What Is Flash Flooding?
Flash flooding is rapidly developing flooding that occurs when water levels rise quickly over a short period. It can develop when intense rainfall produces more water than the ground, waterways or drainage systems can handle.
Unlike slower forms of flooding that may develop over several days, flash flooding can occur with relatively little lead time. Water can quickly cover roads, fill low-lying areas and cause streams or drainage channels to rise.
In Canada, flash flooding is most commonly associated with torrential rain and thunderstorms, but snowmelt, rain-on-snow events and sudden releases or blockages of water can also contribute to rapidly developing flooding.
Quick Facts About Flash Flooding
Main Characteristic: Rapidly developing flooding
Development: Can occur within minutes to hours
Common Causes: Torrential rain, thunderstorms and rapid runoff
Common Locations: Urban areas, low-lying terrain, valleys and small waterways
Major Hazards: Fast-rising water, flooded roads and strong currents
Related Conditions: Heavy rainfall, torrential rain and urban flooding
Canadian Factors: Snowmelt, frozen ground and ice-related flooding can contribute in some situations
What Causes Flash Flooding?
Flash flooding occurs when water enters an area faster than it can drain away, infiltrate the ground or move through nearby waterways.
Intense rainfall is one of the most common causes.
During a powerful thunderstorm, a substantial amount of rain can fall within a relatively short period. If rainfall intensity exceeds the capacity of the soil and drainage system, water begins flowing across the surface.
Runoff collects in low-lying areas, drainage channels, creeks and streams.
When this process occurs rapidly enough, flash flooding can develop.
How Quickly Can a Flash Flood Develop?
Flash flooding is distinguished by how rapidly conditions change.
Water levels may rise substantially within minutes or hours of intense rainfall.
Small streams and drainage channels can respond particularly quickly because they collect runoff from surrounding land.
In urban areas, water can accumulate rapidly because pavement and other impermeable surfaces prevent rainfall from soaking into the ground.
The speed at which flooding develops depends on rainfall intensity, terrain, drainage, soil moisture and the size of the affected watershed.
Flash Flooding vs. River Flooding
The major difference is usually the speed at which flooding develops.
Flash flooding develops rapidly, often following intense rainfall over a relatively small area.
River flooding generally occurs when a river rises beyond its normal channel because of rainfall, snowmelt or other water entering the watershed.
Large rivers may take considerably longer to respond because water must travel from across the drainage basin toward the main channel.
However, smaller rivers and streams can rise very quickly, meaning the distinction is not always based solely on the type of waterway involved.
Flash Flooding vs. Urban Flooding
Flash flooding describes flooding that develops rapidly.
Urban flooding describes flooding occurring within a developed area.
The two can happen simultaneously.
For example, a severe thunderstorm may produce torrential rainfall over a city. Storm drains become overwhelmed and water begins accumulating on roads and in underpasses within a short period.
That event could reasonably involve both urban flooding and flash flooding.
How Does Torrential Rain Cause Flash Flooding?
Rainfall intensity is one of the most important factors.
During light or moderate rain, much of the water may infiltrate the soil or gradually move through drainage systems.
During torrential rain, precipitation can reach the surface faster than those systems can process it.
Water begins accumulating and flowing across the surface.
If the rainfall continues, runoff can rapidly converge in lower terrain and waterways.
This is why a short but exceptionally intense thunderstorm can sometimes produce more immediate flooding than a much longer period of lighter rainfall.
Why Are Thunderstorms a Major Cause?
Thunderstorms can produce extremely high rainfall rates over relatively small areas.
Strong updrafts continuously transport warm, moisture-rich air upward through the storm.
As this moisture condenses, large quantities of precipitation can develop.
Slow-moving thunderstorms are particularly concerning because intense rainfall remains concentrated over the same location.
Several thunderstorms repeatedly crossing the same area can create an even greater risk.
What Are Training Thunderstorms?
Training thunderstorms occur when multiple storms repeatedly move across approximately the same location.
Each thunderstorm adds more rainfall before water from the previous storm has had time to drain away.
Rainfall accumulations can therefore increase rapidly.
Training can produce serious flash flooding even when individual thunderstorms are relatively short-lived.
Meteorologists monitor radar closely for this pattern during heavy rainfall events.
Why Are Slow-Moving Storms Dangerous?
A storm’s movement affects how much rain falls over one location.
