Water Lecture 3- Causes and Consequences of Flooding

Question 1

Is the amount of annual rainfall in the Uk increasing?

Answer 1

Yes

Question 2

What is the physical basis of flooding?

Answer 2

Flow through saturated and unsaturated.

Question 3

Name two not slow flows through soil.

Answer 3

Macropore flow
Pipeflow

Question 4

What is macropore flow?

Answer 4

Due to root or animal activity(note crane fly larvae in image to right and earthworm in image below)

Question 5

What is pipeflow?

Answer 5

Often occur naturally in peat or marl soils
Artificial drainage pipes are commonly found in agricultural soils in the UK

Question 6

What are the 4 effects of catchment characteristics on controlling water flows into channels?

Answer 6

1. Diffuse flows. Hillslope flows get faster as they get deeper (so we get more rapid runoff as the
   amount of runoff increases)
2. Faster flows with steeper slopes (more rapid
   runoff in steeper catchments)
3. Faster flows when the bed surface is smoother or less rough (so runoff is more rapid in urban areas compared to rural areas)
4. Deeper and faster flows will start to form channels (rills and gullies), which are more efficient at moving water

Question 7

What is the discharge equation?

Answer 7

Width x depth x velocity

Question 8

Is water compressible?

Answer 8

No and so density is constant

Question 10

What is discharge proportional to?

Answer 10

Mass
Q1 = Q2

Question 11

Name two critical processes in drainage networks.

Answer 11

Flow accumulation
Flow attenuation

Question 12

What is flow accumulation?

Answer 12

The process of accumulation leads to increases in flood wave peaks – addition of Q

Question 13

What is flow attenuation?

Answer 13

The process of attenuation leads to decreases in flood wave peaks
(Attenuation is the inverse of conveyance)

Question 14

Discuss an example of flow accumulation.

Answer 14

Scenario A – rain falls over entire catchment, so flow depends on catchment area. As flows
combine, Q increases with distance downstream - accumulation.

Question 15

Discuss an example of flow attenuation.

Answer 15

Scenario B – rain just falls in channel headwaters – little inflow further downstream - attenuation
Downstream movement of water is slowed due to secondary circulation, friction, storage etc

Question 16

What causes attenuation?

Answer 16

1. Different timings of sub-catchment response
2. Transfer of water to floodplains
3. Momentum/energy loss within the water wave

Question 17

What is Manning’s n?

Answer 17

Manning’s n is an empirical parameter that allows us to relate flow depth to velocity or discharge

Question 18

What does increased roughness lead to?

Answer 18

Slower flow and then deeper flow.

Question 19

What is velocity equation?

Answer 19

v = depth 2/2 slope 1/2 divided over manning’s n

Question 20

Is attenuation carefully managed in rivers?

Answer 20

Attenuation is carefully managed
in rivers:
May be introduced immediately upstream of towns and cities to take the ‘tops’ of flood waves
May be used further upstream to slow flood waves (natural flood management)

Question 21

What happens with flood defences which can cause problems?

Answer 21

If you defend a town/river by stopping water going onto the floodplain you locally reduce
attenuation
* → has the effect of increasing flood risk downstream

Question 22

What does dredging involve?

Answer 22

Deepening of the river bed

Question 23

What are pros of dredging?

Answer 23

Improve land drainage and flow conveyance
Improve navigation
Economic use for sand and gravel extraction

Question 24

What are cons of dredging?

Answer 24

Can increase flooding downstream, not effective at reducing flood risk
Flow continuity/mass balance – floodplains cover large areas so channels would need to be much
deeper/wider to contain the same amount of flow
Cost:
* Scour and fill – short lived impact, need to repeat after each major flood
* Stabilization of bridges, culverts etc.
* Where to put dredged silt (may be contaminated)
Increase subsequent erosion (and thus siltation) by destabilizing banks
Disturbance of fish spawning grounds/other parts of ecosystem (and thus related ecosystem services)

Question 25

How many people do flood events impact per annum?

Answer 25

> 300 million

Question 26

What were financial losses in 2021 due to flooding?

Answer 26

$105 billion

Question 27

What is the flood risk equation?

Answer 27

Flood risk = flood hazard x vulnerability

Question 28

What does risk depend on?

Answer 28

Demographic changes and economic development in flood prone areas

Question 29

How many people are exposed to 1 in 100 year flood risk?

Answer 29

1 in 5 people (1.8 billion). This distribution however is disproportionate and higher in lower income households.

Question 30

Name 5 ways humans may be making themselves more vulnerable to floods.

Answer 30

1. Urbanization: the increased occupation of flood-risk land (not just floodplain) 2. Urban impacts: how this urbanization happens 3. Infrastructure impacts: the things we rely upon day-to-day that make us more vulnerable (e.g. water, gas, electricity, satnav, mobile phones) 4. Buildings and contents: more things to repair and more expensive things to repair 5. Social impacts: the changing response of society to more flooding (a feedback) (e.g. the ghettoization of high flood-risk communities)

Question 31

In 2020 what was the economic damage from flooding in the UK?

Answer 31

£2 billion

Question 32

How many people in Britain live in areas at significant risk from flooding?

Answer 32

1.9 million

Question 33

Does cost of damage to goods or buildings increase faster?

Answer 33

Goods

Question 34

Has there been an increase in damages at all flood depth?

Answer 34

Yes

Question 35

Name 3 social impacts of flooding.

Answer 35

Inequality in flood-risk exposure Health issues Differential access

Question 36

Do people believe the responsibility for preparing for flooding lie with national and local governments or individuals?

Answer 36

Governments