Water Flashcards
1
Q
closed system
A
inputs and outputs are cycled and there are no loses or gains in the system
the water system is dynamic because it is continually changing through transitions
2
Q
biosphere
A
the living system (plants and animals)
3
Q
cry sphere
A
the ice system (ice sheets and glaciers)
4
Q
residence times
A
how long water stays in a particular store
5
Q
fossil water
A
water that is no longer being naturally replenished and it may have been stored for a long time
6
Q
water stores and transfers
A
- nearly 2/3 of this water is stored in polar ice caps, snowpack and glaciers making it inaccessible for long periods of time. Has to melt
- low lying regions can form atmospheric rivers to transport water horizontally (high moisture and winds)
- clouds shield earth from the sun when there is heavy enough rain
- availability of water affects the type and abundance of vegetation
- when the air temperature is warmer it can hold more water
- only small amount available for human use
7
Q
stores
A
- cryosphere
- ocean (most abundant 97%), residence time of 3600 years
- rivers and lakes
- groundwater
- atmosphere
- vegetation (least abundant, 0.0001%), residence time of one week)
- soil moisture
8
Q
fluxes
A
- evapotranspiration
- ocean precipitation
- land precipitation
- surface flow (40(103km3)
- ocean evaporation (413(103km3)
9
Q
what forces drive the system?
A
- solar energy- heats water causing it to evaporate and then condense and precipitate
- gravitational potential- earths gravitational pull is converted into kinetic energy, accelerating water through the cycle
10
Q
future implications of water security
A
- water is generally considered a renewable resource but humans extract fossil water which isn’t recharged (e.g. the Sahara)
- climate change is altering the budget, the cryosphere is melting and increasing the proportion stored in oceans. many populations rely on glaciers to feed rivers
- global population is rising. currently around 7 billion but this is projected to ties to 10 billion by 2055
11
Q
consequences of water scarcity
A
- conflicts (transboundary)
- drought and famine
- environmental refugees
- price of water increases
- more use of technology (e.g. desalination causing further carbon emissions)
12
Q
orographic precipitation
A
relief rainfall
- caused when air is forced to rise and then 3Cs when it meets the land (esp mountains and hills)
- this is typical in the Lake District
13
Q
frontal precipitation
A
- caused when warm air meets cold air and forces warm air to rise (3Cs)
- typical in the UK
14
Q
convectional precipitation
A
-caused when moisture evaporates and rises when heated by the sun
15
Q
saturated overland flow
A
-surface run off caused when soil is saturated
16
Q
what are basin wide factors?
A
factors that affect the whole basin
- inputs
- flows
- outputs
17
Q
basin inputs
A
- highest inputs found at the topics due to the ITCZ, maximum heating at the equator causes rain and sinking air at 30 degrees north and south. it migrates N and S as the earth tilts creating monsoons and tropical rain belts
- this results in convectional precipitation (average rainfall at its highest is Mawsynram, India with 11,873mm of rain per year)
- lowest precipitation inputs are found in stable areas of high atmospheric pressure (e.g. Atacarna desert receives less than 0.2mm of rain per year)
18
Q
how do the continents affect basin inputs?
A
- the distribution of precipitation is influenced by continentality (distance from the sea), as continental interiors such as the Gobi desert in Asia of the Alice spring region in Australia are far from moisture of maritime air masses
- relief, such as mountains and prevailing winds complicate air patterns with more precipitation occurring where prevailing winds are forced to rise over higher altitudes, forming orographic precipitation
19
Q
what is interception dependent on?
A
- vegetation and precipitation
- interception is greater when precipitation is light and of short duration as dry leaves have greater water storage capacity
- coniferous intercept more than deciduous as it is denser and does not lose leaves during winter
20
Q
what factors affect infiltration capacity?
A
- slope gradient
- precipitation intensity
- vegetation cover
- soil and rock type
- water table depth
- how saturated the soil is (infiltration capacity, maximum rate that soil can absorb precipitation)
- once in the soil water moves both vertically and laterally by the process of through flow. This is a downslope movement aided by gravity.
21
Q
what are the two types of overland flow?
