-splash erosion

• on flat surfaces raindrops compact the soil and dislodge particles equally in all directions
• on steep slopes the downward component is more effective than the upward motion due to gravity and so erosion downslope increases with slope angle

Surface wash- occurs when the soil's infiltration capacity is exceeded

Sheetwash- unchannelled flow of water over a soil surface, for example during the boscastle floods 2004, sheetwash from the shallow moorland peat caused steep, narrow gulleys to form

throughflow- water moving down through the soil, channeled into natural pipes in the soil, gives sufficient energy to transport material, and added to its solute load, may amount to a considerable volume

Slides-sliding material maintains its shape and cohesion until it impacts at the bottom of a slope and leads to large, slumped terraces

• large scale movements can kill people e.g. the Vaiont Dam in italy where over 2000 people died on the 9th October 1963

Falls- Rock falls occur on steep slopes , initial cause of the fall may be weathering

• if the fall is short it produces relatively straight scree, if it is long it forms concave scree

Slumps

• slumps occur on weaker rocks especially clay, and have a rotational movement along a curved slip plane-clay absorbs water, become saturated, exceeds it liquid limit 
• frequently the base of the cliff has been undercut and weakened by erosion reducing its strength

Climate

• slopes in temperate environments are rounder, due to chemical weathering
• slopes in arid environments are jagged or straight because of mechanical weathering

Rock type and structure

• slopes are also influenced by rock types
• the Tees-Exe line is an imaginary line running from the river tees to the river exe
• it divides Britain into hard and soft rock. To the north and west are old, hard. resistant rocks such as granite, basalt and carboniferous limestone forming upland rugged areas
• to the south and east are younger weaker rocks such as chalk and clay forming subdued low-lying landscapes

Aspect

• is the direction a slope faces
• in the Uk north facing slopes remain in the shade
• during cold periglacial times, temperatures rarely rose above freezing
• in contrast, south-facing slopes experienced many cycles of freeze thaw
• solifluction and over-land runoff lower level of the slope and streams remove debris from the valley
• the result is asymmetric slope such as the River Exe in Devon

Weathering is the decomposition and disintegration of rocks in situ, weathering is important for landscape evolution as it breaks down rock and facilitates erosion and transport by rivers

Mechanical weathering:

• Freeze thaw- occurs when water in joints freezes and expands by 10% causing rock to break
• salt crystal growth- when sodium sulphate and sodium carbonate expand by 300%, when the water evaporates, salt crystals are left behind
• Disintegration- found in hot desert areas, rocks heat up by day and contract by night, cause exfoliation to occur
• pressure release- process where overlying rocks removed by erosion cause underlying ones to expand and fracture parallel to the surface

Chemical weathering

• carbonation solution- occurs on rocks with calcium carbonate, rainfall and dissolved carbon dioxide forms a weak carbonic acid, calcium carbonate reacts with the acid water and forms calcium bicarbonate
• hydrolysis- occurs on rocks with orthosclase feldspar
• hydration- process where certain minerals absorb water, expand and change
• oxidation- occurs when iron compounds react with oxygen to produce a reddish brown coating

Biological weathering- involves both mechanical impacts such as growth of roots and chemical impacts such as the release of organic acids

climate

• rate of weathering varies with climate
• peltier's diagram shows how weathering is related to moisture availability and average annual temperature

geology

• rock type influences the rate and type of weathering in many ways due to:
  • chemical composition
  • the nature of cements in sedimentary rock
  • joints and bedding planes

provides upward motion in the flow that allows the lifting and support of fine particles

this will contribute to depositional landforms further down the river

conditions necessary for turbulent flow to occur are:

• complex channel shapes such as meandering channels and alternating pools and riffles
• high velocities
• cavitation in which pockets of air explode under high pressure

it is common in groundwater and in glaciers but not in rivers although it can occur in the bed in the lower course of a river

best condition are:

