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Geography Lecture 10 Part 2 (1. Introduction to floods (Types of floods…
Geography Lecture 10 Part 2
1. Introduction to floods
Rivers morph based on external and internal factors
Flooding is a natural event
Floods replenish soil nutrients and build up floodplains
But they also destroy many things
Impacts of tropical cyclones
Strong winds
Right quadrant has strong winds
Torrential rain
Maximum concentration of rain occurs right outside the core
Storm surge
Storms push water inland when they make landfall
Types of floods
Fluvial (river) floods
Excessive rain from cyclones, snowmelt and ice jam
Pluvial (surface) floods
Heavy rainfall in urbanised areas leading to excessive overland flow
Flash floods
Intense rainfall in short periods of time, overwhelming rivers and prevention methods
Coastal floods
Inundation of land areas along the coast, high high tides, heavy rainfall and onshore winds
Storm surge
Abnormal rise in water level, usually caused by forces from wind, waves and low atmospheric pressure
Flood characteristics
Magnitude
Refers to the amount of water discharged by a river during a flood
(amount of floodwaters)
Frequency
Probability of how often a flood will occur
Recurrence interval
Time till the next occurrence of a flood of a particular magnitude
Small = more frequent
Influence of man on floods
By living in low-lying areas close to rivers, humans have subjected themselves to higher flood risk
Modification of the climate and land for agriculture and other things have also impacted floods, both positively and negatively
2. Factors influencing occurrence of floods
Climatic
Excessive Precipitation
Prolonged excessive rainfall
In Tropics, usually are monsoon and hurricanes
Basin
Basin Size
Smaller basins respond more quickly than large basins
Basin Shape
Rounder basins respond faster and are more likely to flood
Relief
Steeper slopes encourage rapid runoff
No. of tributaries
Higher number of tributaries = faster response
Other Physical Characteristics
Thin soils
Difficult infiltration, promote overland flow
Rocks
Impermeable rocks encourage overland flow
Antecedent moisture
Saturated soil = more overland flow
Channel Aggradation
Deposition of load on the river beds lead to high water levels, leading to higher flood risk.
Rough channel also causes more energy loss through friction = more deposition = reinforce channel aggradation
Human
Urbanisation
Highly impermeable surfaces
Smooth drains lead to faster flow = higher flood risk
Modification of river channels (bridges and such) reduce carrying capacity
Agriculture
Cattle ranching - over grazing can encourage overland flow
Agriculture can reduce infiltration capacity and increase soil erosion
Deforestation
Reduces infiltration capacity and increase overland flow
Loosens soil, leading to higher erosion
Upstream flood control
Dams can cause floods if too much water is released
Others
Tides
High tides increase water level, leading to higher chance of flooding
Coastal storm surges
Hurricanes can cause storm surges that can lead to floods
Landslides
Blocked river channels can flood as the water has nowhere to go
Earthquakes and dam failures
Failure of defense structures can lead to floods (usually are caused by earthquakes)
3. Effects of floods
(NEGATIVE) Usually grouped in 3 groups
Environmental
Erosion of land
Receding waters result in massive erosion in the surrounding lands and may result in loss of farmland, as well as weaken building foundations
Destruction of ecosystems
Natural vegetation will be destroyed and pollutants in the water will make things worse in the future
Changes to river morphology
The river that flooded will change its shape somehow
Pollution
Pollutants will contaminate lands and rivers
Salinisation
Cyclones can bring salt water inland
Can affect drinking sources and irrigation
Social
Death
Floods can kill, and this is one of the most severe social impacts
Health Hazards
Sewage and waste pipes are broken, leaking waste into floodwater
This contaminates floodwaters and drinking water, which leads to spread of water-borne diseases
Sewage may get into homes where it is difficult to clean
Insects are more active and tend to spread more diseases
Bangladesh 2004, Floodwaters contaminated, caused diarrhea
Loss of homes
This can cause both short and long term psychological impact
Bangladesh 1998, 25 million people lost their homes
Disruption of essential services
Wiring and pipes may be damaged, causing disruption of important services
Shortage of food supplies
Can lead to famines, especially in LDCs