A
saturated overland flow and infiltration excess overland flow
- saturated overland flow occurs when water accumulates in the soil until the water table reaches or ponds onto the surface (common when there are thin soils of moderate permeability)
- concavities near a stream or riverbank often have high moisture levels and can produce saturated overland flow early in rain storm cycle
- infiltration excess overland flow occurs when the rainfall intensity exceeds the infiltration capacity, so the excess water flows over the ground surface, delivering water to down stream river channels
22
Q
percolation and drainage basin flows
A
- water fills spaces in permeable and porous rocks creating ground water storage and an aquifer
- this happens where the permeable layer lies above an impermeable layer so water can’t permeate any further and creating a saturated zone
- porosity = total volume of pore space (greatest in corse grain rocks like sandstone)
- perviousness= rocks like limestone have joints and bedding planes along which water can flow
- rate will also increase according to the angle of the rock strata as a steeper gradient will allow gravity to operate more effectively.
23
Q
outputs of a drainage basin
A
- factors affecting evapotranspiration
- wind will increase the rate of evaporation by reducing the relative humidity and preventing saturation of the air
- soil moisture content will determine the amount available for transpiration
- vegetation cover will increase transpiration. vegetation with low albedo (reflectivity) such as dark forests will absorb more solar radiation, increasing evaporation
- higher temperature, more evaporation
- channel flow is the water collected to flow in a rivulet, stream or river. The discharge is dependent on the amount of precipitation falling directly into the channel
24
Q
what is actual and potential evaporation?
A
- actual evaporation is the amount of evapotranspiration that takes place given the actual water availability
- potential evaporation is the amount of evapotranspiration that could take place given unlimited supplies of water in an environment
25
main factors affecting drainage basins
- precipitation
- relief
- temperature
- lithology
- wind
- continentality
- vegetation
26
how does over abstraction impact drainage basins?
- In some locations, ground water is abstracted from aquifers faster than it is replaced, causing reduced groundwater flow and a lower water table. For example, ground water is used to irrigate 40% of China’s farmland and provides 70% of the drinking water available for northern regions, leading to a loss of 2.5 billion cubic metres of water each year.
- In other areas, reduced industrial activity has increased groundwater storage amplifying the risk of groundwater flooding if the water table reaches land surface. This can increase the system’s output through processes like evaporation.
- Aral Sea dried up draining bodies of water and leading to breathing difficulties due to salt in the air
- groundwater conurbation in major UK conurbations as a result of reduced abstraction for industries
27
how can the creation of impoundments impact drainage basins?
- dams restrict the flow of a river, holding water in reservoirs behind built impoundments. This causes water to pool outwards, dispersing over a larger surface area than in a river and meaning that more water comes into direct contact with sunlight. Subsequently, more energy is transferred to water particles, increasing the rate of evaporation and leading to greater outputs from the drainage basin system.
- Lake Nassar behind the Aswan Dam in Egypt has lost approximately 10-16 billion cubic metres of water per year through the process of evaporation.
- can also have a direct impact on the drainage basin inputs as it means more moisture is retained in rainclouds, increasing precipitation levels.
- cause a river’s velocity to decrease. This is because, sediment carried in the river’s load is deposited in still water as there is a loss of energy. This means that the velocity is slower and so the rate of flows through the drainage basin will also decrease.
- e.g. Grand Canyon led to beach erosion as starved of sediment
28
how can deforestation impact drainage basins?
- rainforests add water to the atmosphere via interception and evapotranspiration. Here, this moisture then contributes to the formation of rainclouds increasing precipitation and therefore the system’s inputs. When forests are cut down less water goes into the atmosphere and rainfall declines sometimes leading to drought.
- less interception, decreasing processes like through flow and ground flow whilst increasing surface run off as there is less time for water to be absorbed by the ground. -Rainfall has a global affect, travelling around the world and therefore unlike more sever than other local factors. For example, rainforests in the Congo are shown to have affected rainfall in Midwest America.
