3. Exeter Flooding and Human Impacts on Hydrological Cycle

Question 1

Which physical factors make Exeter prone to flooding?

Answer 1

Physical factors:

• Large catchment area (1500km2)
• Steep slopes around Exeter / Tiverton lead to rapid runoff
• Areas of impermeable rock (clays and grits) lead to flashy tributaries (Culm)
• Confluence of tributaries of Exe, Creedy and Culm north of Exeter.

Question 2

Which human factors make Exeter prone to flooding?

Not all of these factors focus on the catchment!

Answer 2

Human factors:

• Extensive urbanisation of catchment area in last 50 years
• Building on flood plain with inpenetrable surfaces e.g. Marsh Barton
• Sparse forest - little interception
• Increased arable land - compaction by heavy machinery reduces pore amounts
• Field drainage channeled into streams and the river rather than infiltrating into the soil

Question 3

List the features of the River Exe channels which reduce the flood risk:

Answer 3

• Enlarged and straightened trapezoidal channel lined with concrete
• Flood relief channel constructed at St Davids
• Radial flood gates to draw water into flood diversion channel

Question 4

What features do Exeter’s river banks / embankments have to reduce the flood risk?

Answer 4

• Raised banks / levees
• Reinforced banks (gabions under grass)
• No vegetation at sides
• Land use zoning along embankment

Question 5

How have Exeter’s bridges developed to reduce flood risk?

Answer 5

• Higher and wider Exe bridges to allow water to quickly pass under and prevent debris from becoming stuck

Question 6

Give some examples of flood resilient land uses:

Answer 6

Flood resilient land uses:

• Allotments
• Skate parks
• Playing fields
• Playgrounds
• Car parks
• Cycle paths

These land uses all recover quickly after flooding

Question 7

Give an example of an “afforestation project” at the source of the Exe

Answer 7

• Maintaining peat bogs - blanket bogs are the most common type of bog on Exmoor
• Re-wetting peat bogs by blocking drainage ditches

Question 8

Give an example of a hard engineering solution to Exeter flooding

(Outside of Exeter)

Answer 8

• Store and release dam on Exmoor (Wimbleball Reservoir)
• Water is pumped in at times of high flow and stored until needed / low rainfall

Question 9

How are personal flood resistance measure encouraged by the government, and what do they entai?

Answer 9

Devon County Council gives grants for personal flood defences i.e sandbags / high visibility clothing

Other personal flood defences include:

• Flood gates installed on private property
• Non-return valves in sewer pipes
• Raised electrical sockets
• Separate circuit for upstairs and for downstairs
• Kitchen units on legs
• Boiler units on higher floors

Question 10

What caused the floods in Exeter on the 20th-25th November 2012?

Answer 10

• Intense antecedent rainfall for 8 weeks prior saturated soil
• 250mm of rainfall fell in 6 days - 85mm fell on 25th November

(Antecedent - preceeding)

Question 11

Which areas were affected by the November 2012 floods?

Answer 11

• 450 residential / commercial properties affected
• Cullumpton worst affected
• Exeter Quay flooded

Question 12

Which transport links were destroyed by the November 2012 floods?

Answer 12

• Railway severed at Cowley bridge - no trains to Taunton or Barnstaple for 5 weeks
• Road to Tiverton flooded at Stoke Canon and Bickleigh

Question 13

Why weren’t the 1960s flood defences enough to stop the November 2012 floods?

Answer 13

The 1960s flood relief were built to deal with a maximum 700 cumecs, however the original plan was 900. The flood defences have filled 4 times since being built.

Furthermore, the flood defences were built 50 years prior to the flood, and were thus not robust enough to deal with a 1/100 year flood event.

Question 14

How has the risk of damage from flooding increased since 1960?

Answer 14

• 1000 houses were flooded in the October 1960 floods which inspired the major development in flood defences
• However, flooding today could affect an estimated 3,270 properties in Exeter and 700 people, as well as two care homes, an electricity sub-station and schools and health centres in Saint Thomas.
• The amount of property at risk is over 3x greater than in 1960

Question 15

What are recurance intervals?

Answer 15

Recurance intervals are a statistical calculation that gives the probable frequency of a flood of a particular scale.

A 1/100 year flood means a 1% probability of happening in a given year, and helps to decide risk.

Question 16

How has climate change created a need for new flood defenses?

Answer 16

• Climate change has increased the likelihood of large rainfall events, increasing the frequency and size of flooding - the soil is more often saturated and rain cannot infiltrate.

Question 17

How has Exeter’s catchment created a need for new defences?

Answer 17

• More of Exeter’s catchment area has been urbanised

Question 18

How have Exeter’s development created a need for better flood defences?

Answer 18

• Much of Exeter’s new housing is below the flood level because planners were confident in the 1960s scheme
• Exeter Quay acts as a flooding chokepoint / bottleneck and reduces the volume of water that can flow at any time

Question 19

Where is the Major Flood Risk Mile for Exeter?

