The human impact Flashcards
Deforestation
The cutting down of trees mainly to develop agriculture
Reduces interception and evapotranspiration which increases surface runoff
Decrease in channel capacity due to more sediment
Loss of root systems causes soil erosion to occur rapidly so sediment washes into rivers, increasing risk of flooding
Afforestation
The planting of trees mainly to develop agriculture
Increases interception and reduces surface runoff so lower peak discharge and longer lag times
e.g. River Exe - afforestation led to reduced winter and spring flows by 16% by planting trees in the catchment zone
Urbanisation
Natural river channels restricted by bridges or roadside facilities
Highly impermeable surfaces such as roads and pavements increase surface runoff
River channel straightening leads to fast delivery of water downstream
Angles roofs allow rainfall to run off quickly
Dams
Large increase in evaporation due to the construction of large dams
Water loss can be reduced by using chemical sprays on the water, by building sand-fill dams and covering them with plastic
Abstraction
The removal of water for human use
e.g. Irrigation, Drinking water, Domestic use
Saltwater Intrusion
Happens when abstraction occurs too much
Movement of saline water into freshwater aquifers, which can lead to groundwater quality degradation, including drinking water sources, and other consequences
Lowers the water table and increases the salinity of groundwater through overuse and building of wells
Water Storage
Downstream of the dam the flow rate in the river will depend on the amount of compensation flow
However, water volume is considerably reduced during the dry season to fill up the reservoir
Downstream may change to pools alternating with dry stretches during summer months
Sedimentation leads to clear water erosion downstream
When do floods occur
Flooding occurs when the discharge of a river is so high that the river overflows its banks onto the floodplain (over bankfull discharge)
Heavy rainfall can also lead to rapid surface runoff if the rainfall is too intense for infiltration to occur which results in a flash flood due to the sharp rise in discharge
Physical causes of flooding
Heavy rain
Banks burst and levees fail
Impermeable rocks
Steep sides of surrounding land
Drainage system
Low lying banks
Aggradation + sedimentation
Saturated soil or dried soil
Snowmelt from mountains
Human causes of flooding
Deforestation decreases interception
Urbanisation
Climate change
Exacerbated snowmelt and rainfall
Dam and flood scheme failure
Derna floods
10.9.2023
2 dams broke
6,000 deaths and 10,000 missing
30 million cubic meters of water
Entirely human caused
Flood Risk
Likelihood of a certain magnitude flood occurring
Magnitude
The size of the flood (CUMECS)
Frequency
How often a flood of a certain magnitude occurs
Recurrence interval
The interval at which particular levels of flooding will occur
Exponential relationship between the annual maximum discharge and recurrence interval
It has decreased due to climate change - what was a 1 in 100 year flood now could be every 80-90 years
Forecasting floods
Extreme floods are harder to model as there is very limited data available to forecast with
Without a flood forecasting model, you can’t make a forecast until the river’s discharge exceeds the capacity
Now, a flood event that might have occurred every 2 years now occurs every 1.8 years
How to build a flood forecasting model
Observations of existing conditions like rainfall and discharge
Application of physics and meteorology to observe patterns of rainfall
Application of hydrology to estimate how much rainfall enters the river
Creation of a hydrograph to understand how rainfall will change water level in river
Negatives to floods
Primary physical impacts - harm to people, damage to property and infrastructure
Secondary economic effects - disrupted infrastructure and damage to property
Man made levees
Reduce biodiversity by removing natural pool / riffle sequence
Very expensive and don’t last long
Look unnatural and increase sedimentation as sediment isn’t deposited on floodplains
Floodwater pools behind barriers and stagnates creating water-borne diseases
Increase flooding downstream
Dredging
To combat sedimentation and increase channel capacity
Used in combination with man made levees
Channel Straightening / Channelisation
Channel is straightened, widened and deepened
