Daily Stack 20250622
Flood Risk
“Nearly one third of the world's population is exposed to flood risk, and flood is by far the most frequently occurring natural hazard.” Swiss Re.
Image from:
https://www.ctvnews.ca/photos/2025/02/19/deadly-and-devastating-floods-in-us-southeast-states/
Cause of flooding: polar front (blue line with triangles) in combination with warm moist air from the Gulf (light blue station readings with wind direction) to the east of the cold front. A low is over the southeast states.
https://www.weather.gov/lmk/KYFlooding
CAUSES OF FLOODING
Flooding is excess water pooling in an area where normally this does not happen. There are three types of flooding: (1) pluvial flooding which is too much rainfall; (2) riverine flooding excess waters in rivers (scientifically referred to as fluvial flooding); and (3) coastal flooding due to inundation by a large body of water such as a lake or ocean.
Pluvial floods can be caused by heavy convective storms during the summer that dump large amounts of rain in a short period of time causing flash floods. A stalled or slow moving low associated with a cold and warm front can also lead to flooding. It can also be caused by atmospheric rivers that typically affect the Pacific coast of North America and western Europe or by monsoons that affect Asia, Africa and Australia.
Slow moving or stalled lows associated with cold fronts are becoming more common during winter as our climate changes. As described in our earlier post on Polar Vortexes, during mid-February, the south and southeastern US experienced severe weather including extreme cold temperatures, heavy snow and torrential rain causing flooding. The image and weather map above show the polar front with moist Gulf air moving up the southeast causing a low to form and significant rainfall.
Atmospheric rivers are narrow corridors of high concentrations of water vapour that are transported around the world at mid-latitudes. They are associated with the trough (southern part) of low level jet streams. When this jet stream stalls, and is combined with a low pressure front over a mountainous region, this enhancement of the atmospheric river causes significant rainfall and flooding. In North America, this phenomenon is known as the Pineapple Express which has caused severe flooding. In February 2024, California experienced heavy flooding due to the Pineapple Express (see image below), and the US Pacific Northwest and British Columbia, Canada experienced severe flooding in 2021. Monsoons are caused by the temperature difference between the ocean and continents. When warm moisture laden air is transported over land it produces heavy rainfall that results in seasonal flooding.
Riverine flooding when the river overflows its banks is also caused by excess rainfall either in the region or upstream of the river, snowpack melt upstream or something more catastrophic like a dam burst. Coastal flooding can be caused by storm surges (hurricanes, typhoons, tropical cyclones), king tides, or tsunamis.
https://abcnews.go.com/US/californias-historic-storm-numbers/story?id=106989350
CONSEQUENCES
The consequences of flooding are immediate. This includes property damage both personal and infrastructure, potential loss of life, and contaminated water supply. All of this requires an emergency response including evacuation and aid. Longer term consequences include impacts to the local economy, loss of livelihood, cost or burden of recovery including repairs to damage and restoration of essential services, increased insurance, and the spread of disease and illness.
The reinsurance/insurance industry has been monitoring the consequences of floods over the last two decades. Losses have been growing exponentially over this period and global annual losses are in the tens of billions of US dollars.
https://www.swissre.com/institute/research/sigma-research/sigma-2022-01/five-charts.html
In wealthy countries or regions, recovery can be achieved fairly quickly. However, in poor nations or regions, recovery can be slow or non-existent. This link between economic health and the capacity to recover can also be seen among different neighbourhoods within an overall wealthy city.
ROLE OF CLIMATE CHANGE
With our climate changing, we have witnessed increasingly severe and frequent flooding across the world. As our planet warms, there is greater capacity to hold moisture and energy in the atmosphere resulting in more severe storms and precipitation. The tails of the distribution of precipitation events are lengthening. This is causing the intensity, duration and frequency of precipitation amounts to become more extreme.
The following graph shows the distribution of daily rainfall over two 15-year periods, one from 1960 to 1974 and the other from 2010 to 2024, for Toronto, Canada. We can see the tail of the distribution for the later period has skewed to the right with a few more extreme rainfall events in comparison to the earlier period.
Data source: https://climate.weather.gc.ca/historical_data/search_historic_data_e.html
WHAT CAN BE DONE TO PREPARE - Look Up, Look Down, Look Around
Look up: We often have fair warning of about a day of incoming severe weather associated with summer convective storms. For hurricanes or typhoons we may have a few more days of warning. The same can be said of atmospheric rivers with a good idea of the potential flood risks. For all of these cases, weather reports and advisories are given of progressing fronts and storms. If you have a favourite source for such information, please let us know and we will add to our resources page. However, for tsunamis and dam breaks, we have very little warning.
