W5 - Flooding

Question 1

What are floods?

Answer 1

Water that submerges land that is usually dry

Question 2

Types of floods:

Answer 2

• Areal –> Flooding over a massive area
• Riverine (fluvial)
• Estuarine and coastal
• Urban
• Catastrophic failure –> Dam failure etc.
• Snowmelt
• Flash floods
• Single/multi-even

Question 3

Why do floods occur?

Answer 3

Water balance is all about the balance of water inputs into the system and water outputs
Floods occur when inputs exceed storage capacity and outputs

Question 4

What is river (fluvial) flooding?

Answer 4

A rapid thaw or heavy rains cause rivers to inundate their floodplains, often affecting large areas

Question 5

What is surface (pluvial) flooding?

Answer 5

Periods of heavy rain overwhelm drainage systems and concreted surfaces, preventing water from draining away
(Excceed infiltration capacity)

Question 6

What is flash flooding?

Answer 6

Intense rainfall makes water rise quickly and flow at a high speed for a short amount of time, often carrying away heavy objects

Question 7

What is groundwater flooding?

Answer 7

The water table in permeable rocks rises to enter cellars or comes up above the surface and can last weeks or even months.

Question 8

What is coastal (tidal flooding)?

Answer 8

Severe storms, strong winds, and high tides cause large waves that break down defences and flood coastal areas

Question 9

Recent examples of flooding:

Answer 9

Saudi Arabia - devastating period of floods between November 22 and January 23 (Middle East Eye, 2022)
Philippines - driven by cyclones, December to Feb (Pinlac, 2022)

Question 10

How can you categorise factors contributing to flooding?

Answer 10

1. Water/atmosphere
2. Physical landscape
3. Human/Biosphere
   or
4. Underlying causes
5. Intensification factors

Question 11

What are the meteorological factors that contribute to flooding?

Answer 11

Rainfall
Cyclonic storms
Small-scale storms
Temperature
Snowfall and snowmelt

Question 12

What are the physical landscape factors that contribute to flooding?

Answer 12

Soil moisture level
Groundwater level prior to storm
Natural surface infiltration rate
Presence of impervious cover
Channel cross-sectional shape and roughness
Presence or absence of overbank flow, channel network
Synchronisation of runoffs from various parts of the watershed
High tide impeding drainage

Question 13

What are the human/biosphere factors contributing to flooding?

Answer 13

Land-use changes (e.g. surface sealing due to urbanisation, deforestation) increase runoff and may be sedimentation
Occupation of the floodplain obstructing flows
Inefficiency or non-maintenance of infrastructure
Too efficient drainage of upstream areas increases flood peaks
Climate change affects magnitude and frequency of precipitation and floods
Urban microclimate may enforce precipitation events
WMO/GWP, 2008

Question 14

Meteorological factors - rainfall

Answer 14

• Rainfall and high flows are intermittent and vary through time
• Peak in high discharge but not high rainfall could be caused by snow melt

Seasonal
Especially during monsoon seasons
Can predict flooding through looking at seasonal patterns
(Karki et al., 2017)

Cyclic
In UK we have decrease in summer precipitation and increase in winter precipitation
(Dadson et al., 2017)

Question 15

Meteorological factors - snowmelt

Answer 15

In spring, discharge can’t be predicted by precipitation as it is driven large part by snowmelt (López-Moreno and García-Ruiz, 2004)

Question 16

Physical landscape factors: soil moisture/water table wet vs dry antecedent

Answer 16

After prolonged dry conditions
* Water tables depressed
* Ground has greater storage capacity
* Little impact on streamflow
* Direct channel inputs/throughflow
After prolonged wet conditions
* Ground is saturated
* Little/no storage capacity
* Streamflow highly reactive
* Saturation-excess overland flow

Question 17

Physical landscape factors: catchment characteristics

Answer 17

Steeper slope more prone to flooding as water has less time to infiltrate – stream velocity driven by surface slope

Less distance for water to flow = smaller catchment  flashier the hydrograph

High drainage density and connectivity = more pathways to river – water reaches main river faster = higher storm flow and risk of flooding
Shape could be argued both ways

Question 18

Physical landscape factors: channel characteristics

Answer 18

Rougher the channel, slower the speed of water –> trapezoidal channel more likely to flood as water can move quickly, more likely in urban areas

Question 19

Physical landscape factors: floodplain attenuation

Answer 19

• Difference between inflow and outflow
• Volume of water remains unchanged
• Travel times vary
• At outflow peak lowered and slightly delayed
• Shallower flows
• Slows flow of water
• Higher hydraulic roughness
• Blöschl et al. (2015)