29
how can urbanisation impact drainage basins?
creation of impermeable surfaces reduced infiltration snd surface run off increasing river discharge
-this increases flood risk in areas like Winchester and Mindhead in the UK
30
how can cloud seeding impact drainage basins
- attempts to change the amount of precipitation by dispersing substances into the air that serve as cloud condensation nuclei (hydroscopic nuclei)
- new technology and research claims to have produces results that makes it a dependable and affordable solution to water deficits in certain regions
- for many regions, effectiveness is being debated
- China used cloud Seeding before 2008 olympics in Beijing to clear the air of pollution
- used in Alpine Meadows Ski Resort, California to improve snow cover
- 2015 in Texas to combat drought
31
water budget equation
```
p= Q+E(+/-)S
precipitation = discharge + evaporation (+/-) change to strores
```
32
soil moisture surplus
precipitation is greater than potential evapotranspiration and the soil water stores is full so there us a surplus of soil moisture for plant use, runoff and recharging groundwater.
-soil is at field capacity
33
soil moisture utilisation
potential evapotranspiration increases and exceeds precipitation, so there is more water evaporating from the ground surface and being transpired by plants than is falling as rain
-water is also drawn up from the soil by capillary action and the water is gradually used up
34
soil moisture deficit
- the soil water store has been used up by high rates of evapotranspiration and low precipitation
- plants can only survive if they are adapted to periods pf drought or are irrigated
35
soil moisture recharge
-this occurs when potential evapotranspiration decreases so that it is lower than precipitation and the soil starts to fill up again
36
field capacity
- the soil is now full of water and cannot hold any more
| - further rain could lead to surface run off
37
potential evapotranspiration
-the amount of evapotranspiration that occurs as a result of temperature and vegetation (time of years) as long as there is water available
38
river regime
described the annual variations in discharge of a river at a particular point
-cumecs
39
simple regimes
occur when the river experiences one main factor leadings o a period of high and a period of low discharge (e.g. snowmelt in summer or a rainy season/monsoon)
40
complex regimes
more likely to occur in longer rivers that cross more than one type of catchment which results in more than one type of factor influencing the pattern of discharge
41
Mekong example (river regime)
- very large catchment area and therefore experiences different climates across the basin
- it will also have varying terrains and vegetation cover adding to the complexity of the pattern
- this river passes through the mountainous relic close to its source in the Tibetan plateau as well as tropical areas with dense vegetation cover and monsoon climates closer to its mouth
- Mekong is 2600 miles long, passing through six different countries before reaching the south china sea.
42
how does human intervention influence river regimes
- building impoundments such as dams, a river’s flow can be impeded.
- For example, the Aswan Dam was constructed in the River Nile. Not only has this reduced the flow by about 65% but it has also regulated natural fluctuations between the seasons by reducing flood peaks in September months.
- Similarly, trapped water behind a dam, pools outwards forming a reservoir and increasing the surface area of water that is directly exposed to sunlight. This can increase precipitation and therefore reduce a river’s discharge, further impacting its regime.
43
how can climate influence river regimes
- In a humid tropical climate, the lowest discharge tends to be linked to the dry season, occurring in September months as there is less precipitation to maintain discharge levels.
- Peak discharge tends to be during warm and wet months as higher temperatures have the potential to melt nearby glaciers, supplying the river with more water but monsoon seasons also increase discharge through precipitation.
- For example, the Amazon river has the greater discharge in April-May as there is the most rainfall and it is also when snow from the Andes melts.
- tundra areas have lower discharge levels throughout the year due to frozen conditions and low precipitation, both of which reduce the amount of water present in a river.
- For example, the Yukon river has a low discharge from December to May.
44
how can the size of the catchment affect river regimes
Complex regimes are more likely to occur in longer rivers that cross more than one type of catchment as multiple factors will influence the pattern of discharge.
- different points of the river will experience different climates, terrains and vegetation cover. All of these components can influence a river’s discharge by affecting levels of precipitation, evapotranspiration and flows.
- For example, if the climate is hotter water particles will have more energy and so there will be more evaporation. If the terrain is steeper water will have a greater velocity reaching the sea sooner. If there is more vegetation there will be more interception and therefore evapotranspiration.
45
other factors affecting river regimes
- temperature
- precipitation
- geology and soil type. water stored in groundwater in permeable rocks steadily feeds the base flow all year, reducing fluctuation.
