Core Theme 1 - Flooding In Drainage Basins Flashcards
What is a drainage basin
An area of land that is drained by the river and its tributaries
How does water enter a basin?
Through precipitation (rain, snow or sleet condensed into the ground)
If the water isn’t intercepted or evaporated, it will then flow through the drainage basin in a number of different ways.
What is interception and evaporation?
Interception - when water is absorbed and collected by vegetation
Evaporation - water turning into vapour
What is overland flow?
When the water moves across the surface of the Earth, becoming a stream, tributary or river
What is infiltration?
When the water soaks into the soil and passes through the soil and then travel through the soil out to the river channel by a process called through flow
What is through flow?
The downwards movement of water through soil
What is percolation?
When the water moves into the soil into the pores of the rocks where it moves by ground water flow into the channel
What is ground water flow?
When water is able to move through soil and rocks into the sea
What is evapotranspiration?
Water vapour is evaporated from the trunk and the leaves of trees and other vegetation, back into the atmosphere.
When does a flood occur?
A flood occurs when a river bursts its banks and the water spills onto the floodplain.
What are the physical factors of flooding?
Climate
Vegetation
Impermeable rock
Steep sided channel
How is climate a cause of flooding?
The faster the rainwater reaches the river channel, the more likely it is to flood.
If the climate of an area has high temperatures, this will mean more water is lost through evaporation, resulting in less overland flow and less water reaching to the river, causing a lower river discharge.
How is a steep-sided channel a cause of flooding?
A river channel, surrounded by steep slopes, causes fast surface run-off.
How is vegetation a cause of flooding?
Trees and plants intercept precipitation, ie they catch or drink water. If there is little vegetation in the drainage basin then surface run-off will be high.
How is impermeable rock a cause of flooding?
A drainage basin, consisting of mainly impermeable rock will mean that water cannot percolate through the rock layer, and so will run faster over the surface.
How is geology a cause of flooding?
The geology or rock type also influences the speed and quantity of water that reaches a river. Rocks that are permeable or porous allow water into them (this is called percolation) and slowly flow through them, (known as groundwater flow), meaning less water gets to the river and at a slower rate, reducing the discharge of a river.
Rocks that are impermeable do not allow water into them, so they produce greater overland flow. As a result, rainwater gets to the river quicker, causing the discharge of the river to be higher.
What are 2 human factors of flooding
drainage basins in urbanisation areas and deforestation
What process does the building of impermeable surfaces stop?
Percolation
What are the types of impermeable surfaces found in urbanised areas?
Roof tops, pavements, roads, streets etc…
Does urbanisation cause surface runoff to increase or decrease?
Increase because there is more volume instead of percolating through, it instead becomes over-land flow
Why does a greater amount of rainwater get to the river channel in an urbanised area?
Because there is more impermeable surfaces built everywhere, decreasing the chances of infiltration and percolation and more over-land flow reaching the river channel quickly
How is deforestation a cause of flooding?
Because if there is less vegetation, roots and plants, they will not be able to absorb the water reducing interception. Therefore, this increases the chances of over land flow and with less water taken up by the trees, a greater volume of water reaches the river, causing a greater discharge.
1.3.2 A detailed study of the causes and effects
of flooding in at least one location in Wales or elsewhere in the UK.
E.g. Describe and explain the causes and effects of the flooding in Boscastle [8]
Write this down in Geography Book for revision and compare your answer with the model answer on the other side.
The Boscastle flooding had many human and physical causes and effects.
One physical causes was the climate as there was over 60 mm of rainfall (typically a month’s rainfall) fell in two hours. This led to a flash-flood event. Also, a depression brought a thunderstorm on 16 August with over 200mm of rain in 24 hours. This was higher than the expected rainfall. Therefore, the soils became saturated due to the previous 2 weeks of rainfall so water couldn’t infiltrate into the ground. This increased the chances of overland flow, which caused an increase in the river discharge which caused the flood. Also, another cause was the that Boscastle has many areas of impermeable slate which caused fast surface run off. This contributed to overland flow again causing a flood. The flood also coincided with a high tide making the impact much worse. Also, the land had high localised temperatures and moist winds from the sea. This when combined with the effects of coral reef created a quick uplift of moist and warm air which caused an unstable environment with storm development and intense rainfall (1422 million litres of rain onto the flood plain)
Moreover, some human factors were that Boscatsle had very few trees. This is because many trees had been cut down for farming. This limited interception and water went straight into the river channel. Another cause were the few old arch bridges. These bridges got blocked up with the flood debris. This caused the bridge to act like a dam and eventually it smashed through causing a surge of water up to 5m high.