Contaminated floodwaters can destroy farmland and kill livestock
Economic
Damage to property
Private housing and property is damaged or destroyed
DCs tend to have higher damage as things are expensive
LDCs have lower damage as things are worth less
Bangladesh, 1998, 7 million homes were destroyed
Damage to infrastructure
Power lines and other Infrastructure may be damaged, causing decline in economy as businesses are unable to operate
Bangladesh,1998, more than 400 factories closed
Unemployment
If businesses are unable to recover, unemployment can be a very big problem
This problem is more apparent in DCs due to the higher value of infrastructure
Difference between DC and LDC
DC
Tend to have less social impact, but economic impact tends to be higher
LDC
Higher social impact (more people die + other effects) and economic impacts tend to be smaller
Need to remember that there are primary and secondary effects
Remember to talk about vulnerability
(POSITIVE) Effects of floods
In LDC, NGOs and other countries may provide funding for rebuilding, which allows them to build better infrastructure and create new economic opportunities
The sediments of a flood may be very fertile, and that will aid in agriculture
Floodwaters may create new habitats for animals to live in
4. Strategies to manage floods
PMR Model
Prediction
Forecasting using Flood Recurrence Interval
Using past experiences, we can determine how likely is a flood going to occur, and what magnitude it will be at
Satellite Imagery
Satellites monitor the flooding and data collected is used for future predictions
Weather forecast
Finding out how much rainfall can help determine the chance of flooding
Radar system for flood warning
Detects rises in water level and gives out warning
LIMITATIONS
Climate change can cause flood patterns to change, making it less predictable
LDC tend to be less effective as authorities tend to be slower
If can implement short response time = good
Mitigation
Hard Measures
Dams and reservoirs
Can reduce impact of floods in the immediate region, as well as having many other uses such as providing hydroelectricity
Channel realignment and diversions
Modify the channel by realigning or re-sectioning
This diverts floodwaters away from vital areas
Cutting a new channel for the floodwaters to flow while leaving the original flow undisturbed
Channelisation
Line the river with concrete to keep the velocity high and flow faster to the lower course
Channel Dredging
Keeps river free from sediment, increases river capacity
Flood-proofing
Temporary measures to reduce losses due to flood
Blocking entrances
Sealing doors and windows
LIMITATIONS
While these may protect a certain region, it may worsen the flood in another region
May have unexpected outcomes
Levees
Raised river banks to increase the height of river banks - prevent bankfull discharge
Low cost, easy to construct. Can also be used to redirect water on low lying lands
Soft Measures
Flood abatement
Reforestation helps to reduce the amount of water going into the river, thus helping with floods
Terracing of slopes to reduce runoff
Preservation of natural water stores
Land-use zoning
Divide the floodplain into areas with differing flood risk
Restrict land use based on those areas to reduce flood risk
Evacuation
People and property are removed from the hazard area
Loss sharing
Disaster aid and insurance can help reduce impacts of floods
LIMITATIONS
These measures may not work on their own and require hard measures as well
Some of these also encourage people to continue living in flood-prone areas
Response
Initial response
Local authorities provide short term food and shelter
Bangladesh 1998 government provided 350,000 tonnes of food and 5000 shelters
Search and rescue
Bangladesh government worked with police, civil defense and fire brigade
Foreign aid
Japan contributes US$8.2 million to provide aid to Bangladesh for the 2004 flood
Secondary response
Local authorities may relocate the victims
Restart the economy by helping businesses
Further flood protection
Foreign aid
LIMITATIONS
REMEMBER:
Scale of measure
Stakeholders
Cost-benefit
Moral considerations
EVALUATION:
Reduction in vulnerability?
Strengths and weaknesses
Reconstruction and restoration of basic services
Long term/short term?
5. Evaluation of current flood management
All 3 (PMR) must work together to help reduce flood risks
Ayubia National Park in Northwestern Pakistan
WWF and Coca-Cola helped build new infrastructure to battle floods
Reforested with pines and other trees, as well as build walls for slope stabilisation and dams