Question 20

Physical landscape factors: flood wave sychronisation

Answer 20

Don’t want to do something that will affect the flood peaks in other areas

When thinking about flood management and prediction, downstream needs to be in the forefront of our minds

Question 21

Human factors: landcover/channel change

Answer 21

Really straight smooth channels will let the water flow rapidly through the catchment  who knows what will happen downstream
Soil compression from equipment or fires can increase overland flow

Question 22

Human factors: occupation of floodplain

Answer 22

Nice flat bit of land next to rivers  urban areas have built around rivers for trade etc.
Increased exposure to floods since the post-war period
Population increases since 1950 in the UK
o Substantial expansion of housing/industry onto floodplains
Between 2001 – 2014, >250,000 homes built in at-risk areas.
o Wilfully putting people at risk/in harm’s way

Question 23

Human factors: inefficient infrastructure

Answer 23

• Inefficient management of drains
• Drainage design in urban area may not have been built for extreme events
• Poor design

Question 24

Human factors: overly efficient infrastructure

Answer 24

• Moving a problem downstream
• Taking a catchment based approach is necessary  always look downstream

Question 25

human factors: climate change

Answer 25

Average could stay the same but more extreme weather More precipitation with climate change

Question 26

What is a 1 in 100-year flood?

Answer 26

The chances of a 1 in 100-year flood occurring in any given year are 1%

Question 27

How do we see long-term flooding records?

Answer 27

We can look at sediment records to extrapolate further back Yellow lines are periods of rapid sediment influx – larger quantity of sediment = larger volume of water

Question 28

What is stationarity?

Answer 28

everything stays the same, no other factors driving changes in the volume of rainfall or discharge

Question 29

Are hydrological systems stationary?

Answer 29

land use changes, decadal cycles, and climate changes are important  they create non-stationarity * Even though we are a small country, variations in flow are immense – noisy data * We can’t predict how land use will change in the future * Trying to predict high flow events is extremely difficult

Question 30

What is a hazard?

Answer 30

"any natural process or phenomenon that may cause loss of life, injury or other health impacts, property damage, loss of livelihoods and services, social and economic disruption or environmental damage" - UNDRR (No, Date)

Question 31

What is a risk?

Answer 31

"the potential loss of life, injury, or destroyed or damaged assets which could occur to a system, society or a community in a specific period of time, determined probabilistically" (UNDRR, No Date)

Question 32

What is exposure?

Answer 32

“the situation of people, infrastructure, housing, production capacities and other tangible human assets located in hazard-prone areas” (UNDRR, No Date)

Question 33

What is vulnerability?

Answer 33

“the conditions determined by physical, social, economic and environmental factors or processes which increase the susceptibility of an individual, a community, assets or systems to the impacts of hazards” (UNDRR, No Date)

Question 34

What is flood mapping used to identify?

Answer 34

inundation extents at local scale – site specific flood management strategies for certain flood scenarios (e.g., 1 in 100-year event)

Question 35

what can we map to give an idea of areas most at risk

Answer 35

o Water velocity o Depth o Duration o Debris

Question 36

What are the two types of hazard maps?

Answer 36

* Map of the inundation area for a single return period (e.g. the 100-year flood), showing the distribution of the water depth within the flooded area. * Map of inundation areas for events with different return periods, e.g. overlaying the flood limits of the 10-, 20-, 50-, 100-, 200- and 1000-year event.

Question 37

What is depth and velocity mapping?

Answer 37

When you combine depth and velocity, that’s where risk to life comes in The deeper the water, the more dangerous

Question 38

What is the debris factor?

Answer 38

Floods contain more than just water: * Sediment * Raw, un-treated, sewage * Debris (e.g. logs, cars) * Dead animals and bodies * Chemical pollutants It is often not just the water that causes the main problems but what is also in the water

Question 39

What is the hazard rating equation?

Answer 39

HR = depth of flooding * (velocity of floodwaters (m/s) + 0.5) + debris factor (scale of 0-1)

Question 40

What is vulnerability mapping?

Answer 40

* Vulnerability maps that inform about the flood exposure o e.g. Map of the 1000-year inundation area and the buildings which would be flooded * Vulnerability maps may illustrate the susceptibility of specific elements at risk o e.g. Map of particularly susceptible elements, that need special consideration in case of floods, e.g. hospitals, nursing homes etc.

Question 41

What are flood hazard maps?

Answer 41

o Data on flood extent for floods with different average recurrence intervals o Sometimes also data on water depths, velocity etc