46
features of a flashy hydrograph
- high peak discharge
- short lag time
- steep rising limb
- rainfall reached the river quickly
- more likely to cause flooding
47
features of a subdued hydrograph
- low peak discharge
- long lag time
- gentle rising limb
- rainfall reached river slowly
- less likely to cause flooding
48
factors increase flooding
- urbanisation
- saturated ground
- frozen ground
- rural areas
- impermeable surfaces
- steep terrain
- high drainage density
- more rainfall
- circular shape
- deforestation
49
factors decreasing flooding
- hot temperatures
- forested areas
- permeable surfaces
- flat land
- less rainfall
- low drainage density (not many tributaries to collect rain water and have the potential to flood, this reduces the volume of water)
- elongated shapes
- dry soil can both increase and decrease
50
impact of dry soil on flooding
- dry ground would normally have lots of pore spears, encouraging infiltration until the soil becomes saturated again
- in some cases, it may have experienced high evapotranspiration which may have drawn salts up through the soil via capillary action and left a salty crust on the surface. This makes it initially impermeable until washed away.
51
players involved in the hydrological cycle
- within the UK, planners are required to determine how any proposed development (e.g. new building estates) will influences flood risk. the national planned policy framework sets out strict guidelines
- decisions vary depending on how much weight is given to environmental factors (e.g. maintaining unaltered river flow) or economic factors which change flow.
52
SUD water systems
- sustainable drainage systems
- introduced to reduce run off produced from rainfall
- permeable pavements (delay run off by creating gaps between slabs)
- rainwater harvesting (collecting and reusing roof rainwater)
- filter drains( trenches filled with gravel to take away runoff water)
- detention basins (delay storm run off by a couple of hours)
- wetlands (retention areas covered with vegetation)
- sock away (a channel dug out to disperse surface water from ground)
53
drought
an extended period (season, year or several) of deficient rainfall relative to the average of an area.
-it is a slow onset hazard
54
socio-economic famine drought
widespread failure of crops and natural vegetation. demand greater than supply
55
meteorological drought
occurs when long term precipitation is lower than normal region specific
56
hydrological drought
deficiencies in surface and sub surface supplies (rivers, reservoirs, lakes and groundwater) can lead to salinisation in high temps
57
the order that droughts occur
meterological
agricultural
hydrological
socio economic/famine
58
how does global atmospheric circulation lead to drought
- the areas 30 degrees north and south of the equator are drought prone (sub tropical ridge area) as air sinks creating arid conditions
- sometimes the sub tropical high pressure zone (STR) associated with the descending Hadley cell air blocks the high humidity, rain bearing masses that arrive with the ITCZ. it is so strong and dense making conditions stable.
- this can cause severe drought in areas like the Sahel
59
how to blocking anticyclones create drought
- Depressions move from west to east in the mid latitudes as a result of the Coriolis effect and their track is directed by the polar front jet stream.
- The loops of the Jetstream occasionally stabilise or break up allowing high pressure areas from the subtropics to move northwards.
- this brings stable weather conditions to mid latitude countries, but it also forces depressions to migrate away. -Anticyclones are extremely stable due to their sinking air and calm conditions, meaning that they can persist and block western weather systems for up to two months.
- If this continues over the space of a few months, normal precipitation is greatly reduced and can cause drought. For example, in 2018 England experienced a period of prolonged below average rainfall due to blocking anticyclones that displaced mid latitude depressions.
60
what are the normal equatorial conditions
- At the equator, trade winds converge and migrate westwards due to the Coriolis effect. This pulls warm surface water in the direction of travel (east to west) causing heat to gather in the western side of the Pacific Ocean.
- the east hand side, upwelling occurs whereby cold water formerly located in deeper ocean is forced upwards to replace migrating water. This sets up a self- perpetuating ocean-atmospheric circulation system whereby warmer water in the West adds heat to the air causing it to rise and induce unstable weather conditions in Asian areas.
- It then starts to sink in the east creating stable, arid conditions in countries such as Peru.
61
how can the el Nino effect cause drought
- The El Nino effect is often caused by large volcanic eruptions such as Pinatubo in 1991 (ash can create solar shielding, increasing surface temperatures) or due to slow oceanic changes around the equator.
- can reverse the normal weather patterns in the equatorial pacific, creating potential drought in South East Asia as oppose to South America.
