Earth Oddities • Hydrology • Extreme Water
A flood can creep across a floodplain for days, explode through a canyon in minutes, overwhelm a city beneath a single cloudburst, push inland from the sea, burst from behind an ice jam, or reshape an entire landscape after the failure of a natural dam.
Floods occur when water reaches places, depths or flow rates that the landscape, river channel, drainage network, coastline or infrastructure can no longer contain. They range from sudden flash floods and overwhelmed cities to slow-rising river floods, coastal inundation, desert torrents, ice-jam floods, catastrophic outburst floods and ancient megafloods. This master guide explains the major flood types, how they form, why similar rainfall can produce radically different outcomes, how flooding is forecast and classified, and which specialist Strange Sounds guides explore each mechanism in depth.
Updated:
• StrangeSounds Flood Master Pillar
Floods: Quick Facts
- A flood is water covering land that is normally dry or exceeding the normal capacity of a channel, drainage system or coastal boundary.
- Flooding can develop in minutes, hours, days or weeks.
- Flash floods are defined largely by rapid onset and dangerous flow.
- River floods usually develop as runoff accumulates across a drainage basin.
- Urban flooding occurs when built drainage systems and paved surfaces cannot handle incoming water.
- Coastal flooding occurs when seawater inundates normally dry land.
- Storm surge is one mechanism capable of producing coastal flooding.
- Desert floods can be extremely fast because hard, dry ground often produces rapid runoff.
- Ice jams can block rivers and cause abrupt water-level changes.
- Snowmelt and rain-on-snow events can trigger large basin floods.
- Dam or levee failure can produce high-energy downstream flood waves.
- Megafloods are exceptionally large floods capable of reshaping landscapes.
- Heavy rainfall is a major flood trigger, but it is not itself a flood type.
- The same rainfall total can produce radically different flooding depending on intensity, soil saturation, terrain, drainage and land use.
- A 100-year flood does not mean one flood every 100 years.
- A 100-year flood has approximately a 1% annual exceedance probability.
- Flooding can combine with landslides, debris flows, erosion, infrastructure failure and contamination.
- Flood disasters depend on both the physical hazard and the people and infrastructure exposed to it.
What Is a Flood?
A flood occurs when water covers land that is normally dry or rises beyond the normal capacity of a river, drainage system, coastal boundary or other landscape feature.
Floodwater can come from many places:
- rainfall;
- rivers and streams;
- snowmelt;
- groundwater;
- lakes and reservoirs;
- the ocean;
- glacial lakes;
- failed dams or levees;
- blocked channels;
- or several sources at the same time.
Flooding Is About Exceeding Capacity
The basic principle is remarkably simple:
Water input > storage, infiltration, drainage or channel capacity = flooding.
But the details determine whether the result is a shallow puddle, a slow river flood or a wall of debris-charged water capable of moving vehicles and buildings.
Floods Do Not Behave the Same Way
A major river may rise gradually for several days.
A desert wadi can go from dry to deadly in minutes.
A coastal city may flood from seawater even while little rain falls locally.
A subway system can flood because drainage fails while a nearby river remains inside its banks.
Flood Source vs Flood Mechanism vs Flood Outcome
Flood terminology becomes much easier to understand when three ideas are separated:
| Concept | Meaning | Examples |
|---|---|---|
| Water source | Where the water originates | Rain, river discharge, snowmelt, ocean water, groundwater, reservoir water |
| Mechanism | How the water escapes normal containment | Drainage exceedance, river overflow, storm surge, ice blockage, dam breach |
| Flood outcome | The resulting inundation | Urban flood, flash flood, river flood, coastal flood, downstream breach flood |
One flood can involve several mechanisms at once.
For example, a tropical cyclone can simultaneously produce:
- extreme rainfall;
- flash flooding;
- river flooding;
- urban drainage flooding;
- storm-surge-driven coastal flooding.
That overlap is why major flood disasters rarely fit perfectly into a single box.
Why Do Floods Happen?
Flooding begins when water arrives faster than the environment can absorb, store, drain or transport it.
Extreme Rainfall
Intense rainfall can exceed soil infiltration and drainage capacity, producing runoff faster than streams, gutters or storm drains can carry it away.