Due to these causes there were many effects such as homes, businesses and cars were swept away, affecting more than 1,000 people, local businesses were destroyed and income from tourism was lost. This had an impact on livelihoods and the local economy. The short term effects were that tourism decreased, houses were soaked and cars were recovered. Some long term effects were that it would take time to get insurance, the river to lower and valuables to retrieve back.
What is a hydrograph?
A hydrograph is a graph that shows the response of a river to a rain event in its drainage basin
What is peak discharge
Peak discharge is the point at which the river reaches its highest level.
What is peak rainfall
The highest amount of rainfall (most rain falls)
What is lag time
The time difference between peak discharge and peak rainfall.
What is the rising limb
Rising limb is the time when the river discharge is increasing.
What is the falling limb
The Falling limb is the time when the river discharge is falling.
What is base flow
Base flow is the normal flow of the river before the rain event.
What factors can affect the hydrograph
Vegetation, type of slope and soil type
Vegetation hydrograph. If a line is bare or void of vegetation. How will the line be? And if there is a forest how will the line be?
A very quick rising limb and falling limb and shorter lag time as it will quickly reach the river. This is known as a flashy hydrograph
For the forest line, the water will be intercepted by the leaves, so it would take its time to filter through into the soil into the river. This is why the line would have a longer lag time and the line would be much flatter. This is known as a non flashy hydrograph
Type of slope also affects the line on a hydrograph. How would the steep line be and how would the gentle line be?
If it is steep, the water will fall very quickly into the water. This would have a very fast rising limb and then a falling limb and has a short lag time. Flashy hydrograph
If it is a gentle slope, it would have a longer lag time before it then falls. Non flashy hydrograph
Soil type hydrograph. How would the impermeable line be and how would the permeable line be?
If there is permeable soil, there would be rapid infiltration and little overland flow, resulting in a less steep rising limb. This is a non-flashy hydrograph
If it is impermeable, there would be a steeper rising limb and short lag time as there would be lots of overland flow. This is flashy hydrograph
What is hard engineering?
Hard engineering is where man-made structures are used to prevent or control natural processes.
What is soft engineering?
When a river is managed in such a way so that it needs limited building work and floods are more prevented than managed. These measures are usually more sustainable
Give some examples of hard engineering methods
Flood walls, river engineering, dams, embankments, concrete channel and enlarge the channel
What are dams
Give the advantages and disadvantages as well
Built along the course of a river to control the amount of discharge. Water is held back by the dam and released in a controlled way.
Water is usually stored in a reservoir behind the dam. This water can then be used to generate hydroelectric power or for recreation purposes.
However it is very expensive and settlements and agricultural land can be lost (flooded)
What is river engineering
Give the advantages and disadvantages as well
A river channel may be straightened so that water can travel faster along the course. The channel course can be altered to divert floodwater away from settlements.
However, there can be a greater risk of flooding downstream, as the water is carried there faster.
What are embankments?
Give the advantages and disadvantages as well
Embankments are a large area of material that is built up alongside river to prevent flooding
They can hold more water, cheaper than flood walls and last for a long time (100 years)
They are still very expensive
What are flood walls
Give the advantages and disadvantages as well
A wall built along the river bank to prevent flooding
They can be used to raise the height of a river to a level where the river may not burst its banks
Very expensive
What is enlarging the channel
Give the advantages and disadvantages as well
Making the river widen or bigger so it can carry more water
Very little chance of flooding
Valuable farming land can often be lost at the side of rivers
What is a concrete channel
Give the advantages and disadvantages as well
The bottom of the river bed is rough so building concrete on it can smooth the flow of the river and make water flow faster. This is good for areas which are protected but land further down can experience erosion which can effect peoples homes and farming
Give some examples of soft engineering methods
Afforestation, no more building on flood plains, marshal areas, do nothing and give out sandbags
What is afforestation
Give the advantages and disadvantages as well
Tress are planted near river. This means greater interception of precipitation, slowing down how quickly it gets to the river and lowering the discharge.