• shallow channels
• smooth straight channels
• low velocities

waterfalls

• waterfalls form where a band of hard rock meets softer rock. the soft rock is eroded more than the harder rocks, causing a 'step' in the river bed
• the water flowing over the step speeds up due to the lack of friction as it drops over the step. this increase in speed gives the water greater erosive power, causing further erosion of the soft rock and undercutting of the harder rock
• as the hard rock is undercut by hydraulic action, 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 e.g. niagara falls on the USA/canada border

potholes

• small circular hollows in the river bed
• formed by abrasion as turbulence swirls a river's bedload round in a circular motion, causing it to rub and scrape out holes

Rapids

• rapids are relatively steep sections of river with turbulent flow where there are several sections of hard rock
• they are a bit like mini waterfalls

v-shaped valleys

• weathering and mass movements occur on the valley sides while the river erodes the base of the slopes
• the angle of the v-shape depends on
  • the rate of downward erosion by the river
  • the resistance of the rocks to weathering, mass movements and erosion
  • climate
  • the location along the course of the river

Meanders= large sweeping curves in a rivers middle and lower stages

• meanders form when alternating pools (areas of deep water) and riffles (shallow water) develop at equally spaced intervals along the stretch of a river, the distance between is 5-6 times the width of the 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 because of 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 water begins to twist and coil, the thalweg (maximum velocity of the river) begins to flow from side to side
• this causes a corkscrew-like currents in the river called helicoidal flow which spiral from bank to bank between pools 
• the helicoidal flow causes more erosion and deepening of the pools.

• also causes eroded material to be deposited on the inside of the next bend where the river loses energy creating slip-off slopes
• the combination of erosion and deposition exaggerates the bends until large meanders are formed. the combined processes also create the meanders' distinctive asymmetric 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 ox-bow lake

channel characteristics and meanders

• curved part of the meander is generally 6-10 times the width of the river channel and/or the discharge
• meandering is more pronounced when the bed load is varied
• meander wavelength increases in streams that carry coarse debris
• meandering best develops at or near the bankfull stage

ox-bow lakes

• result of erosion and deposition
• lateral erosion, caused by centrifugal forces is concentrated on the outer, deeper bank of a meander
• during times of flooding, erosion increases causing the river to break through and create a new, steeper channel.
• in time, the old meander is closed off by deposition to form and ox-bow lake

Braided channels/ braiding

• river becomes braided when the main channel separates into a number of smaller interlocking channels
• occurs when the river is carrying a vast amount of eroded sediment
• if the river's velocity drops, or the sediment kload 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 join to form a single channel
• conditions needed:
  • a channel gradient that is slightly steeper than that of a meandering stream
  • a load that contains high proportions of coarse material
  • a highly variable discharge
• they are especially common where they drain from glaciers in in semi-arid areas where all the conditions are met.

Flood Plains

• when a river overflows its banks and floods the flat land either side of the river (flood plain) there's an increase in the wetted perimeter and reduction in hydraulic radius caused by increased discharge as a result of heavy precipitation leading to the bankfull stage
• the continued friction, reducing the velocity of the river and causing fine silt and sand (alluvium) to be deposited
• floodplains are also formed by the erosion of bluffs

Levees

• =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 (e.g. sand and gravel) is dropped first, closest to the river channel- stratified
• over time this builds up on the river bank, creating a levee

Deltas

• when a river reaches the sea the energy of the river is absorbed by the slower moving water of the sea causing the river to deposit its load
• the deposits build up in the sea bed, until the alluvium rises above sea level, partially blocking the mouth of the river
• for deltas to form the river needs to carry a large volume of sediment and enter a still body of water
• deposition is increased if the water is salty as the salt particles group together becoming heavier- flocculation
• vegetation also increases the rate of deposition by slowing down the water

• processes of erosion and deposition in a river are controlled by location relative to the base level of the river- usually the sea
• the river, in its attempts to become graded will develop low gradients as it approaches any base level
• base levels are static and do not change
• uplift or a fall in sea level will produce a negative change of base level, while coastal submergence produces a positive change in base level

as the potential energy of the river increases due to the greater difference in altitude between the source and base level, the river responds by increasing its velocity and downcutting, following features arise:

