W5 - Flooding Flashcards
What are floods?
Water that submerges land that is usually dry
Types of floods:
- Areal –> Flooding over a massive area
- Riverine (fluvial)
- Estuarine and coastal
- Urban
- Catastrophic failure –> Dam failure etc.
- Snowmelt
- Flash floods
- Single/multi-even
Why do floods occur?
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
What is river (fluvial) flooding?
A rapid thaw or heavy rains cause rivers to inundate their floodplains, often affecting large areas
What is surface (pluvial) flooding?
Periods of heavy rain overwhelm drainage systems and concreted surfaces, preventing water from draining away
(Excceed infiltration capacity)
What is flash flooding?
Intense rainfall makes water rise quickly and flow at a high speed for a short amount of time, often carrying away heavy objects
What is groundwater flooding?
The water table in permeable rocks rises to enter cellars or comes up above the surface and can last weeks or even months.
What is coastal (tidal flooding)?
Severe storms, strong winds, and high tides cause large waves that break down defences and flood coastal areas
Recent examples of flooding:
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)
How can you categorise factors contributing to flooding?
- Water/atmosphere
- Physical landscape
- Human/Biosphere
or - Underlying causes
- Intensification factors
What are the meteorological factors that contribute to flooding?
Rainfall
Cyclonic storms
Small-scale storms
Temperature
Snowfall and snowmelt
What are the physical landscape factors that contribute to flooding?
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
What are the human/biosphere factors contributing to flooding?
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
Meteorological factors - rainfall
- 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)
Meteorological factors - snowmelt
In spring, discharge can’t be predicted by precipitation as it is driven large part by snowmelt (López-Moreno and García-Ruiz, 2004)
Physical landscape factors: soil moisture/water table wet vs dry antecedent
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
Physical landscape factors: catchment characteristics
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
Physical landscape factors: channel characteristics
Rougher the channel, slower the speed of water –> trapezoidal channel more likely to flood as water can move quickly, more likely in urban areas
Physical landscape factors: floodplain attenuation
- 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)
Physical landscape factors: flood wave sychronisation
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
Human factors: landcover/channel change
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
Human factors: occupation of floodplain
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
Human factors: inefficient infrastructure
- Inefficient management of drains
- Drainage design in urban area may not have been built for extreme events
- Poor design
Human factors: overly efficient infrastructure
- Moving a problem downstream
- Taking a catchment based approach is necessary always look downstream
human factors: climate change
Average could stay the same but more extreme weather
More precipitation with climate change
What is a 1 in 100-year flood?
The chances of a 1 in 100-year flood occurring in any given year are 1%
How do we see long-term flooding records?
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
What is stationarity?
everything stays the same, no other factors driving changes in the volume of rainfall or discharge
Are hydrological systems stationary?
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
What is a hazard?
“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)
What is a risk?
“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)
What is exposure?
“the situation of people, infrastructure, housing, production capacities and other tangible human assets located in hazard-prone areas” (UNDRR, No Date)
What is vulnerability?
“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)
What is flood mapping used to identify?
inundation extents at local scale – site specific flood management strategies
for certain flood scenarios (e.g., 1 in 100-year event)
what can we map to give an idea of areas most at risk
o Water velocity
o Depth
o Duration
o Debris
What are the two types of hazard maps?
- 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.
What is depth and velocity mapping?
When you combine depth and velocity, that’s where risk to life comes in
The deeper the water, the more dangerous
What is the debris factor?
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
What is the hazard rating equation?
HR = depth of flooding * (velocity of floodwaters (m/s) + 0.5) + debris factor (scale of 0-1)
What is vulnerability mapping?
- 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.
What are flood hazard maps?
o Data on flood extent for floods with different average recurrence intervals
o Sometimes also data on water depths, velocity etc
