5.5 Surpluses in the hydrological system

Question 1

What is flooding?

Answer 1

if the discharge is of sufficient quantity to cause a body of water to overflow its channel and submerge the surrounding land, flooding is deemed to have occurred.

Question 2

What are the four metrological causes of flooding?

Answer 2

-Intense storms leading to flash flooding
-Unusually heavy or prolonged rainfall
-Extreme monsoonal rainfall
-Snowmelt

Question 3

What are some examples of intense storms leading to flash flooding

Answer 3

e.g. November 2013 in Sardinia thunderstorms caused flash flooding resulting in 18 deaths and US$14.14 billion in damage.

Question 4

What is intense rain that leads to flash flooding

Answer 4

Flash floods happen very quickly, often without warning. Occur in the UK in summer. Intensity of the rain exceeds capacity of the river. Short lag time – minutes or hours.

Question 5

What is snowmelt?

Answer 5

Occurs in higher latitudes or mountainous areas due to a sudden rise in air temperature in spring. Flooding occurs snow melts and the water cannot infiltrate into the soil.

Question 6

What are examples of snowmelt?

Answer 6

-e.g. 2009, Red River in North Dakota - due to increased air temperature, impermeable ground and additional precipitation.
-In Iceland glacial outburst floods occur due to volcanic activity known as jokulhlaup.
-In winter in 2013 Norfolk Police reported flooding caused by snowmelt after mild wet weather caused rapid thawing.

Question 7

What is extreme monsoon rainfall and where does it become a problem specifically?

Answer 7

A seasonal change in the direction of the prevailing winds of a world region. Occurs across South and Southeast Asia between May and September when around 70% of the annual rainfall occurs in about 100 days, due to warm air from the south-west Indian Ocean blowing towards India.

Question 8

What are some examples of extreme monsoon rainfalls?

Answer 8

-July 2015 usually heavy monsoon rains in Myanmar caused 103 deaths and critically affected over 1 million people.
-In July 2010 in Pakistan 9000mm of rain in one week caused landslides and flooding.

Question 9

What causes unusually heavy or prolonged rainfall? How does this then lead to flooding

Answer 9

In the UK this can be associated with the passage of low-pressure systems (depressions), and usually occurs in autumn and early winter. Depressions have 2 bands of rain – showers and rain with the warm front then heavier rain with the cold front. Initially the ground will absorb the rain, but this will soon become saturated, leading to increased overland flow.

Question 10

What is an example of unusually heavy or prolonged rainfall?

Answer 10

In the UK there was a succession of very intense storms between October and December 2015. It resulted from a sinuous jet stream in a fairly constant track which mean that high pressure was blocked.

Question 11

What factors can affect flooding levels?

Answer 11

soil depth
vegetation
slope
rock type
drainage density

Question 12

What is land use now like and how does this affect flooding levels

Answer 12

we have barer, drier soils; straightening and dredging channels; faster runoff; reduced lag times, higher peak discharge. Rainwater now reaches the floodplain quicker – and is where we find many impermeable surfaces. Due to climate change, we also have higher levels of rainfall.

Question 13

What did land use used to be like and how did this affect flooding levels?

Answer 13

trees absorbed and slowly released water; meandering channels slowed the flow, and bogs held back water.

Question 14

How does farming cause flooding?

Answer 14

-Ploughing compacts soil - overcultivation
-Grazing animals compact soil - over grazing
-Sprinkling of groundwater on to arable crops - saturated groundwater storage

Question 15

How does damming and bridges cause flooding

Answer 15

-Dams built to supply towns with water, but can suddenly burst
-Impeding channel flow by building alongside the river

Question 16

How does changing the course of a river cause flooding?

Answer 16

-Natural streams meander and have marshy areas; channelization does not
-Sewers feed water into channels
-Streams channelled into culverts to aid rapid drainage of farmland

Question 17

Why is channel straightening seen as mismanagement?