It is very cheap but it cannot prevent flooding. It only reduces the seize of a flood and the likelihood of flooding
What is no more building on flood plains
Give the advantages and disadvantages as well
When construction is banned on a floodplain
Damage would be reduced
What are marshal areas
Give the advantages and disadvantages as well
This is a certain area of land next to a river.
Provides a great habitat for plants and animals and helps soak up the flood waters.
However, this land cannot be built on
What is do nothing
Give the advantages and disadvantages as well
This is just to do nothing and take no precautions and wait for a flood to happen and clean up afterwards
Insurance companies may refuse to insure people
What is giving out sandbags
Give the advantages and disadvantages as well
This is when local residents are given sandbags to protect their homes and businesses from flood water
It is arranged and distributed by the government
Short term only
What is land use zoning?
Managing the floodplain around a river
Local authorities and the UK government introduce policies to control urban development close to or on the floodplain. This reduces the chance of flooding
Low value land use such as farmland and campsites will be closest to the river
Land use of some value such as higher value agricultural land will be the second closest to the river
Highest value land use including settlements, industry and retail will be the furthest away from the river
What have scientists predicted?
Scientists have predicted our climate will become stormier producing more flash floods, as seen in Boscastle.
What has been suggested for future river management
That we should leave the river alone and look at how we could do this differently, e.g. not building on floodplains and allowing them to flood naturally. Also, in upper parts of the drainage basin, do afforestation, to reduce the amount of water reaching the river.
What is the new river management in the future
Pushing back embankments slow down the flow of the river and communities are better protected. When urban land is impermeable, embankments create narrow passages which makes rivers overflow and towns flood. Also, we can reconnect to natural floodplains with its alluvial forests. This way the river can connect to less vulnerable land and helps protect the communities better. A overlay narrow river runs too quickly which increases flood risk down stream. Also, a river which naturally meanders improves water quality, enriches biodiversity and can reduce flood risk. This causes flood flows to be slowed and flood peaks are delayed which gives more time to prepare. Straightened rivers can accelerate the flow of the river which can causes more increased flooding downstream. Also, by allowing natural sediment transport connectivity is good. As repeated flooding can make rivers without natural bed material more fragile. This causes the bank to deteriorate which can cause bridges and wildlife to be destroyed. By these ways, floods can be prevented
Name some conventional methods of controlling a river that floods? which stakeholders would support
House builder: Floodplains are the easiest land to build on as they are flat and people like living by the river. There are many ways to stop a river flooding, and we can design flood-proof houses these days. The Government says we need to build millions more homes.
Secretary of State for Environment, Food and Rural Affairs: Climate change means that in the future more money will have to be spent on expensive river flood defences. The government will not have funds to flood proof all rivers and will need to look at more long-term options that control the flow of water to the river. Changing planning regulations to stop building on floodplains will also help.
Riverside resident: My house is built on a floodplain, so it is up to the council to protect my home from flooding. I pay taxes and these should go towards stopping the river flooding.
What has Boscastle done to prevent flooding in the future?
£4.5 million has been spent on a flood defence scheme.
The scheme stretches along the valley, incorporating drainage, sewerage systems and land re-grading.
Boscastle car park has been raised in height, which will stop the river from bursting its banks so easily.
New drains allow water to run into the lower section of the river quickly.
The river channel has been made deeper and wider so that it can accommodate more water.
What do Governments and developers often favour
Large hard engineering potion such as dams as it can generate profit and income
What do environmental groups and local residents often prefer
Like planting trees as it causes little damage to the environment