Prolonged Rainfall
Long-duration rainfall can gradually saturate soil and increase river discharge across entire drainage basins.
River Overflow
Rivers flood when incoming water exceeds channel capacity and spills onto the surrounding floodplain.
Snowmelt
Rapid warming can release large volumes of stored winter precipitation into rivers and drainage networks.
Rain on Snow
Warm rain falling onto snow can combine direct rainfall with accelerated snowmelt.
Coastal Water
Elevated ocean levels can cross onto normally dry land during storm surge, extreme tides or combinations of coastal processes.
Ice Jams
Floating river ice can pile up and temporarily block water flow, causing levels to rise rapidly upstream.
Dam or Levee Failure
Failure of infrastructure retaining water can release large volumes downstream with little warning.
Natural-Dam Failure
Landslides, glaciers, ice dams and volcanic deposits can temporarily block water before failing catastrophically.
Blocked Drainage
Debris, sediment, undersized culverts, blocked storm drains and damaged infrastructure can turn otherwise manageable rainfall into destructive flooding.
The Main Types of Floods
Floods can be grouped by their dominant mechanism, speed and landscape setting.
| Flood Type | Main Mechanism | Typical Onset | Deep Guide |
|---|---|---|---|
| Flash Flood | Rapid runoff, intense rainfall or sudden water release | Minutes to hours | Flash Floods Explained |
| Urban Flood | Rain exceeds drainage and sewer capacity | Minutes to hours | Urban Flooding Explained |
| River Flood | River discharge exceeds channel capacity | Hours to weeks | River Flooding Explained |
| Coastal Flood | Ocean water inundates normally dry land | Hours to days | Coastal Flooding Explained |
| Desert Flood | Rapid runoff through arid terrain and dry channels | Minutes to hours | Desert Flooding Explained |
| Ice-Jam Flood | River ice blocks normal flow | Minutes to days | Ice-Jam Floods Explained |
| Outburst Flood | Sudden release from a natural lake or dam | Minutes to hours | Megafloods & Ancient Floods Explained |
| Dam-Failure Flood | Failure or overtopping of engineered water-retaining infrastructure | Minutes to hours | Dam Failures & Infrastructure Collapse |
| Megaflood | Exceptional natural outburst or basin-scale water release | Catastrophic | Megafloods & Ancient Floods Explained |
How Are Floods Classified?
There is no single flood classification system that captures every event.
Hydrologists, meteorologists and emergency agencies may classify flooding according to different features.
By Source
- rainfall;
- river discharge;
- snowmelt;
- groundwater;
- coastal water;
- stored reservoir or lake water.
By Speed of Onset
- flash flooding;
- rapid flooding;
- slow-rising river flooding;
- long-duration inundation.
By Landscape
- urban floods;
- river floods;
- coastal floods;
- desert floods;
- mountain floods;
- delta floods.
By Mechanism
- pluvial flooding;
- fluvial flooding;
- coastal inundation;
- ice-jam flooding;
- outburst flooding;
- dam-break flooding.
Flash Flood vs River Flood vs Urban Flood vs Coastal Flood
| Feature | Flash Flood | River Flood | Urban Flood | Coastal Flood |
|---|---|---|---|---|
| Typical speed | Very fast | Usually slower | Fast | Variable |
| Main mechanism | Rapid runoff | Channel overflow | Drainage exceedance | Marine inundation |
| Typical setting | Canyons, small basins, wadis, steep terrain | River valleys and floodplains | Cities and built environments | Low-lying coasts, bays, deltas |
| Warning time | Often very short | Often longer | Often short | Can be hours to days depending on cause |
| Deep guide | Flash Floods | River Flooding | Urban Flooding | Coastal Flooding |
Flash Floods
Flash floods develop rapidly, often within minutes or a few hours of intense rainfall or sudden water release.
They are particularly dangerous because both depth and flow velocity can increase before people have time to react.
Where Flash Floods Are Most Dangerous
- narrow valleys;
- slot canyons;
- mountain basins;
- wadi systems;
- burn scars;
- urban underpasses;
- small steep watersheds.
Urban Flooding
Urban flooding occurs when water overwhelms the drainage capacity of a built environment.
Roads, roofs, parking lots and other impermeable surfaces prevent rainfall from soaking into the ground efficiently.