- occurs every 2-7 years in the Pacific Ocean
62
how can the la Nina cycle cause drought
- occurs when the warm mass of water is pushed even further west than normal, causing drought in Peru and California
63
examples of el Nino effects
- ENSO is known to cause global variations in rainfall patterns by changing the global atmospheric circulation, creating droughts and floods throughout the world. For example, the severe East African drought of 2011 was attributed to La Nina.
- this is thought to have caused the 2015 droughts which led to widespread agricultural failures and forced Indonesia to import 1.5 million tonnes of rice to compensate for this.
64
features of an anticyclone
- upwards air movement
- clockwise winds
- dry weather
- very stable, not early moved and can stay put for several weeks
65
human influences on drought
- over abstraction of groundwater (hydrological drought)
- climate change
- growing affluence of the population thus increasing demand for goods
- poverty (root cause)
- over cultivation and over grazing
- deforestation
- development (root cause)
- population rise
66
Brazil case study for drought
- experienced worst drought for 80 years in 2014-15
- water supplies were so low that HEP dams were depleted (some to 1% of capacity) and so suspended, there were agricultural crises and urban taps ran dry (3 days a week in some towns)
- water rationing for 4 million people
- a series of high pressure systems diverted rain bearing winds further north away from the Amazon and also prevented them from diverting southwards from the Andes
67
how did deforestation affect Brazilian drought
- faced droughts 2000. by 2014 some now argue it has reached a tipping point meaning hydrological patterns have changed permanently
- thinning forests have reduced soil water storage and evapotranspiration
- replaced the area with savannah like grasslands
- will threaten Brazil's dependancy on HEPs
68
how has over abstraction impacted Brazilian drought
- in Sao Palo states, industries, domestic users used groundwater as rivers ran low
- tap water was also at high fees of (us$3000)
- this meant many started to drill illegal wells (70% in 2014). This meant water wasn't regulated and so groundwater levels shrank rapidly
- illegal wells are also shallow and less filtered by bedrock so they contain industrial pollutants and higher levels of bacteria perhaps leading to socio-economic drought/famine
69
drought in the Sahel
- drought since 1960s
- CISRO suggested they were caused by air pollution generated by Europe and NAmerica that caused atmospheric cooling, changing the global heat budget and circulating, causing drought
- in 2005, NOAA indicated that the drought could be a result of higher sea surface temp caused by anthropogenic climate change as rain bearing winds move over the Sahel when sea temp is higher
- el nino effects region
- region has one of highest poverty rates and lowest development. this has increased demand leading to cultivation and desertification, increasing susceptibility to drought as reduced vegetation cover also reduces soil moisture
- poorer populations tend to live less of a consumer lifestyle and therefore are less likely to contaminate the atmosphere with pollutants. Given that these pollutants are known to alter the global heat budget and therefore cause drought, I therefore believe poverty is not the main factor.
70
Australian drought
- 1997-2009
- largest uninterrupted series of years with below median rainfall in SE Australia since 1900 (rainfall 12.4% below mean of the century)
- el niño was partly responsible (explaining 2/3 of the rainfall deficit)
- strengthening of the high pressure belt accounts for 80% as this ridge blocked depressions forcing them to higher latitudes and reducing frontal rainfall
- research indicates that this is linked to anthropogenic climate change by increasing global surface temperatures. it reduces the temperature gradient between the poles reducing the energy available for mid latitude storms and polar front jet streams
- there is insufficient data to prove this
71
components of an ecosystem
- biotic (living species)
| - abiotic (non living species, water, rocks and atmosphere)
72
processes in an ecosystem
-nutrients cycling, hydrocycle, weathering, food webs,
energy transfers
-ecosystem functioning = the biological, chemical and physical processes that take place within an ecosystem
73
ecosystem resilience
the capacity of an ecosystem to recover from disturbance to withstand ongoing pressure
74
impacts of drought on wetlands
- less clean water can be provided for streams
- persistent dry conditions can encourage wild fires
- soil moisture is reduced, becoming vulnerable to erosion and less able to store water in times of flood. organic soil can also oxidise and break down releasing carbon into the atmosphere
- depleted food supply for non aquatic birds making, it harder for them to find invertebrates. this means less resilient species may be eliminated from the ecosystems affecting the food web
- less river and groundwater flow into the wetland leads to areas of an open water shrinking and drying up. this can lead to a loss of habitat (e.g. aquatic birds like moor hen)
