Studied by 0 people
Just flashcards to help revise my notes
What does steepness of slopes contribute to flooding?
Very steep slopes prevent water from soaking into the ground, causing it to run straight down into valleys and rivers, increasing flooding risk.
How does the geology (rock type) of an area affect flooding?
Some rocks let water into them (permeable) while others don't (impermeable). If the rock is permeable it can act like a sponge - soaking up the water. If it is impermeable however, it can't soak downwards so it flows across the land and into rivers.
What role do previous weather conditions play in flooding?
The ground (e.g. soil) often acts as natural 'sponges'. When it rains, they fill with water and stop it from reaching the river quickly (reducing chances of flooding). However, if it has rained lots in the days leading up to the storm, the soil may be 'full' or 'saturated' meaning no more can fit in. As a result, the water flows straight into the river.
How does urbanization affect flooding?
Concrete can't soak up water and is the main material used in building. As the rain lands, it therefore runs straight off into the river through drains and over the ground and into the river. This means that the water reaches the river quickly and increases the chances of flooding.
How does deforestation affect flooding?
By cutting down trees, the natural ability to intercept and absorb rainfall is lost, allowing water to reach rivers faster and increasing the risk of flooding.
Why is flooding worse in an LIC than in an HIC
Flooding is worse in Low Income Countries (LICs) than in High Income Countries (HICs) for several reasons. In LICs, there isn't enough money to repair damaged homes or support people, so recovery is much harder. Governments in LICs also struggle to provide aid and support, unlike HICs, where help is available. LICs are less educated on how to handle floods, while HICs know what to do.
Homes in LICs are often not strong enough to withstand floods, but HICs have warning and evacuation systems to keep people safe. Finally, food and water sources are often unreliable in LICs, making it harder for people
Dams - Hard engineering
A dam is a large barrier built across a river in order to limit the amount of discharge (water volume) that flows downstream. Dams are effective because water can be very accurately regulated, but also they can be multi-purpose. Dams can be used to generate electricity (like the 3 Gorges Dam in China) or the reservoir be used for leisure activities (eg boat trips, water sports etc). The 3 Gorges Dam has reduced the likelihood of a flood from 1 to 10 to 1 to 100 years and also was expected to produce 10% of China's electricity demands. However, there are many people who oppose the building of dams. Firstly, they cost a huge amount of money (3 Gorges Dam $37 billion), but also they require a huge area of land to be flooded behind the dam meaning that people living there need to move or agricultural land is lost.
Preparation - Soft engineering
Preparation is about individuals and families preparing themselves for cases of flooding. This can be about making changes to your house, keeping sandbags, storing torches and valuables in a high place, and having plans for what to do in the case of a flood. Positives of this idea are that the impact of flooding can be reduced to a certain extent. People are less likely to worry about flooding and it can be relatively cheap for certain preparations eg sandbags. Negatives of this idea are that they can provide people with a false sense of security. It also doesn't necessarily protect effectively from serious flooding, and any larger modifications to houses would cost a lot. In Osney (Oxford) more than 2000 sandbags were given to residents prior to the floods of 2007.
Afforestation - Soft engineering
Trees are planted in the catchment area of the river, especially upstream, in order to intercept (catch) rainwater and stop it from reaching the channel. This helps because not only will it catch water on its leaves, but absorb it with its roots from the soil. Furthermore, the roots from trees help keep the soil together and stop it from eroding and being washed into rivers. This means more water will also be stored in the soil. This is being done in many parts of the world where quick deforestation has taken place, e.g. in Nepal afforestation is taking place to reduce the flooding of the Ganges delta. Other benefits of this scheme is that it is sustainable, low-cost and improves the quality of the environment. Drawbacks of such a scheme can be that it can look unnatural if densely packed trees, and it can increase the acidity of the soil. In some cases and environments, it can be a fire risk.
Flood Walls - Hard engineering
Flood walls are walls of concrete built either side of the river bank. They are built so that more water can fit in the river channel, and to stop it from flowing out onto the flood plains. Although not as expensive as dams, they an be quite costly, and if not assembled very well they can collapse which would mean they are not effective. This type of protection is best used where there is very little space, and where the area either side of the river is of high value and needs to be protected from flooding. Along the Mississippi in New Orleans there were flood walls to try and help eliminate flooding- although these were broken when Hurricane Katrina struck.
Warning Systems - Soft engineering
One way to try and manage flooding is by having in place warning systems to let people know when a flood is likely. This can be done by sirens, on TV, via radio, text messages or email. This would allow people to be able to prepare for flooding, evacuate, or move valuables to a higher level. This is especially effective in MEDC’s where people are easily contactable by these technologies. A huge advantage of this is just how low-cost it is. It requires very little money to install. However, concern is still raised about whether simply 'knowing' that a flood is about to happen will really help solve the problem of flooding, and it is possible that sirens could be vandalised. In the UK the Met Office carefully monitors predicted weather and rainfall, and has a different set of traffic coded warnings to allow people time to prepare in advance of potential flooding.
What are cause action response
Cause: An action that makes an event happen.