Water instead concentrates into:
- storm drains;
- sewers;
- underpasses;
- basements;
- subway tunnels;
- parking structures;
- roads and low points.
Pluvial Flooding
Urban flooding caused directly by intense rainfall is often called pluvial flooding.
A city can experience major pluvial flooding even when no nearby river overflows.
Why Cities Flood So Quickly
Impervious surfaces accelerate runoff while drainage networks have finite capacity.
Once rainfall intensity exceeds that design capacity, water can accumulate extremely quickly.
River Flooding
River flooding, also called fluvial flooding, occurs when river discharge exceeds the capacity of the channel and water spreads onto surrounding land.
Unlike many flash floods, large river floods can build over days as runoff accumulates across a drainage basin.
Major Causes
- prolonged rainfall;
- repeated storms;
- snowmelt;
- rain on snow;
- saturated soils;
- tributary flooding;
- ice jams;
- levee failure.
Floodplains
Floodplains are naturally part of river systems.
During high flow, water spreads across these relatively flat areas and temporarily stores part of the flood volume.
Development inside floodplains therefore converts a natural hydrological process into a major exposure problem.
Coastal Flooding
Coastal flooding occurs when ocean or sea water inundates land that is normally dry.
It describes the flood outcome rather than a single physical mechanism.
Coastal Flood Drivers Include
- storm surge;
- storm tide;
- high astronomical tide;
- wave setup;
- large waves and run-up;
- persistent onshore winds;
- compound interaction with rivers and rainfall.
Storm Surge Is One Important Driver
During hurricanes, typhoons and major extratropical storms, strong winds can push ocean water toward shore and create a large storm surge.
But not every coastal flood is a storm-surge flood.
Drainage Can Reverse
Elevated coastal water can also prevent rivers, storm drains and canals from emptying efficiently into the sea.
This can worsen flooding inland even when the ocean itself does not travel very far from the shoreline.
Desert Flooding
Deserts may receive little rain overall, but that does not make them safe from floods.
In fact, some arid landscapes are extremely efficient at converting intense rainfall into runoff.
Why Dry Ground Can Flood Quickly
Desert surfaces may be:
- hard;
- crusted;
- sparsely vegetated;
- steep;
- poor at absorbing intense rainfall quickly.
Wadis and Dry Washes
Normally dry channels can collect runoff from distant thunderstorms.
Water can therefore arrive in a wadi even when little or no rain has fallen directly overhead.
Temporary Lakes
Closed desert basins can collect runoff and form temporary lakes that may persist for days, months or even longer depending on evaporation and drainage.
Ice-Jam Floods
Ice-jam floods occur when river ice obstructs normal flow.
Breakup Ice Jams
During spring thaw, broken slabs of river ice can move downstream and pile up at:
- river bends;
- bridges;
- islands;
- shallow reaches;
- channel constrictions.
Water backs up rapidly behind the temporary ice dam.
Jam Failure
When the blockage suddenly releases, water and ice can surge downstream.
Large ice slabs can add major impact forces to the floodwater.
Snowmelt and Rain-on-Snow Floods
Snow acts as temporary water storage.
When temperatures rise, that stored water is released into soil, streams and rivers.
Gradual Snowmelt
Slow spring melting may produce manageable increases in river flow.
Rapid Thaw
Sudden warming can release water faster than rivers and floodplains can accommodate.
Rain on Snow
Warm rainfall can simultaneously:
- add new water;
- accelerate snowmelt;
- reduce snowpack storage;
- produce rapid basin runoff.
Frozen Ground
If the soil remains frozen while the snow melts, infiltration can be limited and more water travels directly over the surface.
Groundwater Flooding
Flooding does not always arrive from a river or the sky.
After prolonged wet periods, groundwater levels can rise until they reach or exceed the land surface.
Groundwater Flooding Can Affect
- basements;
- low fields;
- roads;
- underground infrastructure;
- springs and seepage zones.
It is often slower than flash flooding but can persist long after rainfall ends.
Outburst Floods
An outburst flood occurs when water stored behind a natural barrier is suddenly released.
Possible Natural Barriers
- glacial ice;
- moraines;
- landslides;
- volcanic deposits;
- lava flows;
- river sediment;
- ice jams.