75
impacts of drought on forests
- forests are highly valued as they partake in water storage, the regulation of hydrological processes, timber production, habitats and carbon sequential and recreational opportunities
- GFN calculated that forests store an average of 0.73 tonnes of carbon per hectare per year
- between 2002-3 severe drought hit SW USA. hot dry conditions made the piñons more susceptible to pine bark beetle attacks. in some areas more than 90% of the piñons died. warmer winters also meant more survived and reproduced
- research suggests the an average of 1300 forests worldwide have trees that take an average of 2-4 years to recover and resume normal growth rates following a period of drought (lower resilience for species with a higher water intake like pine). it can also cause long term issues due to stress (foliage loss, impairing growth and water transport, disease, damage to vascular tissue)
76
different types of flooding
fluvial flooding - river discharge exceeds bank full conditions and overflows from the river
pluvial - surface flooding
e.g. -urban flooding (drains overflow and water comes up)
-water table rising to surface (ground saturation = saturated overland flow = groundwater flooding)
-infiltration excess flooding (rain is falling faster than infiltration capacity)
77
depression formation
- depressions cause frontal rain when warm air is forced to rise over cold air
- e.g. when WA from the tropics meets CpolarA over the Atlantic ocean the tropical forms a wedge into the polar air forming the front.
- depressions move eastwards across the Atlantic as this is the direction of the jet stream
- there are low pressure, rainy and windy systems
78
what do depressions look like on a synoptic chart
-cold front is blue with triangles
-warm front is red with semi circles
-black circular lines are isobars (close together)
on a satellite image they look like swirling clouds
79
how can depressions lead to flooding
- steady rain can lead to saturated overland flow
- intense rain can lead to infiltration excess overland flow
- of the jet stream stays put over a region for a long period of time, this place can receive more depressions than usual and therefore fee at greater risk of flooding
- there is uncertainty over how the jet stream might be affected by climate change. it is suggested that the jet stream may become weaker as the Artic warms and the tropics expand.
- this may mean it stays put in one areas for longer leading to more extreme conditions
80
cumbria floods December 2015
- NW of the UK is familiar with heavy rain. The combination of warm west westerly winds plus upland topography creates orographic rainfall.
- flooding was made worse by already saturated ground conditions (November had been the second wettest November since 1910)
- caused by deep Atlantic low pressure systems. Associated fronts stretched across norther Britain, bringing prolonged and heavy rainfall via a mechanism known as the 'warm conveyor'. The jet stream also remained over the NW longer than usual bringing in rain laden depressions from the Atlantic-over 400mm of rainfall at Thilmere in 38 hours
- 5200 homes flooding
- 61,000 homes lost electricity due to an electrical substation was flooded
- landslide closed section of the west coast mainline
81
economic impacts of flooding
- structural damage to infrastructure
- higher insurance premiums/refusal of insurance
- lower property value
- loss of livelihoods
- drop in tourism
82
social impacts of flooding
- stress and grief
- sewage mixing with flood water creating secondary hazards
- tropical countries can mean that poisonous snakes start swimming around (e.g. in Bangladesh)
- psychological and physical loss of death
83
environmental impacts of flooding
- natural, moderate flood can be beneficial to wetland ecosystems by encouraging nutrient dispersal, breeding and migration
- recharge of groundwater supplies
- environments already degraded by human activities means flooding becomes more negative
- excessive overland flows brings too much sediment and washes too many nutrients into ecosystems leading to eutrophication or pollution
84
how did changes to land use affect storm Desmond
- environmentalist George Manbiot suggests that farmers make matters worse via overgrazing as this creates bare slopes reducing the benefits of forests and meandering channels
- now we have bare drier soil, straighter and dredged channels and faster run off, creating reduced lag time.
- rainwater reaches flood plains quicker
- impermeable surfaces have increased due to urbanisation
85
how did river mismanagement affect storm Desmond
- before 2005, a combination pf raised river banks, pumping stations and diversion channels carried surplus water away from built up areas in Cumbria and these were reinforced after the 2005 floods
- design is based on the flood return period (an estimate of how often a flood of a certain magnitude will occur) based on past floods (e.g. a 1-in-a-100 year flood)
- climate change is likely to cause further changes to land use meaning there will be more extreme storms and flooding, overcoming similar protection schemes.