A powerful thunderstorm moving rapidly may produce torrential rain but leave the area before extremely large accumulations occur.
A slow-moving storm can continue depositing intense rainfall over the same area.
If the storm becomes nearly stationary, localized rainfall totals can become very large.
This substantially increases the potential for flash flooding.
Why Are Cities Vulnerable to Flash Flooding?
Urban development changes how water moves across the landscape.
Natural surfaces such as soil and vegetation can absorb some rainfall.
Cities contain large areas covered by:
- Roads
- Parking lots
- Sidewalks
- Buildings
- Rooftops
These impermeable surfaces cause rainfall to become runoff more quickly.
Water is directed toward storm drains and drainage infrastructure.
If rainfall becomes more intense than the drainage system can accommodate, water may accumulate rapidly on streets and in low-lying locations.
Why Do Underpasses Flood So Quickly?
Road underpasses are lower than the surrounding terrain.
Water naturally flows toward lower elevations.
During intense rainfall, runoff from nearby roads and surfaces can therefore collect within an underpass.
Drainage systems are designed to remove this water, but extremely intense rainfall can exceed their capacity.
A flooded underpass can become dangerous because water depth may be difficult to determine from inside a vehicle.
How Does Terrain Affect Flash Flooding?
Steep terrain can significantly increase the speed of runoff.
Rain falling on hillsides moves downhill under gravity and can become concentrated in valleys, gullies and drainage channels.
Mountainous regions can therefore experience rapidly changing water conditions during intense rainfall.
Narrow valleys can be particularly hazardous because water from a larger surrounding area may become concentrated within a relatively confined space.
Can Flash Flooding Occur on Flat Terrain?
Yes.
Steep terrain is not required.
Flat urban areas can experience significant flash flooding when rainfall overwhelms drainage systems.
Low-lying agricultural land can also collect water rapidly.
The important factor is whether water is arriving faster than it can infiltrate, drain or flow away.
How Does Saturated Ground Increase Flash Flood Risk?
Soil can absorb a portion of incoming rainfall.
However, its ability to absorb water decreases as it becomes saturated.
After several days of rain, the ground may already contain substantial moisture.
Additional rainfall then produces more surface runoff.
As a result, a thunderstorm occurring after an extended wet period can produce flash flooding more easily than the same storm occurring when soils are relatively dry.
Can Dry Ground Increase Flash Flood Risk?
In some circumstances, yes.
Very dry or compacted soil does not always absorb water efficiently at the beginning of an intense rainfall event.
If rainfall arrives faster than water can infiltrate the surface, substantial runoff can occur.
Therefore, both saturated ground and certain very dry soil conditions can contribute to rapid runoff.
Can Frozen Ground Cause Flash Flooding?
Frozen ground can reduce the amount of water that infiltrates into the soil.
This can become important in Canada during late winter and early spring.
If heavy rain falls while the ground remains frozen, a larger proportion of the rainfall may travel across the surface.
When combined with melting snow, the amount of runoff can increase substantially.
This can contribute to rapidly rising streams and localized flooding.
Can Snowmelt Cause Flash Flooding?
Rapid snowmelt can contribute to rapidly developing flooding, particularly when several factors occur together.
A sudden period of warm weather can melt snow quickly.
If rain simultaneously falls onto the snowpack, additional water is introduced while rainfall may also accelerate melting.
Frozen or saturated soil can further limit infiltration.
This combination can send substantial runoff into drainage systems and waterways.
What Is a Rain-on-Snow Event?
A rain-on-snow event occurs when rain falls onto an existing snowpack.
Rain can add liquid water directly to the landscape while warm temperatures and atmospheric conditions accelerate snowmelt.
The combined rainfall and meltwater can produce much more runoff than rainfall alone.
These events can contribute to flooding when soils are frozen or saturated or when nearby waterways are already elevated.
For Snowstorms.ca, this is an especially useful internal link between your winter and flood content.
Can Ice Jams Cause Rapid Flooding?
Yes.
An ice jam occurs when pieces of river ice accumulate and restrict the normal flow of water.
Water can back up behind the blockage and cause river levels to rise.
If an ice jam forms, shifts or suddenly releases, water levels can change quickly.
Ice-jam flooding is particularly relevant to Canadian rivers during freeze-up and spring breakup.
Because this is a distinct process, it deserves its own future condition page under your Rain & Flood category.