Glacial Lake Outburst Floods
Lakes dammed by ice or glacial debris can fail suddenly and release enormous volumes downstream.
Landslide-Dam Floods
Landslides can temporarily block rivers and create lakes.
If the landslide barrier overtops or collapses, downstream flooding can be severe.
Megafloods and Ancient Floods
Megafloods are extreme floods with discharges and erosional power far beyond ordinary modern river floods.
Many of the best-known examples occurred during or after the Ice Ages.
What Can Cause a Megaflood?
- failure of giant glacial lakes;
- collapse of ice dams;
- catastrophic lake drainage;
- failure of natural landslide dams;
- rare basin-scale outburst events.
Landforms Left Behind
Ancient megafloods can leave:
- giant ripple marks;
- scoured bedrock;
- oversized channels;
- dry cataracts;
- gravel bars;
- scablands.
Dam Failures, Levee Breaches and Infrastructure Floods
Some flood disasters begin with the failure of infrastructure designed to control water.
Dams and levees can fail because of:
- overtopping;
- internal erosion;
- foundation failure;
- structural damage;
- extreme inflow;
- landslides into reservoirs;
- poor maintenance;
- earthquakes or other external stresses.
Why This Topic Crosses Two Hierarchies
The resulting downstream event is clearly a flood.
But the initiating mechanism may be an infrastructure failure rather than a purely natural hydrological process.
For that reason, dam and levee failures connect this flood pillar with the site’s human-impact and infrastructure-disaster coverage.
Extreme Rainfall: A Major Flood Trigger, Not a Flood Type
Extreme rainfall causes many of the world’s worst flood disasters, but rainfall and flooding are not the same phenomenon.
Rain is an atmospheric input.
Flooding is the hydrological response.
Extreme Rainfall Can Produce
- flash floods;
- urban floods;
- river floods;
- desert floods;
- landslides and debris flows;
- compound flooding when coastal water is also elevated.
Important Rain-Producing Systems
- training thunderstorms;
- slow-moving convective systems;
- tropical cyclones;
- monsoons;
- atmospheric rivers;
- stationary fronts;
- orographic rainfall;
- cloudbursts.
Compound Flooding: When Several Flood Drivers Arrive Together
Some of the worst flood disasters occur when multiple water sources overlap.
Coastal Water + River Flow
Elevated sea levels can prevent river water from draining efficiently.
Storm Surge + Extreme Rainfall
A tropical cyclone can simultaneously push seawater inland while dumping extreme rainfall over the same area.
Rainfall + Snowmelt
Warm rain can combine with rapid melting to produce exceptionally large runoff.
River Flood + Dam or Levee Failure
High water can place additional stress on infrastructure and worsen downstream flooding if a barrier fails.
Why Compound Flooding Matters
Flood systems are often interconnected.
A drainage network designed for heavy rain may fail if river or coastal water is already blocking its outlet.
What Makes a Flood Worse?
Rainfall amount alone does not determine flood severity.
| Factor | Why It Matters |
|---|---|
| Rainfall intensity | Very heavy rain can exceed infiltration and drainage capacity quickly. |
| Rainfall duration | Long rainfall events saturate soil and fill river basins. |
| Antecedent soil moisture | Already-wet ground absorbs less additional water. |
| Terrain slope | Steep terrain accelerates runoff and concentrates flow. |
| Basin shape | Some watersheds deliver runoff to the same outlet very efficiently. |
| River capacity | Narrow or restricted channels overflow more easily. |
| Urban paving | Impermeable surfaces reduce infiltration and accelerate runoff. |
| Drainage capacity | Undersized or blocked drains increase local flooding. |
| Snowpack | Rapid melt adds stored winter water to runoff. |
| Coastal water level | High sea levels can block drainage and create compound flooding. |
| Land use | Deforestation, channelization and wetland loss can alter runoff and storage. |
| Infrastructure | Dams, levees, roads and culverts can reduce or redirect floodwater — or worsen damage if they fail. |
How Urbanization and Land Use Change Flood Risk
Flood hazard exists naturally, but human development can change how quickly water moves and how much damage results.
Impervious Surfaces
Asphalt, concrete and roofs reduce infiltration and increase runoff.