Answer 19

The Major Flood Risk Mile is the mile between Exeter St Davids Station and Exeter Quay - most assets at risk of flooding are found along here

Question 20

How has the leat by the Customs House been managed?

Answer 20

• Gate on leat return to stop water backing up
• Bridge sides made from strengthened glass to withstand flood buildup

Question 21

How have the sides of the Exe been managed?

Answer 21

• Brick wall rebuilt and heightened
• Flood gates at access points, closed in flood
• Height increased by 1.5m due to data on recent flood events
• Concrete wall encased in aesthetic brick (glass wall used outside Royal Oak pub to retain views)

Overflowing water will flow through Marsh Barton!!

Question 22

How have de-mountable bollards been implemented along the Quay?

Answer 22

• Flood boards hidden in benches slotted between bollards - on-site but not unsightly
• Protect shops at the waterfront as main economic asset but landscape also considered

More considerate approach than economic-focused 1960s defences

Question 23

How has Trews Weir flood relief channel been managed?

Answer 23

• No gates - fixed weir dependent on water levels (less to go wrong!)
• Channel transformed into wetland habitat

Question 24

How has the Trews Weir itself been managed?

Answer 24

• Crest of weir reduced to divert more water out of the Exe and lower the water around the Quay
• Removal of end weir gives more capacity in the channel during a flood event
• Fish pass created ton enable fish back up into river
• Blocks at the bottom of weir dissipate water as it falls

Question 25

How has the Trews Weir itself been managed?

Answer 25

• All designed for a 1 in 100 year flood
• Maintenance and replacing considered e- e.g. concrete has a 100 year lifespan
• All flood defences made as passive as possible to reduce further need for maintenance

Question 26

How does cloud seeding work?

Case Example - Idaho, USA

Answer 26

Cloud seeding is a strategy to encourage precipitation - particulates (usually silver iodine) are released into the atmosphere and act as the condensation nuclei for precipitation.

Question 27

How is cloud seeding being utilised in Idaho, USA?

Answer 27

Since 2017, a team from the National Centre of Atmospheric Research (NCAR) has been collecting / analysing data from a cloud seeding experiment known as SNOWIE

Many of the farmers in the local areas rely on snowmelt from the mountains as a supply of water for irrigation

SNOWIE uses ‘artificial’ clouds to enlarge the snowpack volume in the mountains during the winter to meet the demand for water

Question 28

How did deforestation increase the risk of mudslide in Freetown, Sierra Leone?

Answer 28

In August 2017, after an intense rainy season, 1,141 people were declared dead or missing when a massive mudslide slipped into the Babadori river

As Freetown had expanded, the city had crept up on the surrounding hills leading to the deforestation of areas such as the Central Highlands, leaving soil exposed and weakened.

Question 29

How does the £500,000 ‘Slowing the Flow’ project in the UK help to reduce the peak flow of floods?

Answer 29

‘Slowing the Flow’ - Saved town in Yorkshire from 2016 flooding - reduced peak flow by 20% after 50mm of rainfall fell in 36 hours.

4 Main strategies to reduce flooding:

• Constructing low-level soil embankments (bunds)
• Planting of 40,000 trees along stream sides and in flood plains
• Restoring dams of wood debris in small streams
• Restoring wetlands

Restoration of wetlands increases interception and infiltration rates in upper parts of the drainage basin

Question 30

How did urbanisation lead to significant flooding in Houston, Texas?

Answer 30

In the last 50 years, Houston’s population has nearly doubled to 2.3 million people.

As a result, the city has expanded, destroying 100,000 hectares of wetland and building 7000 homes on floodplains

This was a factor affecting the scale of the flooding from 2017 Hurricane Harvey

Question 31

What are the impacts of afforestation projects in Ireland?

Answer 31

• Ireland has increased its forest cover from 1% in 1900 to 11% today
• However, in the north of Ireland, non-native conifers have been planted, which have had a detrimental impact on the ecology due to their acidic needles, lack of leaf decomoposition and shallow root systems, which leads to less flood protection than other tree species

Question 32

Why is the Grand Ethiopian Renaissance Dam so controversial?

Answer 32

The dam construction began in 2011, with filling beginning in 2020

• 65 million Ethiopians will benefit from the hydroelectricity generated which will enter the national grid
• Surplus electricity will be exported to Sudan during rainy seasons
• However, redued discharge rates downstream may lead to water shortage in countries such as Egypt and could lead to conflict
• Higher evaporation rates from the resevoir will reduce the overall water quantity of the river

Question 33

What is an aquifer?

Answer 33

An aquifier is a body of water-bearing rock or sediment

Question 34

What is the Ogallala Aquifier, USA?

Answer 34

• One of the largest aquifiers in the world - covers approximately 450,000 sq km that underlies 8 states!
• The Ogallala Aquifier is part of the High Plains Aquifier System and provides about 30% of the groundwater for irrigation in the USA

Question 35

How has the Ogallala Aquifer declined?

Answer 35

Since the 1950s, due to a growing population and advances in automatic irrigation systems, water levels have dropped by up to 150ft in some places.