• intrenched/ incised meanders: meanders that develop on a floodplain will maintain its form as the river cuts down, where the process has continued for some time the river will flow in a meander gorge
• river terraces: when the river cuts down into its floodplain, the remnants of the floodplain form a terrace with a steep slope, periodic changes in base level will form a sequence of terraces
• knickpoints: the long profile of a river is never smoothly concave, but shows many sudden changes in gradient, knickpoints occur where the river changes from deposition to erosion and are caused by the negative changes in local base levels due to lithology or human actions

=catchment area within which water is supplied by precipitation or by underground sources is transferred out via stream

• every stream has its own drainage basin, separated by adjacent drainage basin by a watershed (divide)
• drainage basins are open systems there are factors which operate in them and results are created

They provide:

• a source of water, power,
• opportunities for industrial, agricultural (fertile silt) and residential development
• means of transportation
• place for recreation and leisure
• a means of flood defence through floodplains
• a conservation value

Old Basing is in the River Loddon drainage basin used for:

• fishing, walking, swimming, paddling, canoeing, nature walks
  
• walking dogs, playing in the water
• used to be used for water mills
• used for agriculture like growing corn and wheat
• animals like horses and cows drink from it
• houses built on it, cleaned sewage put into river
  
• incinerator built on flood plain
• Basingstoke shopping centre built on drainage basin
• industrial park built on the drainage basin in Chineham
• parks used for cricket and football
  

Guildford in the river wey basin

• used for shopping centre
• car parks built on the drainage basin, which also allow for management when the river floods

is the most developed part of the UK,with a population of about 12 million (5th of nations population) covers an area of 13,000km, includes 14 counties, 58 district councils and 33 local planning authorities

tributaries: Cole, Lee, Kennet, Wey and Loddon

Water supply

• 4700 million litres of water is abstracted every day
• rain falling from the Cotswold can be used up to 8 times before it reaches the Thames estuary
• overuse of water has had a negative effect on some rivers- low flow
• region has seen continued growth of housing and commercial development increasing pressure on land use and water resources
• demand for public water supplies has increased by about 1.7% each yr for 30yrs
• key factors: the use of water in home and garden, losses through leakage from distribution systems and consumers plumbing, population growth and household size, development pressure and economic activity

Industrial development

• area contains much chalk, limestone, sand and gravel creating opportunties for mineral extraction
• river allows for import and export of raw materials and finished goods
• car manufacturing and iron steel works- built because of flat cheap land near water cooling faculties
• e.g. former industries in London Docklands, current: refineries based at Tilbury

Residential development

• built because of benefits- supply of water, potential for trade and communications, flat sites for industrial and residential development
• many settlements are built on raised ground in order to reduce risk of flooding e.g. Oxford
• much of the floodplain of the Thames has not been built raised and has been left for farming and recreational use
• some developments are at risk from flooding- east of London priority
  • 14 zones of change planned, forecasts housing growth to about 100,000 by 2016
• much of the conflict is due to demand for housing- Thames Estuary is important ecological area providing wetlands but plans for building settlement in the area- new settlement will have 120,000 homes below sea level
  • schemes for protection are being considered e.g. building houses on stilts and building roads on embankments- large areas of kent and essex farmland to floodwaters during a tidal surge to prevent London flooding
  • reason for development is the large number of jobs in London region

Farming

• lower part is attractive to farmers due to supply of water
• relatively gentle slopes allow for the use of machines
• upper regions are suitable for livestock farming
• lower Thames basin arable farming predominates, areas close to wealthy urban markets, market gardening is common

Transportation and trade

• Many of worlds greatest cities lie on important trade routes, e.g.London- Thames.
• with trade comes jobs- transport, storage, logistics--> multiplier effects such as demand for housing for workers and retail, education and health service
• more trade, greater jobs
• river valleys especially those in the lower course are relatively flat and make transport routes easy to build e.g. the main railway line from Oxford to London follows the River Thames