Floodplain Development
Building on naturally flood-prone land increases exposure even when the underlying river behavior has not changed.
Wetland Loss
Wetlands can store water and slow runoff. Removing them can reduce landscape storage.
Channelization
Straightening or confining rivers may move water more quickly through one location while transferring risk downstream.
Drainage Networks
Modern drainage systems can rapidly remove ordinary rainfall but may fail abruptly once design limits are exceeded.
How Are Floods Forecast?
Flood forecasting combines atmospheric forecasts with observations of the land and water system.
Rainfall Forecasts
Weather models estimate where, when and how much precipitation may fall.
Weather Radar
Radar helps meteorologists monitor rainfall intensity and movement in real time.
Rain Gauges
Ground observations measure how much precipitation has actually fallen.
River Gauges
River sensors track water level and discharge as flood waves move downstream.
Soil Moisture
Saturated ground dramatically changes how much additional rainfall becomes runoff.
Snowpack
Snow-water equivalent helps estimate how much water may become available during thaw.
Hydrological Models
Models convert rainfall and snowmelt into estimated runoff, river flow and flood levels.
Terrain and Inundation Models
Elevation data can be used to estimate which land may become flooded at different water levels.
Why Flash Floods Are Harder
Large river floods may offer hours or days of warning.
Flash floods can develop faster than local observations and forecasts can fully capture, especially in small steep basins.
What Does a “100-Year Flood” Actually Mean?
The phrase 100-year flood is widely misunderstood.
It does not mean a flood happens once and then cannot happen again for another century.
Annual Exceedance Probability
A 100-year flood corresponds approximately to a flood magnitude with a:
1% chance of being equaled or exceeded in any given year.
Two 100-Year Floods Can Occur Close Together
Each year represents another opportunity for the threshold to be exceeded.
Multiple statistically rare floods can therefore occur within a short period.
Other Common Return Periods
| Return Period | Approximate Annual Probability |
|---|---|
| 10-year flood | 10% |
| 20-year flood | 5% |
| 50-year flood | 2% |
| 100-year flood | 1% |
| 500-year flood | 0.2% |
Return Periods Are Estimates
These values depend on the available historical record and statistical assumptions.
Changing land use, river engineering, measurement records and environmental conditions can also alter flood probabilities over time.
Flood Watch vs Flood Warning vs Flash Flood Warning
Terminology varies between countries, but the general concept is similar.
Flood Watch
Conditions are favorable for flooding.
Flood Warning
Flooding is occurring, expected or sufficiently likely that protective action may be required.
Flash Flood Warning
Rapid dangerous flooding is occurring or expected soon.
Flood Hotspots Around the World
Flooding can occur almost anywhere, but certain landscapes repeatedly concentrate risk.
| Setting | Common Flood Pattern |
|---|---|
| Monsoon regions | Extreme rainfall, river flooding, flash floods and landslide-related hazards. |
| Large river floodplains | Long-duration river overflow and widespread inundation. |
| Deltas | River flooding, coastal flooding and compound water-level events. |
| Arid regions | Fast wadi flooding and temporary desert lakes. |
| Mountain valleys | Flash floods, debris-rich torrents and outburst floods. |
| Dense cities | Pluvial flooding, overwhelmed drainage and underground infrastructure flooding. |
| Cold river basins | Snowmelt floods and ice-jam flooding. |
| Low-lying coasts | Storm surge, tidal flooding and compound coastal inundation. |
Historic Flood Disasters and Scientific Benchmarks
Historic floods are useful because they show how different flood mechanisms create very different disasters.
| Event | Flood Type | Why It Matters |
|---|---|---|
| 1931 China floods | River / basin flooding | Among the deadliest flood disasters in recorded history. |
| 1953 North Sea Flood | Coastal flooding | Landmark storm-surge disaster in northwestern Europe. |
| 1970 Bhola Cyclone | Coastal / storm-surge flooding | Catastrophic flooding across the low-lying Bengal delta. |
| 1993 Mississippi–Missouri floods | River flooding | Major long-duration floodplain disaster in the United States. |
| 2010 Pakistan floods | River / monsoon flooding | Large-scale basin flooding affecting millions of people. |
| 2021 Western Europe floods | Flash / river flooding | Extreme rainfall produced destructive floods in Germany, Belgium and neighboring regions. |
| 2021 Waverly flood | Flash flood | Modern example of extreme short-duration rainfall and rapid destructive runoff. |
| Missoula Floods | Prehistoric megafloods | Ice Age outburst floods that reshaped large parts of the Pacific Northwest landscape. |
Strange and Unusual Flood Phenomena
Floods produce some of the most surreal natural-disaster scenes on Earth.