25% of the main rivers of England and Wales have been channelised
Allows higher volume of water to flow faster, reducing risk of flooding
Allows for agriculture to develop and for human purposes
Advantages of Channelisation / Straightening
Increases efficiency of river as hydraulic radius increases
Averts bed aggradation
Keeps channels navigable
Bank raising and dredging increase capacity
Widely used
Disadvantages of Channelisation / Straightening
Needs maintaining
Expensive and unnatural
Artificial Channel Linings
Concrete lined channels create smoother wetted perimeter and so increase velocity
Water levels drop and flood risk is reduced
Expensive and high maintenance
Dam Construction
Multi-purpose
Controlled release of water - removed flood risk entirely
Effectiveness depends on relative scale
Removes sediment causing clear water erosion, deepening the channel
Advantages of Dams
They trap and store water so reduce surface runoff as water is released in a controlled way
The reservoir lake can be used for recreation e.g. fishing
Very effective
Disadvantages of Dams
If the land upstream of dam is flooded, ecosystems change and residents may have to move
Trap sediment that would normally flow downstream and farmers rely on silt providing fertile soil after flood events
Very expensive
Spillways
Water from a too-high body of water spills into pipes which carry the water into a storage reservoir
Culverts / Flood Relief Channels / Diversion Spillways / Reservoirs
Effectively increases bankfull capacity and diverts flow away from high impact / risk zones
Requires space on floodplains so not always possible
Advantages of Culverts / Flood Relief Channels / Diversion Spillways / Reservoirs
Water can be pumped out of the river and stored in temporary lakes, or diverted during times of high flow
Pumped back in when discharge in river drops
Low impact zones can be recreational land use
Flood water will evaporate or eventually infiltrate, replenishing groundwater supplies
Disadvantages of Culverts / Flood Relief Channels / Diversion Spillways / Reservoirs
Need a large area of available land
Mississippi River hard engineering
Levees built along the whole length of the river
27 control dams built upstream and 100 dams built along the Mississippi’s tributaries and reduce pressure on levees
Channel straightening and channel lining took place
River shortened by over 250km
$7bn spent over 100 years
Soft engineering
Involves the use of the natural environment
Flood Abatement
Afforestation at upper catchment zones increases interception and reduces surface runoff
Lowers peak discharge and increases lag times
Contour Ploughing
Ploughing along contours of a field to reduce surface runoff
More suitable to Europe
Advantages of contour ploughing
Increased water infiltration and soil moisture storage so less overland flow
Easily manageable and a cheap solution
Disadvantages of contour ploughing
Heavy rain causes ground to become saturated so when there is peak rainfall the precipitation hits the ground and runs off into the river anyway
Gradient of slope helps increase velocity of runoff
Land owners may not agree to have their land ploughed along the contours
Terracing
Strip cropping to vary interception and surface runoff
Stores water as surface water
Vertical ploughing technique
Used in Asia as it is better for growing rice
Advantages of terracing
Can prevent flooding from the source of the river
Often is cheap
Disadvantages of terracing
The gradient of the land can increase velocity of surface runoff
Restoration
Aim to restore the river’s natural processes of erosion and deposition
May use floodplains for water to overflow into, all hard engineering removed, meanders re-dug
Complete survey completed to ascertain river’s natural course, then attempt to chive it through mechanical engineering
Advantages of restoration
New meanders can hold more water, therefore reducing the risk of floods as there is more room in the river for floodwater
Cheap
Disadvantages of restoration
Need space for a large floodplain
Flooding still occurs when water moves at high speed
Needs high investment
River Quaggy, Sutcliffe Park
Example of river restoration scheme
85,000 cubic metres of water can now be stored as the park was lowered and reshaped to create a floodplain capable of storing large volumes of water
Made to provide additional flood storage through ponds and lakes, simultaneously creating an attractive open space