Energy development

• Thames not used for hydroelectric power as it is not steep enough
• has been used for water power in the past e.g. mills used for grinding grain at Coleshill near Faringdon
• Thames also used indirectly to generate power- London's waste is carried by barge to incinerators which convert waste into energy

Recreation and Leisure

• Thames used for fishing, swimming, canoeing, rowing, sailing, cruising, guided tours, walking and hiking
• e.g. Cotswold Water Park- made p of many lakes which are used for a wide range of water-based recreation, Thames path runs through the area
• e.g. in the Oxford region used for sports grounds, farmland, city's botanical gardens and allotments, river used for punting and water sports

Flood defences:

• Thames barrier provides a high standard of protection to the 420,000 London properties at risk from Thames flooding
• Thames barrier became operational in October 1982, on average the barrier has to close 3 times a year
• In Oxford- Port Meadow, on a local scale, the restored meanders and floodplain of the Rive Cole is an important local flood relief measure

Conservation

• upper reaches of the Thames basin in the Cotswold Water Park is considered a pressure point because of the variety of developments: mineral extraction, wide diversity of plants and wildlife, SSSI- number of wet meadows
• other parts of the drainage basin are mudflats and estuaries- seen as hotspots because of biodiversity e.g. Thames Estuary and Marshes Special Protection Area (SPA)

The mekong is south-east Asia's largest river and the worlds 8th largest, there are 6 countries which share the Mekong so conflicts occur over the use of this resource

Transportation

• transport is limited because the river it not navigable much beyond Phonom Penh
• in the dry season, when the river is low there are reefs and shifting sandbars
• when the water level rises, the many rapids of Si Phan don, or 'four thousand islands' form an obstacle for shipping

Industrial and economic development

• development has recently accelerated rapidly
• the Mekong river basin is located on the Pacific Rim, and the governments of China and Vietnam are keen to develop their national economies
• the first dam on the river, at Man wan, in China was finished in 1993, the first bridge was built in 1994, between Vientiane in Laos to Nong Khai in Thailand
• the population of the area is growing rapidly but economic growth is faster

Residential development

• home to 55 million people an each year they have to contend with widespread flooding, in 2000, over 800 people died and many people were left homeless
• the population is predicted to rise which will place further demands on the water supply and infrastructure
• the disposal of raw sewage is already a challenge with much of it finding its way into the river system

Recreation and leisure

• As one of the fastest growing tourist destinations in the world, the Mekong river basin is benefiting from economic growth
• the expansion of tourism has the potential to conflict with requirements of local residents and traditional activities such as fishing
• the region is receiving funding from the Asian Development bank to help conserve the natural environment and implement sustainable development strategies

Energy development

• Hydroelectric potential of the Mekong is considerable because of the steep relief and large volumes of water
• only 5% (1600MW) of the Lower Mekong Basin's potiential hydroelectric potiential of approximately 30,000MW has been developed
• dams generate valuable electricity, aid irrigation and regulate flooding  but damages fisheries

Farming and water supply

• 80% of rice production in the Lower Mekong Basin depends on water, silt and nutrients provided by the flooding of the Mekong
• Dams on the upper Mekong affect water flow in the Lower Mekong during the dry season changing the natural cycle of the river
• dams could mean less frequent floods, adversely affecting farming and fishing

