Core Theme 1 - Flooding In Drainage Basins

Question 1

What is a drainage basin

Answer 1

An area of land that is drained by the river and its tributaries

Question 2

How does water enter a basin?

Answer 2

Through precipitation (rain, snow or sleet condensed into the ground)

Question 3

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?

Answer 3

Interception - when water is absorbed and collected by vegetation

Evaporation - water turning into vapour

Question 4

What is overland flow?

Answer 4

When the water moves across the surface of the Earth, becoming a stream, tributary or river

Question 5

What is infiltration?

Answer 5

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

Question 6

What is through flow?

Answer 6

The downwards movement of water through soil

Question 7

What is percolation?

Answer 7

When the water moves into the soil into the pores of the rocks where it moves by ground water flow into the channel

Question 8

What is ground water flow?

Answer 8

When water is able to move through soil and rocks into the sea

Question 9

What is evapotranspiration?

Answer 9

Water vapour is evaporated from the trunk and the leaves of trees and other vegetation, back into the atmosphere.

Question 10

When does a flood occur?

Answer 10

A flood occurs when a river bursts its banks and the water spills onto the floodplain.

Question 11

What are the physical factors of flooding?

Answer 11

Climate
Vegetation
Impermeable rock
Steep sided channel

Question 12

How is climate a cause of flooding?

Answer 12

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.

Question 13

How is a steep-sided channel a cause of flooding?

Answer 13

A river channel, surrounded by steep slopes, causes fast surface run-off.

Question 14

How is vegetation a cause of flooding?

Answer 14

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.

Question 15

How is impermeable rock a cause of flooding?

Answer 15

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.

Question 16

How is geology a cause of flooding?

Answer 16

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.

Question 17

What are 2 human factors of flooding

Answer 17

drainage basins in urbanisation areas and deforestation

Question 18

What process does the building of impermeable surfaces stop?

Answer 18

Percolation

Question 19

What are the types of impermeable surfaces found in urbanised areas?

Answer 19

Roof tops, pavements, roads, streets etc…

Question 20

Does urbanisation cause surface runoff to increase or decrease?

Answer 20

Increase because there is more volume instead of percolating through, it instead becomes over-land flow

Question 21

Why does a greater amount of rainwater get to the river channel in an urbanised area?

Answer 21

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

Question 22

How is deforestation a cause of flooding?

Answer 22

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.

Question 23

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.

Answer 23

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.

Question 24

What is a hydrograph?

Answer 24

A hydrograph is a graph that shows the response of a river to a rain event in its drainage basin

Question 25

What is peak discharge

Answer 25

Peak discharge is the point at which the river reaches its highest level.

Question 26

What is peak rainfall

Answer 26

The highest amount of rainfall (most rain falls)

Question 27

What is lag time

Answer 27

The time difference between peak discharge and peak rainfall.

Question 28

What is the rising limb

Answer 28

Rising limb is the time when the river discharge is increasing.

Question 29

What is the falling limb

Answer 29

The Falling limb is the time when the river discharge is falling.

Question 30

What is base flow

Answer 30

Base flow is the normal flow of the river before the rain event.

Question 31

What factors can affect the hydrograph

Answer 31

Vegetation, type of slope and soil type

Question 32

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?

Answer 32

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

Question 33

Type of slope also affects the line on a hydrograph. How would the steep line be and how would the gentle line be?

Answer 33

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

Question 34

Soil type hydrograph. How would the impermeable line be and how would the permeable line be?

Answer 34

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

Question 35

What is hard engineering?

Answer 35

Hard engineering is where man-made structures are used to prevent or control natural processes.

Question 36

What is soft engineering?

Answer 36

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

Question 37

Give some examples of hard engineering methods

Answer 37

Flood walls, river engineering, dams, embankments, concrete channel and enlarge the channel

Question 38

What are dams

Give the advantages and disadvantages as well

Answer 38

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)

Question 39

What is river engineering

Give the advantages and disadvantages as well

Answer 39

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.

Question 40

What are embankments?

Give the advantages and disadvantages as well

Answer 40

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

Question 41

What are flood walls

Give the advantages and disadvantages as well

Answer 41

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

Question 42

What is enlarging the channel

Give the advantages and disadvantages as well

Answer 42

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

Question 43

What is a concrete channel

Give the advantages and disadvantages as well

Answer 43

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

Question 44

Give some examples of soft engineering methods

Answer 44

Afforestation, no more building on flood plains, marshal areas, do nothing and give out sandbags

Question 45

What is afforestation

Give the advantages and disadvantages as well

Answer 45

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

Question 46

What is no more building on flood plains

Give the advantages and disadvantages as well

Answer 46

When construction is banned on a floodplain

Damage would be reduced

Question 47

What are marshal areas

Give the advantages and disadvantages as well

Answer 47

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

Question 48

What is do nothing

Give the advantages and disadvantages as well

Answer 48

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

Question 49

What is giving out sandbags

Give the advantages and disadvantages as well

Answer 49

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

Question 50

What is land use zoning?

Answer 50

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

Question 51

What have scientists predicted?

Answer 51

Scientists have predicted our climate will become stormier producing more flash floods, as seen in Boscastle.

Question 52

What has been suggested for future river management

Answer 52

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.

Question 53

What is the new river management in the future

Answer 53

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

Question 54

Name some conventional methods of controlling a river that floods? which stakeholders would support

Answer 54

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.

Question 55

What has Boscastle done to prevent flooding in the future?

Answer 55

£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.

Question 56

What do Governments and developers often favour

Answer 56

Large hard engineering potion such as dams as it can generate profit and income

Question 57

What do environmental groups and local residents often prefer

Answer 57

Like planting trees as it causes little damage to the environment