Visits have increased by 73%
New habitats built by introducing wetland pond areas so biodiversity increased
Forecasts and warnings
Improved use of weather satellites and radars can reduce flood damage by up to 40%
Data on rainfall and stream discharge produce accurate predictions of flood surge times
Can be used for authorities to organise evacuations or communities to prepare for floods
Issues with forecasts and warnings
People don’t always listen
Warnings may not be received, like in rural areas of LICs
Not always possible as flash floods have too short a lag time
It is usually much less sophisticated in LICs so they are more likely to be affected by floods
Floodplain zoning - zone 3
The prohibitive zone
No further development is allowed except for essential waterfront facilities
Floodplain zoning - zone 2
The restrictive zone
Only essential development and recreational activities are permitted - all buildings should be waterproof
Floodplain zoning - zone 1
The warning zone
Inhabitants receive warnings of impending floods and are reminded regularly of the flood hazard
Advantages of floodplain zoning
People can live close to a river but not be affected by floods (unless extreme ones occur)
Cost is minimal and once put in place requires little management
Disadvantages of floodplain zoning
No hard structures to prevent flood water so the scheme is flawed when there is heavy rain and high velocity in river
Can only be put into place in an area of the river which hasn’t been built up
Can’t be used where floodplain has already been built up
Causes of Bangladesh floods
80% of Bangladesh is in the delta of 3 rivers - Ganges, Meghna and Brahmaputra so when snowmelt occurs in late spring, early summer, flood risk increases due to high peak discharge
Low lying country with most of it between 5 and 8m above sea level with some coastal areas less than 1m above sea level
70% of land is farmland so vegetation which could intercept surface runoff has been removed
Upper courses of rivers are in other countries so Bangladesh can’t build dams upstream which could prevent flooding downstream
High corruption (149 / 180) and low GDP per capita ($2,600) so much of the little money the country has is pocketed and not spent on flood prevention
Social Impacts 2004
40% of Dhaka underwater
800 deaths and 30m people made homeless
100,000 people in Dhaka suffered from diarrhoea due to dirty water
900,000 families lost their home
3m experienced damage to housing
Social Impacts 2007
9m made homeless
1,000 died from drowning and water-borne diseases
1,100 deaths in total
10.5m displaced
100,000 cases of dysentery and diarrhoea
52 villages in 40 districts affected
Social Impacts 2017
8m affected and 150 deaths
104,000 houses ruined and 630,000 partially damaged
1,000 schools closed and 4,300 schools were damaged
450,000 hectares of farmland flooded
Economic Impacts 2004
Cost of flood damage was $2.3bn
2m acres of agricultural land submerged
5,000km of road destroyed
3,919 livestock dead
Economic Impacts 2007
2.2m acres of damaged cropland
$290m of crops damaged
Total economic loss was $1.3bn
$7bn in damage to schools and hospitals
Economic Impacts 2017
21.49% reduction in annual earnings
90% of crops damaged in Sunamganj district
Rice price increased by 30%
Damaged 220,000 hectares of crops with 16,000 hectares fully lost and 560,000 hectares were partially damaged
Environmental Impacts 2022
53,000 hectares of agricultural land was flooded
Many fish could be seen floating from the flooded ponds and reservoirs so habitats were destroyed
Water couldn’t drain due to the soil already being saturated from a flood the month before
Hard Engineering - Kaptai Dam
Kaptai Dam built in 1962:
Stores 6,477m cubic metress
Cost $22.5m
Produces 230 MW - 5% of national electricity
Funded by US Agency of International Development (USAID)
Displaced 100,000 indigenous people
Submerged 54,000 acres of farmland
Hard Engineering - Teesta Barrage
Started in the 1960s
Aimed to protect coastal zone from storm surges
Manages the area’s water flow
Cost around $150m (funded by World Bank)
4,000km of embankments along coastal areas
139 polders built to protect agricultural land
Manages up to 4,500 cubic metres per second
Provides irrigation to 111,406 hectares of farmland
Planning to generate 67.5 MW
Flooded in October 2023 due to heavy rains in India causing a dam to release excess water