Fishing and water supply

• The Mekong and its tributaries yield more fish than any other river system
• the annual harvest, including fish farms, amount to about 2 million tonnes
• it is home to over 1200 different species of fish
• over 1 million people in Cambodia depends solely on fishing to make a living
• in Laos 70% of rural households supplement their income by fishing
• during the monsoon, the habitat for fish increases by 10 times
• much of the floodplain is forest, providing leaves for fish to feed on, they spawn at the end of the dry season so that the coming floods can carry the fish to the floodplain
• the bigger the flood, the greater the leaves on offer so fish are fatter and more numerous
• more dams, however mean smaller floods- most hydroelectric plants aim to generate the same amount of energy all year round, requiring consistent flow through turbines, which in turn requires rainwater to be held in a reservoir
• rapid development is having a positive impact on energy and industrial production but is having a negative impact on farming and fishing

flooding occurs when the discharge of the river is so high the river spills over its banks onto the flood plain

Main causes:

• prolonged rainfall- after a long period of ran, the ground becomes saturated, any further rainfall can't infiltrate increasing surface run off, which increases discharge
• Heavy rainfall can lead to rapid surface runoff, if the rainfall is too intense for infiltration to occur it can lead to a sharp rise in river discharge= flash flood
• Melting snow an ice also lead to a huge increase in the rivers discharge
  • e.g. melting snow in the Himalayas contributes to the annual summer flooding of the River Ganges in Bangladesh

Sparse vegetation or deciduous trees:

• sparse vegetation in the drainage basin means little rainfall is intercepted so more rain reaches the ground, increasing the volume of water and therefore increasing river discharge
• deciduous trees have no leaves in winter so little rain is intercepted

Circular drainage basins

• water draining into the main river channel will all arrive in a short space of time as all points in the basin are a similar distance from the river- increased discharge

High drainage density

• drainage basins with a high drainage density 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

Impermeable ground

• clay soils and some rocks such as granite and shale are impermeable- they don't allow infiltration of surface water increasing surface runoff, increasing discharge
• if the ground has been baked hard by the heat of the sun in summer, or it's frozen, the same thing happens- water can't infiltrate, increasing surface runoff and discharge

Steep slopes

• if the drainage basin has steep-sided valleys, water will reach the river channel much faster because water flows more quickly on steeper slops. this increases discharge

Urbanisation

• urban areas have large areas of impermeable tarmac and concrete, so when it rains surface runoff is very rapid
• gutter and drains take runoff to rivers quickly
• both of these things reduce lag times and so increase discharge

deforestation

• clearing trees and plants reduces interception and evapotranspiration increasing the volume of water that reaches the channel which decreases discharge
• deforestation leaves the soil loose so the soil is eroded and carried to the river, which raises the river bed reducing the river channel capacity so it takes less water for the river to flood

climate change-could cause and increase in rainfall and more storms in some areas so increased 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

Agriculture

• overgrazing leaves areas with less vegetation, so it has the same effect as deforestation
• overgrazing and ploughing also increase soil erosion

Social:

• people and animals can be killed
• floodwater is often contaminated with sewage which can lead to lack of clean drinking water
• contaminated water can also put people at risk of diseases (e.g. diarrhoea and dysentery)
• possessions can be damaged by floodwater or washed away
• people can be made homeless as their properties are inundated or damaged

social impact is usually higher in poorer countries as flood defences are poorer, people are less able to evacuate, sanitation systems aren't as good and buildings are of poor quality

Economic

• businesses often have to shut down as premises are inundated and power supplies are affected
• rescue work and repairs are usually costly
• insurance premiums usually go up after floods
• unemployment levels often rise as businesses shut down because they can't recover from the flooding
• public transport, road and bridges can be destroyed
• crops can be destroyed which can lead to a rise in the price of food

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 affects the economy more because they have less money to recover

Environmental

• floodwater contaminated with sewage and rubbish can pollute rivers
• river banks are eroded

positive impacts:

• river sediment is deposited on flood plain making the land more fertile
• wetlands can be created e.g. marshes and ponds which are habitats for many species

parts of South Asia flood most years, usually in late summer because:

• south asia has a monsoon climate- 80% of rain falls in 4 months
• must of south asia is low-lying land, particularly Bangladesh where 90% of land is below sea level
• melting snow and ice from the Himalayas in the late summer months increase the brahmaputra river discharge

In July and August 2007, the flooding was particularly severe in Bangladesh and India.