Effect: How people or the environment was changed in a good and bad way.
Response: The help given to manage the problem.
Causes of Boscastle floods
Causes :
Boscastle is found at the bottom of a steep slope near the mouth of the River Jordan drainage basin.
The first two weeks of August had been very wet with rainfall soaking the soil in the drainage basin around the village of Boscastle.
On 16th of August, the big storm, very heavy rain fell on the saturated, steep slopes surrounding the village of Boscastle, causing a devastating flood.
Over 75 mm of rainfall fell in two hours equivalent to a month’s rainfall. In total 200 mm of rainfall fell in the drainage basin over 24 hours.
Two billion litres of water then rushed down the River Valency valley straight into Boscastle.
Effects of Boscastle floods
Huge amounts of rainwater flowed quickly as surface runoff into the River Valency and its tributary the River Jordan (the confluence of these two rivers is in Boscastle).
Floodwater flowed into houses filling them with 2 metres of mud and water, 25 houses were destroyed by the flooding. 150 cars were swept away, 4 bridges were washed away.
Trees were uprooted and swept into peoples' gardens.
The weight of water eroded river banks, damaged gardens and pavements.
No one was killed
The damage caused to people’s homes and businesses ran into millions of dollars.
Responses of Boscastle floods
Responses:
Rescue helicopters quickly rushed to the flooding
150 people were rescued
Causes of Bangladesh floods
CAUSES:
70% of Bangladesh is less than 1m above sea level
Monsoon rainfall occurs within a few months
Deforestation has occurred in the Himalayas
High temperatures melts snow and ice in the Himalayas
35 cm of rain fell in one day on 13th September
If the ground is saturated then run-off will be very high
Effects of Bangladesh floods
EFFECTS:
600 people died as a direct result of the floods
The main food crop, rice was destroyed, along with other crops
Bangladesh experienced its heaviest rain in 50 years
Death toll rose to over 750
Airport, roads and railways flooded, bridges destroyed
40% of Dhaka was underwater and 60% of Bangladesh submerged
100,000 people in Dhaka alone were suffering from Diarrhoea
Damage to schools and hospitals put at $7 billion
30 million people were left homeless
Responses of Bangladesh floods
RESPONSES:
The UN launched an appeal for $74 million but only 20% had arrived by September
Authorities are encouraging people to build their homes on raised mounds of land
Food supplies, clothing and blankets distributed
Flood shelters have been built and flood warning systems established
Embankments have been built
Flood Hydrographs parts
Hydrograph characteristics:
Rainfall shown in mm, as a bar graph
Discharge in m3/s, as a line graph
Rising limb : The rising flood water in the river
Peak flow : Maximum discharge in the river
Recession limb : Falling flood water in the river
Lag time : Time difference between the peak of the rain storm and the peak flow of the river
Flash floods
Flash floods : A burst of very heavy rain can cause a sudden flood called a flash flood. This happens so fast that people get no warning. They can get trapped, and drown. Adding to the flood risk If rain can soak into the soil quickly, there is less chance of flooding. And the reverse is true: anything that slows down or prevents infiltration will increase the flood risk.
Factors that affect flooding
Factors that contribute to flooding
Heavy rain: Rain is the main factor. The heavier it is and the longer it goes on for, the greater the risk of flooding. The soil will be so soggy that the rain can't soak away. Impermeable rock: If there is impermeable rock (such as granite) below the surface, rain will not be able to soak through.
Cutting down trees: Trees help rain to soak into the soil. It is caught by the leaves, and trickles to the ground. It runs into the soil along the tree roots. So if you cut trees down, you increase the risk of flooding.
Steep land : Rain will run down a steep slope quickly - before it has a chance to soak into the soil.
Tributaries: The more tributaries the river has, the greater the risk of flooding. They may be swollen by rain too.
Built up areas: Rain can't soak through concrete. So rain that falls on our streets runs down the street drains, and is usually carried to the river. The river swells. (But if the street drains are blocked, streets can flood quickly just from the rain)
Long term solutions
Long-term solutions
Build embankments (high banks). These embankments along the Thames in London were first built to hold an underground sewage system, and Tube lines. But later, the walls were made higher as flood protection.
Dig new river channels. The Jubilee River above looks natural - but it's not! It was dug out to divert water from the Thames, in order to prevent flooding at Maidenhead, Windsor, and Eton. There are plans for others too.
Take care where you build. Local councils now weigh up the flood risk, before they allow new buildings in the Thames floodplain. They may refuse planning permission. You'd have to find another site.
Let nature help. Allow land along the river to soak up flood water, as nature intended. Plant more trees too. This common land beside the Thames in Oxford is called Port Meadow. It regularly floods.
Short term solutions
Short-term solutions
When we know floods are on the way, here are some things we can do
.1. Put up portable flood barriers. This shows them being fitted in Oxford, next to the river, after a flood warning. They'll be taken down later and stored away.
Put anti-flood shutters on homes. You can buy metal shutters like these to stop water coming in through doors and windows. (Or else try sandbags.)