Water can:
- float houses from their foundations;
- carry cars through streets;
- expose buried coffins;
- strand fish in roads and fields;
- force spiders and other animals into mass movements;
- create temporary rivers through deserts;
- turn highways into channels;
- move buildings, trees and debris far downstream.
These unusual outcomes are often visually spectacular but should still be traced back to the underlying flood mechanism whenever possible.
Not Every Extreme Water Event Is a Flood
Dramatic water disasters can look similar in photographs while originating from completely different physical processes.
Storm Surge
Storm-driven abnormal coastal water rise.
Tsunamis
Long waves created by sudden displacement of large volumes of water, most commonly from undersea earthquakes.
Meteotsunamis
Long waves generated by atmospheric pressure disturbances rather than ordinary flood runoff.
Rogue Waves
Exceptionally large ocean waves occurring within a sea state.
Coastal Erosion
Removal or retreat of shoreline material by waves, currents, storms and long-term coastal processes.
Flood Safety: The Most Important Principles
Flood behavior differs by location, so official local warnings and evacuation instructions always take priority.
Do Not Drive Into Floodwater
Roads may be:
- washed away;
- covered by deeper water than expected;
- crossed by strong currents;
- blocked by debris.
Avoid Moving Water
Flowing floodwater can exert considerable force even when it does not appear especially deep.
Move Away From Low Ground During Flash-Flood Threats
Canyons, wadis, stream beds and underpasses can become dangerous very quickly.
Know Whether Your Risk Is River, Flash or Coastal Flooding
Different hazards require different preparation and evacuation strategies.
Do Not Assume the Rain Must Be Falling Where the Flood Arrives
Runoff and river flow can travel from storms many kilometers away.
Treat Floodwater as Contaminated
Floodwater can contain sewage, chemicals, fuel, sharp debris, animals and electrical hazards.
Flood Glossary
- Flood
- Water covering normally dry land or exceeding the normal capacity of a river, drainage system or coastal boundary.
- Flash Flood
- Rapid-onset flooding developing within a short period, often after intense rainfall or sudden water release.
- Fluvial Flooding
- Flooding caused by rivers or streams overflowing their channels.
- Pluvial Flooding
- Surface-water flooding caused directly by rainfall overwhelming infiltration or drainage capacity.
- Coastal Flooding
- Inundation of normally dry coastal land by seawater.
- Runoff
- Water flowing across the land surface toward streams, rivers, drains or low areas.
- Infiltration
- Movement of water from the land surface into soil.
- Drainage Basin
- Land area from which water drains toward the same river, lake or outlet.
- Floodplain
- Relatively flat land beside a river that is naturally subject to flooding.
- Bankfull Stage
- Water level at which a river approximately fills its channel before spreading across the floodplain.
- Wadi
- Usually dry channel in an arid region that can carry sudden floodwater after rainfall.
- Ice Jam
- Accumulation of river ice that restricts water flow and can cause flooding.
- Outburst Flood
- Sudden release of water stored behind a natural barrier such as ice, moraine or landslide debris.
- Megaflood
- Exceptionally large flood capable of producing major geomorphic change.
- Levee
- Natural or engineered embankment that confines floodwater.
- Overtopping
- Water flowing over the top of a dam, levee or other barrier.
- Compound Flooding
- Flooding produced or intensified by multiple water sources or mechanisms occurring together.
- Return Period
- Statistical expression describing the average recurrence interval associated with a flood magnitude.
- Annual Exceedance Probability
- Probability that an event of a given magnitude will be equaled or exceeded in any individual year.
Frequently Asked Questions About Floods
What is a flood?
A flood occurs when water covers land that is normally dry or rises beyond the normal capacity of a river, drainage system or coastal boundary.