physical factors were the main cause:

• monsoon came early after a very dry, early summer,
• there was heavy rainfall- Assam had a record 169.5mm in 24 hours on the 22nd July and 900mm in total for July
• the long duration of heavy rainfall completely saturated the soil, increasing surface runoff and discharge
• the peak discharges of the River Ganges and Brahmaputra coincided leading to increase in river discharge downstream.

human activities made the flooding worse:

• deforestation in Nepal and the Himalayas meant less rainfall was intercepted, increasing 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

Social impacts:

• over 2000 people died, the death toll was high for many reasons:
  • many people were reluctant to evacuate- would have to leave unattended
  • 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
  • over 100,000 people caught water-borne diseases like cholera and dysentery
• an estimated 25 million people were made homeless
• 112,000 houses were destroyed in India as porous mud bricks became saturated by floodwater
• Dhaka 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

Economic:

• cost of the flood was estimated at about US$1 billion including damage to crops and property
• factories were closed around Dhaka, due to flood damage and loss of raw materials (e.g. rice) many of the poorest workers became unemployed
• there was widespread loss of livestock (e.g. cattle)
  • since 80% if Bangladeshis rely on agriculture, many lost livelihoods
• 550,000 hectares of land couldn't be planted with rice at peak times because of flooded fields
  • lower rice crop meant the world price of basmati rice rose by 10%
• 10,000km of roads were destroyed. landslides blocked roads in the highlands of Nepal and Assam
• Debt increased, both individually (e.g. farmers borrowed money for food and seeds0 and nationally (e.g. governments imported food and medicine)

Environmental:

• the floods deposited fertile silt on the flood plain
• rivers were polluted with sewage

human factors made 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 officials diverted aid money away from the people most in need

the River Eden runs through North Cumbria and reaches the sea near Carlisle

• the drainage basin of the river Eden is very large so it catches a large volume of rainfall leading to high river discharge
• some parts of the basin have steep sides, so water runs quickly down to the river
• there are many streams that drain quickly into the river, making the lag time short

on the 8th January 2005, the River Eden flooded Carlisle

Physical factors were the main cause:

• there was heavy rainfall on the 6th January for 36 hours, 200mm of rainfall was recorded, which was equivalent of 4 months rain
• rain fell on saturated ground so the water didn't soak into the ground but ran straight into the river
• this caused a very high peak discharge (over 1520 cumecs) compared to the average 52 cumecs

human factors making the flooding worse:

• Carlisle is a large built-up area, with impermeable concrete and tarmac surfaces, and little soil or vegetation which means 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

Social Impacts

• 3 people died
• over 3000 people were made homeless for up to a year and thousands of personal possessions were damaged. living in temporary accomodation disrupted lives in many ways:
  • travel arrangements were disrupted
  • people were separated from community networks and friends
  • they had problems receiving post
• children lost out on education as 4 schools were severely flooded, Newman Catholic School didn't reopen until Easter
• there was an increase in stress-related illness following the floods

Environmental impacts:

• the flooding increased river bank erosion in some areas
• rivers were polluted with rubbish and sewage

Economic:

• it took a year to repair the damage to homes and repairs cost over £100 million
• 350 businesses had to shut down as there was no electricity, telephone service or transport, trade services from Carlisle railway station were suspended
• United Biscuits, the largest employer in Carlisle, was flooded with 3m of water than 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 (most of the public transport fleet) were destroyed. Many roads and bridges were damaged e.g. Warwick Road

There is not enough money to protect everywhere from flooding:

• aim off flood management is to protect homes, businesses and the environment from flooding
• this is because flooding can have severe social, economic and environmental impacts
• it is tricky to manage flooding as there is not enough money to protect everywhere
• large settlements and important industrial sites (like power plants) are more likely to be protection than small settlements or farmland

Hard engineering defences are man-made structures that reduce flooding, general disadvantages include:

• 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 occur 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