Look Down: We can keep tabs on any weather forecast. We are then able to determine if the risk of flooding is high or low depending on your location. For example, if you are situated in a low lying area relative to the neighbouring areas or near the coast your risk may be high. Also there may be risk of flood water channeling as was seen and experienced in Europe in 2024 as a number of storms passed over the summer and fall.
https://www.cnn.com/2024/10/31/europe/spain-deadly-flash-flooding-wwk-intl/index.html
We need to anticipate and plan how to evacuate from such potentially dangerous locations. Flood water channeling is of particular concern if you are downstream of a dam or river. We should also be aware of prior conditions: has there been consistent rain in the past few days that has saturated the ground so that the ground is unable to retain more moisture; or has it been very dry for an extended period so that the ground is also unable to retain moisture ? These conditions cause water channeling and flash floods. This antecedent condition of saturated landscape contributed to the deadly and devastating flood experienced in North Carolina in 2024 when hurricane Helene hit the area.
Look Around: A few of the things we need to think about are terrain, transportation, and health and safety. If our homes are nested within mountainous areas we have to worry about the possibility of water channeling and mudslides or avalanches. Low lying areas and coastal areas are prone to flooding. Floods are usually a consequence of storms which will down power lines, creating local outages and live wires pose a fire and safety risk. Floods will cause sewers to back up and surcharge resulting in contaminated waters that pose a health hazard. Transportation can also be impacted with dangerous flooded roadways. All of these negative consequences can increase insurance rates.
WHAT CAN BE DONE WHEN FLOOD HITS
Where available, there are many local and municipal emergency management programs that we can use as a resource or guide. Key items to know are what civil agencies are responsible for what activity, where are the shelters, plan escape routes and/or adhere to local authority’s advice on escape routes, prepare and protect your home for flooding, ensure all important documents and valuables are secured, keep a personal life vests and perhaps a boat if possible. These are just some basic ideas. Please feel free to leave other suggestions or ideas in the comments.
EXPOSURE MANAGEMENT AND ADAPTATION
To limit our risk of floods, we need to first manage (reduce) our exposure and adapt to the new normal. By managing our exposure, we need to think of flood protection measures such as dykes, coastal flood barriers, flood water conveyance infrastructure or spillways, and overall hardening our infrastructure against floods. These measures are costly at both the regional/municipal scale and homeowner scale. Hardening our infrastructure can also involve raising them above flood water levels and buttressing them against the force of moving water damage. This type of hardening is usually reserved for existing dykes, bridges, and harbours.
In order to meaningfully harden our infrastructure or protect it, we must conduct flood risk analysis using climate models to project the potential risk of damage into the future to say 2080s. This gives us an idea of how severe floods can become and plan and build accordingly. We must estimate the new frequency or return period of floods of a given intensity given climate change. We have seen that the historical 1 in 100 year flood we have designed infrastructure to withstand now occurs more frequently. For example, such floods can now have a return period of 1 in 25 years, and this means that the historical 100 year flood now has a 4% chance of happening each year. Furthermore, the new 100 year flood is now more severe than before, perhaps reaching what was a 1 in 500 year flood. After conducting such a flood risk analysis, we may discover that the location of our asset is no longer safe and may require a complete relocation in the near future. When building new assets, we need to account for updated flood maps and future sea level rise that will exacerbate king tides and storm surges.
The buildings we work and live in were designed under historical building codes and standards that did not account for our increasing extremes in weather. These standards now need to be updated to reflect our current reality given our changing climate.
As our climate changes we need to adapt to the new reality of more severe floods. Aside from hardening our infrastructure, we need to change our activities and behaviours. For example, our emergency response, business continuity planning and health and safety protocols need to account for climate change. Critical business activities and infrastructure need to guard against flood risk and severe weather. For example, server rooms need to be flood proofed, electrical substations need to be elevated or flood barriers erected, safe evacuation of personnel during a flash flood need to be practiced, and safety protocols enacted when flood risk is anticipated.
As noted earlier, the ability to recover is linked to economic strength. As our climate rapidly changes we will see more hits to our economy from floods. This will erode our ability to be resilient resulting in the internal displacement of people migrating to areas that are less flood prone. Poorer neighbourhoods will be the hardest hit.
Resources:
Atmospheric Rivers:
https://earth.org/atmospheric-rivers/
https://www.scientificamerican.com/article/what-are-atmospheric-rivers-and-how-are-they-changing/
https://en.wikipedia.org/wiki/Pineapple_Express
Monsoon:
https://weathermonths.com/monsoon/
Music:
Kansas Joe McCoy and Memphis Minnie When the Levee Breaks - Famous 1927 Mississippi River Flood
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