86
storm Desmond mitigating risk
introduced a range of soft engineering strategies
- reforestation upland reduces rapid surface run off
- restoration of river channels allows meandering
- restoration of floodplains to natural absorbent states soaring more flood water
- refusal of planning permission to built near rivers
- e.g. Keswick flood defences were built to allow the river to rise by 5m (height reached in 2009) but in 2015 river rose 5.9m. the height had been decided based off a 1-in-a-100 year event and so they weren't prepared.
87
snowmelt tsunami flooding
- Melting snow in late spring regularly causes extensive flooding throughout Asia, especially in the region of Siberia due to rivers such as the Ob and Yenisei.
- This is because, the quick transition from winter to spring upstream causes rapid snow melting while lower reaches remain frozen.
- This impedes infiltration but also means that flood water is often blocked by ice dams, causing it to pool out in one specific area and cause flooding.
- snow melt in the Himalayas can also cause flooding, glacial outburst floods occur when ice dams melt, leading to the catastrophic draining of glacial lakes.
- this can be exacerbated by volcanic activity which generates melt water beneath the sheets, triggering instability and releasing sudden Jokulhalps.
88
how can monsoons cause Asian flooding?
- Intense solar heating during the summer can cause the ITCZ to more north during this period. The temperature and pressure gradient that this creates coupled with the earth’s rotation often yields the c-shape of boreal summer monsoons.
- this carries evaporated water from the Indian ocean to mainland India (e.g. the Bay of Bengal) before turning westward at the monsoon trough in north India.
- The formation of monsoonal rainfall can lead to flooding as it is thought to create a 10% variation in heavy rainfall, with higher amounts expected year on year.
- For example, in 2016, heavy monsoon rain in the Philippines led to flooding, landslides and the evacuation of villages.
- That said, if it wasn’t for monsoonal rain, much of India would face agricultural drought and therefore despite significant it is not the most catastrophic form of flooding.
89
how do monsoons form?
- seasonal reversal of wind direction caused by temperature differences between land and sea
- (spring/summer) large land masses heat up rapidly drawing humid air in from the surrounding ocean. as the moisture laden air reaches the warm land, and the moisture condenses as rain
- (winter) land masses become cooler than the surrounding ocean causing cold dry air to flow from the land out over the ocean
90
how can deforestation and the removal of natural vegetation cause flooding
- for agriculture or urbanisation
- reduced interception and evapotranspiration, increasing surface runoff (saturated overland flow) and resulting in precipitation reaching her river quicker (flashy hydrograph)
- increases soil erosion, resulting in higher sediment load and deposition in channels, reducing river capacity, further increasing flood risk
- e.g. deforestation in Nepal and Tibet is increasing flood risk in Bangladesh from the Ganges and Brahmaputra rivers
91
how can mismanagement of rivers cause flooding
- using these structures can sometimes just transfer the problem of flooding elsewhere
- building artificial levees increases channel capacity but discharge is funnelled downstream instead
- e.g. the River Mississippi where channelisation (making it wider, deeper and straighter) and levees have actually increased flood risk by constricting the water and putting pressure on the levees until they first
- this happened in 1993 and 2011
92
how can urbanisation cause flooding
- expansion of impermeable surfaces (road, roofs and patios) increases surfacer run off Ito rivers via the urban drainage system
- lag times are reduced as drainage systems aim to deliver water quickly to water courses so that streets don't flood
93
how can floodplain drainage cause flooding
- done to provide land for agriculture or to expand urban areas
- this reduces the natural storage capacity of the floodplain especially where natural wetlands are lost
- land also shrinks when it is drained, making it more low lying and vulnerable to flooding
94
Haitian floods
- more than 98% of its forests are gone
- 800 people are dead
- removing vegetation means the top soil washes away making it harder to absorb water.
- yet, less tree coverage means less regular rain and so water table drops damaging farmers and communities
95
Missouri floods
- 1993 flood triggered by same discharge as 1903 but was 12feet higher.
- this wasn't meant to happen as it was protected by 29 locks and dams north of St Louis, hundred of channels and artificial embankments
- constrained artificial channels caused the flooding