Answer 17

This may speed up water flow but it doesn’t take a greater capacity. It would be more efficient to dredge the river, which involves widening and deepening the channel to increase efficiency by increasing capacity and moving water at a faster rate (although at an environmental cost)

Question 18

What are the four main human causes of flooding?

Answer 18

-deforestation
-increased urbanisation
-climate change
-increased population

Question 19

How has urbanisation caused flooding?

Answer 19

-60% of the population have decided to turn their gardens into driveways to accommodate more than one car
-urban drainage systems look to be as efficient as possible
-more bridges have been built for better access
-channelization in urban areas increases the velocity of a river

Question 20

How does a growing population cause flooding?

Answer 20

-more space is needed for living
-wetland areas near towns have been drained for living space
-need to provide more land for food so land is required for agriculture
-change was quick and therefore lacked planning and thought

Question 21

How does climate change cause increased flooding?

Answer 21

-higher levels of rainfall

Question 22

How has deforestation caused flooding?

Answer 22

-tree roots helped to bind soil together; this no longer happens
-less interception and evapotranspiration
-animals trample land and compact soil
-bare soil is more susceptible to erosion

Question 23

When did Storm Desmond happen and what happened?

Answer 23

It hit Cumbria in December 2015, particularly affecting the towns of Keswick and Cockermouth
341mm of rain fell in 24 hours

Question 24

What caused storm Desmond?

Answer 24

deep Atlantic low pressure system. Low pressure of 946 millibars meant its fronts brought prolonged heavy rainfall
-brought through a mechanism known as the conveyor belt
-the jet stream remained over the north west for longer than usual- 48 hours; an atmospheric river was detected bringing warm air across from the Caribbean in a narrow band of enhanced water vapor transfer - warmer air holds moisture for longer

Question 25

What types of rain was storm Desmond?

Answer 25

convectional and orographic rainfall over the lake district

Question 26

What were some of the factors that meant towns like Keswick were not well prepared for Storm Desmond?

Answer 26

Too much rainfall fell for an already saturated ground to absorb any water. Overland flow carried water to rivers , which flooded and steep slopes accelerated this process
Impermeable surfaces and blocked sewers meant water rapidly flowed into already filled channels

Question 27

What were the social impacts of Storm Desmond?

Answer 27

5,200 homes flooded
a landslide closed a section of the west coast mainline between Preston and Carlisle
61,000 homes lost power
One found dead after falling in a river outside Kendall
-40 schools closed

Question 28

What were the economic impacts of Storm Desmond?

Answer 28

Dublin and Leeds Bradford airport closed
Storm Desmond and Eva cost 1.3 billion together
Insurance claims caused by flooding across the UK in 2015 exceeded £6billion
House prices fell in areas at risk

Question 29

What were the environmental impacts of Storm Desmond?

Answer 29

-Many river banks were eroded, adding to future flood risk
-rivers were choked with debris and contaminated sewage
-saturated ground led to landslides and decomposition of dead plants and animals- giving off poisonous gases

Question 30

What were the responses to storm Desmond?

Answer 30

8 response and rescue centres set up
the government emergency committee - Cobra- released £50 million to local councils

Question 31

Why was the flood defenses in Keswick not appropriate?

Answer 31

There were hard engineering defences in Keswick. However, these weren’t always appropriate given changes to land use and climate change. After the 2009 floods a defense wall was built to 5m, but the flood in 2015 reached 5.9 metres and so the water went over the top

Question 32

What soft engineering techniques do people in Cumbria think would be the way forward?

Answer 32

-reafforestation
-restoration of river channels in their natural meandering state
-restoration of flood plains
-refusal of planning permission to build or expand developments near the river

Question 33

Why was the government criticised in Storm Desmond

Answer 33

there was an anger over the lack of flood defences, but there are questions over whether it would have actually done anything as 2005 flood was half a metre less
Additionally they were criticised for failing to apply for EU funding for clean up