=are huge walls built across rivers, a reservoir forms behind the dam, flood water is caught by the dam, preventing flooding downstream, the water is released at a steady flow throughout the year

benefits

• turbines are often built into dams, which generate electricity
• steady water release allows irrigation of land below the dam throughout the year
• people can use the reservoir for recreational activities e.g. sailing

Disadvantages

• they're very expensive
• land is flooded when a reservoir is created, often destroying farmland and forcing people to move
• they affect wildlife e.g. can prevent salmon migrating upstream to breed
• they trap sediment which can cause the dam to fail but also causes increased erosion downstream as less protective sediment is deposited

=where meanders are removed by building artificial cut throughs making water flow faster, reducing flooding because water drains downstream more quickly and doesn't build up to a point where the river channel can't contain any more

Benefits:

• takes less times to navigate the river because it has been made shorter

Disadvantages:

• flooding may happen downstream instead as floodwater is carried there faster
• more erosion occurs downstream because the river flows faster
• altering river channels disturbs wildlife habitats

=embankments built along rivers. the river can hold more water without overflowing and so it floods less often

benefits

• allow the flood plain to be built upon

disadvantages

• they are quite expensive
• there's a risk of severe flooding if the levees are breached

= channels that take water elsewhere if the water level in the river is too high

• the water is normally diverted around an important area 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

disadvantages

• 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 area not used to flooding, which could cause even bigger problems

soft engineering defences use knowledge of the whole river basin and its processes, to try to work with nature. general advantages are:

• they're cheaper to maintain than hard engineering defences- especially important in poorer countries
• 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 schemes

Soft engineering is more sustainable than hard engineering

• hard engineering is often expensive and disrupts natural process
• soft engineering tends to be cheaper and requires much less time and money to maintain than hard engineering
• soft engineering is designed to integrate with the natural environment and creates areas like wetlands which are important habitats
• therefore soft engineering is more sustainable because it has lower economic cost and environmental impact

• planning restrictions prevent buildings or roads being constructed on 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

benefits:

• there are no new buildings or roads on the flood plain to be damaged so the impact of flooding is reduced
• it provides recreational opportunities e.g. football fields

disadvantages

• restricts development, is especially a problem where there's a shortage of housing
• it can't be used in areas which are already urbanised

• wetlands store flood water and slow it down reducing flooding downstream
• so conserving or re-establishing wetlands gives natural protection
• planting trees and shrubs along the river bank increases interception and lag time, reducing flooding and decreases flooding

benefits

• vegetation protects the surface from soil erosion
• the vegetation provides habitats for wildlife

disadvantages

• less land is available for farming

• involves making the river more natural e.g. by removing man-made levees
• means the flood plain can flood naturally
• as the water spreads out over the flood plain the river's discharge is reduced which reduces flooding downstream

benefits

• little maintenance is required as the river is left in its natural state
• the river provides a better habitat for wildlife

disadvantages

• local flood risk can increase, especially if nothing's done to prevent major flooding

• building porous pavements or soakaways increases infiltration which reduces rapid surface run off to the river channel which increases lag time which reduces discharge and flooding

benefits

• any pollutants in the water are filtered out by the soil before the water reaches the channel

disadvantages

• its expensive

the impacts of flooding can be reduced by weather forecasts and flood warnings:

• the environment agency monitors weather forecasts, rainfall and river discharge
• they warn people about  floods through TV, radio, newspapers adn the internet
• means people can evacuate before the flood happens, saving lives
• people can also more possessions and use sandbags to help reduce damage if flooding occurs

disadvantages:

• some people might not be able to access the communication network
• flash flood cannot be predicted and are too fast for warnings
• people may ignore warnings if they have been inaccurate in the past