What causes floods?
Floods can be caused by extreme rainfall, river overflow, snowmelt, groundwater rise, storm-driven coastal water, ice jams, natural outburst floods, dam or levee failure and combinations of several mechanisms.
What are the main types of floods?
Major flood types include flash floods, urban or pluvial floods, river or fluvial floods, coastal floods, desert floods, ice-jam floods, groundwater floods, outburst floods and dam-failure floods.
What is the difference between a flood and a flash flood?
Flood is the broad term for water inundating normally dry land. A flash flood is a rapid-onset flood that develops over a short period and can produce dangerous fast-moving water.
What is the difference between a flash flood and a river flood?
Flash floods develop rapidly, often in small or steep drainage basins. River floods usually develop as runoff accumulates across a larger watershed and pushes a river above channel capacity.
What is urban flooding?
Urban flooding occurs when rainfall or another water source overwhelms drainage systems, sewers, culverts and other infrastructure in a built environment.
What is pluvial flooding?
Pluvial flooding is surface-water flooding caused directly by rainfall exceeding the ability of the ground and drainage systems to absorb or remove it.
What is fluvial flooding?
Fluvial flooding is flooding caused by a river or stream overflowing its channel.
Why do deserts flood?
Intense rainfall can run rapidly over hard, dry or sparsely vegetated terrain and concentrate into normally dry washes and wadis.
Can flooding happen without rain?
Yes. Floods can result from snowmelt, ice jams, groundwater rise, coastal inundation, dam failure or sudden release from a lake or natural barrier.
Can it flood where it is not raining?
Yes. Runoff, river flow and flash-flood water can travel from rainfall occurring far upstream or many kilometres away.
What is an ice-jam flood?
An ice-jam flood occurs when river ice blocks normal water flow and causes levels to rise upstream. A sudden jam release can also send water and ice downstream rapidly.
What is an outburst flood?
An outburst flood is the sudden release of water stored behind a natural barrier such as glacial ice, moraine material or a landslide dam.
What is a megaflood?
A megaflood is an exceptionally large flood with enough discharge and erosional power to reshape major landscapes.
Are coastal floods and storm surge the same thing?
No. Coastal flooding is the broad outcome of seawater inundating normally dry land. Storm surge is one important physical mechanism that can cause coastal flooding.
Is extreme rainfall a type of flood?
No. Extreme rainfall is an atmospheric trigger that can produce flash floods, urban floods, river floods, desert floods and other flooding.
What is compound flooding?
Compound flooding occurs when multiple water sources or flood mechanisms overlap, such as storm surge combined with river flow and extreme rainfall.
What makes flooding worse?
Flood severity depends on rainfall intensity and duration, soil saturation, slope, watershed shape, river capacity, drainage infrastructure, snowmelt, coastal water levels, land use and the amount of development exposed to flooding.
Why does pavement increase flooding?
Roads, roofs and other impermeable surfaces reduce infiltration and send rainfall rapidly toward storm drains and low areas.
What does a 100-year flood mean?
A 100-year flood is a flood magnitude with approximately a 1% chance of being equaled or exceeded in any given year. It does not mean such a flood can occur only once every 100 years.
Can two 100-year floods happen in consecutive years?
Yes. The probability applies independently to each year, so statistically rare floods can occur close together.
How are floods forecast?
Flood forecasts combine rainfall forecasts, radar, rain gauges, river gauges, soil moisture, snowpack observations, terrain data and hydrological models.
Why are flash floods difficult to predict?
Flash floods can develop very quickly in small basins where rainfall intensity, terrain and runoff interact on short timescales.
Are tsunamis floods?
Tsunamis can cause coastal inundation, but they are long-wave phenomena generated by sudden displacement of water and should be distinguished from ordinary flood mechanisms.
Are rogue waves floods?
No. Rogue waves are unusually large individual ocean waves. They can cause coastal damage but are not themselves a flood type.
What is the most dangerous flood type?
There is no universal answer. Flash floods are especially dangerous because of their speed, while river and coastal floods can affect much larger populations and areas.
Where do floods happen most often?
Floods are common in river floodplains, monsoon regions, deltas, low-lying coasts, mountain valleys, dense cities, arid wadis and snow-covered river basins.