Can Dam or Infrastructure Failures Cause Flash Flooding?
Rapid flooding can also result from sudden releases of stored water.
A dam, levee or other water-control structure failure can release a large volume of water downstream.
These events are different from rainfall-generated flash floods but can produce similarly rapid and dangerous increases in water levels.
How Deep Can Flash Flood Water Become?
There is no standard depth.
Flash flooding ranges from relatively shallow water covering streets to deep, fast-moving flows capable of causing extensive damage.
Water depth can also change rapidly as additional runoff enters the flooded area.
This unpredictability is one reason flooded roads are hazardous.
A roadway may also be damaged or washed out beneath water that obscures the surface.
Why Is Moving Floodwater Dangerous?
Moving water exerts force against people, vehicles and structures.
As water becomes deeper or flows faster, that force increases.
Floodwater can also contain debris such as branches, rocks, garbage and damaged building materials.
Visibility through muddy floodwater is often poor, making it difficult to identify holes, damaged pavement or other hazards beneath the surface.
Why Is Driving During Flash Flooding Dangerous?
Vehicles can encounter several hazards during flash flooding.
Standing water can reduce tire contact with the road and contribute to hydroplaning.
Deeper water can interfere with vehicle operation or cause a vehicle to become stranded.
Moving water may also push vehicles away from the roadway.
Drivers cannot reliably determine whether pavement beneath floodwater remains intact.
For these reasons, flooded roads should not be treated as ordinary wet-road conditions.
How Is Flash Flooding Forecast?
Forecasting flash floods requires meteorologists to consider both the expected rainfall and the characteristics of the affected area.
Forecasters examine:
Weather models to determine where heavy rainfall may develop.
Weather radar to monitor rainfall intensity and storm movement.
Rain gauges to measure how much precipitation has already fallen.
Soil moisture to estimate how much additional rainfall may become runoff.
Terrain and watersheds to determine where water is likely to collect.
River and stream levels to identify waterways already experiencing elevated conditions.
How Does Weather Radar Help Detect Flash Flooding?
Weather radar allows meteorologists to monitor intense precipitation as storms develop.
Repeated scans show where the heaviest rain is occurring and how quickly storms are moving.
Radar is particularly valuable for detecting slow-moving or training thunderstorms.
Meteorologists can estimate rainfall accumulation using radar observations and compare those estimates with measurements from rain gauges.
However, radar primarily observes precipitation. Flooding itself is also assessed using stream gauges, reports and other observations.
Why Is Flash Flooding Difficult to Predict?
Flash flooding can depend on very localized conditions.
Thunderstorm rainfall can vary dramatically across only a few kilometres.
Small differences in storm movement may determine whether one neighbourhood receives a brief downpour or several consecutive periods of torrential rain.
Local drainage, terrain and soil conditions create additional differences.
Forecasts can identify an environment favourable for flash flooding, but predicting exactly which streets or small waterways will flood can be much more difficult.
Flash Flooding in Canada
Flash flooding can occur across Canada wherever intense rainfall produces rapid runoff.
Summer thunderstorms are an important cause, particularly when storms move slowly or repeatedly cross the same area.
Urban areas can experience rapid flooding when drainage infrastructure becomes overwhelmed.
Mountainous terrain can concentrate runoff into valleys and waterways.
Canada’s winter climate introduces additional factors, including frozen ground, rapid snowmelt, rain-on-snow events and river ice.
The exact causes and flood behaviour therefore vary substantially between Canadian regions and seasons.
Flash Flooding and Climate
Flood risk depends on many factors beyond rainfall, including land development, drainage infrastructure, soil conditions, snowpack and watershed characteristics.
A warmer atmosphere can hold more water vapour, which can contribute to heavier precipitation when atmospheric conditions are favourable.
However, the effect on flooding in a specific location depends on how precipitation changes interact with local terrain, waterways, infrastructure and land use.
How to Stay Safe During Flash Flooding
Monitor forecasts and official alerts when torrential rainfall is expected.
Avoid entering flooded roads, underpasses or trails because water depth and current strength can be difficult to determine.
Move away from rapidly rising streams, drainage channels and low-lying areas.
During thunderstorms, remember that flooding may occur alongside lightning, hail and strong winds.
If authorities issue evacuation instructions or other emergency directions, follow the information provided for your area.