• the Yangtze river flows through China, 6380km long- 3rd longest in the world
• seasonal flooding is common- China has a rainy season from June until August and the huge increase in river discharge during this season often causes flooding
• flooding can cause major problems because there's lots of farmland and loads of major cities next to the river e.g. Wuhan and Nanjing
• five major floods have happened in the last century- in 1931, 1935, 1949, 1954 and 1998
• the flood of 1954 covered 193,000km2 of land and killed 33,169 people and over 18 million people had to move, it covered the city of Wuhan for over 3 months
• the flood in 1998 killed around 3000 people and made 14 million people homeless
• flood protection is mostly done through hard engineering defences

There are many dams on the Yangtze that reduce flooding (46 are planned or under construction at the moment)

the biggest dam is the Three Gorges Dam

• work began on the 101m high Three Gorges Dam in 1994
• A reservoir is building up behind the dam  (takes years to build up as the dam is huge) the reservoir catches any flood water which can be slowly released over time. The reservoir can store around 22km3 of flood water
• It's also the largest hydroelectric power station in the world. the flow of water turns 26 turbines built into the dam
• locks have been built alongside the dam so ships can get past it

There are many Levees along the river

• e.g. there are 3600km of levees along the middle and lower parts of the river

positive effects of the Three Gorges Dam:

• it is thought 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- capacity is likely to reach 22.5gigawatts (enough to supply 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

Negative effects of the Three Gorges Dam:

• people have had to relocate as the water level in the reservoir has reason
  • thought that between 1.3 and 2 million people in total will have to relocate by the time it is full
  • 13 cities and 1352 villages will be submerged
• the reservoir will also flood farmland, 657 factories and 1300 sites of cultural and historic interest
  • e.g. The Temple of Zhang Fei will be submerged
• a huge amount of sediment normally carried down the river will be trapped which could lead to failure of the dam and cause catastrophic flooding
• the dam could destroy habitats and endanger species
  • e.g. the endangered siberian crane spends winter in the wetland below the dam, which are expected to be affected by less flooding. also 100 baiji dolphins are left, the dam could reduce their food supply
• the Three Gorges Dam doesn't protect everyone- rising water levels in the reservoir will increase flooding along the tributaries leading to it e.g. Daning river
  • will also increase erosion of riverbanks causing collapses and landslides

• Abingdon, south-east England was built on the flood plains of the River Thames and the River Ock
• 1500 properties in Abingdon have a 1% chance of flooding in a given year
• Abingdon has had regular floods over the years- in 1947, 1968, 1977, 1979, 1992, 2000 and 2007
• intense storms in July 2007 caused bad flash floods, the river Thames and Ock burst their banks, flooding 660 properties in Abingdon. increased surface runoff in built-up areas made the flooding even worse
• hard engineering defences have been considered but have been rejected for various reasons
  • e.g. a diversion spillway to transport Ock floodwater south of Abingdon was too expensive
  • flood barrier to protect properties along the Ock would increase flood risk downstream
• flood protection is mostly done through soft engineering defences

• Gravel soakaways built along the A34 road
• Low-value land is allowed to flood e.g. tilsley park sport ground is beinf considered as an additional flood storage area
• there are planning restrictions on housing developments 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 area at risk and provides a 24 hour floodline
• there are restrictions on land use e.g. planning permission was refused for building on the Thames flood plain
• Improvements have been made to riparian buffers along smaller rivers. planting trees reduces the volume of water reaching the Thames and Ock rivers where flood problem is greater
• Local voluntary wardens communicate advice and flood warnings
• there's detailed advice on the internet about reducing flood damage e.g. by raising cupboards and using water resistant plaster

Soft engineering reduces damage but floods still happen

• It's difficult to measure the success of flood defences because it is hard to firgure out if any reduction in flooding was because of the success of new defences or because the weather was less severe
• several flood warnings were issued by the Environment Agency in early 2008
• the Ock flood plain, which has developments on, didn't get flooded. the Thames flood plain did get flooded but it is largely clear of development, due to land use management and planning restrictions
• the 2008 floods did less damage than previous years, with minimal cost, little disruption to the community services, no lives lost and only a few injuries- however, flooding does still happen in Abingdon.