Earth Oddities • Floods • Coastal Hazards
A coast can flood beneath a hurricane, during an unusually high tide, when waves pile water against shore, when a swollen river cannot drain into the sea — or when several of those processes arrive together.
Coastal flooding occurs when ocean, sea or estuarine water inundates land that is normally dry.
It can be driven by storm surge, high astronomical tides, persistent onshore winds, wave setup and runup, storm tide, rising relative sea level, river backwater effects, heavy rainfall or several flood drivers interacting at once.
This guide explains why coasts flood, how coastal water levels are built from several interacting components, why storm surge and coastal flooding are not the same thing, how high-tide and compound flooding develop, why estuaries and deltas are especially vulnerable, and how coastal inundation is forecast and managed.
Updated:
• StrangeSounds Coastal Flooding Guide
Coastal Flooding: Quick Facts
- Coastal flooding occurs when seawater inundates land that is normally dry.
- It affects beaches, roads, homes, ports, wetlands, estuaries, deltas, barrier islands and low-lying coastal cities.
- Storm surge is a flood driver, not a synonym for coastal flooding.
- Storm tide is the observed coastal water level produced by astronomical tide plus storm-related water-level effects.
- High-tide flooding can occur without a major storm.
- Very high astronomical tides are often informally called king tides.
- Wave setup raises the mean water level near shore as breaking waves transfer momentum toward land.
- Wave runup describes how far waves rush vertically and horizontally up beaches or structures.
- Waves can overtop dunes, seawalls and other coastal barriers.
- Barrier islands can experience overwash when water and sediment cross the island during high-energy events.
- Elevated coastal water can prevent rivers and storm drains from emptying efficiently.
- This drainage restriction can worsen flooding well inland from the immediate shoreline.
- Compound flooding occurs when two or more flood drivers interact.
- Common combinations include storm surge + rainfall, river flow + high tide, and rainfall + elevated groundwater + coastal water.
- Relative sea-level rise raises the baseline from which tides, waves and storm surge operate.
- Land subsidence can increase relative sea-level rise even where ocean rise itself is unchanged.
- Saltwater flooding can damage soils, vegetation, freshwater systems and infrastructure after the visible floodwater disappears.
What Is Coastal Flooding?
Coastal flooding is the temporary or persistent inundation of normally dry land by seawater or water strongly influenced by coastal water levels.
It can affect:
- beaches;
- dunes;
- barrier islands;
- marshes and wetlands;
- harbors and ports;
- roads and railways;
- waterfront neighborhoods;
- estuaries;
- deltas;
- low-lying islands;
- coastal cities.
The defining feature is not the storm, tide or wave that produced the event.
It is the fact that coastal water reaches land that is normally dry.
How Does Coastal Flooding Happen?
A coast floods when the total coastal water level rises high enough to cross the elevation of beaches, dunes, roads, seawalls, wetlands or other land surfaces.
That total water level can be built from several components operating at the same time.
Tide + surge + waves + regional water-level anomalies + long-term baseline = total coastal water level.
Rainfall, river discharge and groundwater can then add additional flooding inland.
A Few Centimeters Can Matter
Coastal flooding often behaves like a threshold problem.
If a road surface sits only slightly above ordinary high water, a relatively small additional increase may be enough to cross it.
This is why relatively modest changes in baseline sea level can greatly change how often a particular flood threshold is exceeded.
What Controls Coastal Water Level?
Coastal flooding is easier to understand if the total water level is separated into its components.
| Component | What It Does |
|---|---|
| Astronomical tide | Regularly raises and lowers ocean levels due primarily to gravitational interactions involving the Moon and Sun. |
| Storm surge | Adds abnormal water-level rise associated mainly with storm winds and atmospheric pressure. |
| Wave setup | Raises mean nearshore water level as breaking waves push water toward shore. |
| Wave runup | Allows individual waves to rush farther and higher onto land or structures. |
| Regional ocean anomaly | Ocean circulation, winds and large-scale climate patterns can temporarily elevate or depress coastal water levels. |
| Relative sea level | Sets the longer-term local baseline relative to the land. |
| River discharge | Adds freshwater pressure from inland near estuaries and deltas. |
| Rainfall | Adds pluvial water that may become trapped when coastal outlets are blocked. |
| Groundwater | A shallow water table can reduce subsurface storage and contribute to emergence flooding. |
What Causes Coastal Flooding?
Coastal floods can result from one dominant mechanism or from several drivers occurring simultaneously.
1. Storm Surge
Strong storm winds can push ocean water toward shore and raise coastal water levels.
Tropical cyclones can produce devastating surge, but major extratropical storms can also generate substantial coastal flooding.
Explore Storm Surge Explained →
2. High Astronomical Tides
Some coastal areas flood simply because the astronomical tide becomes high enough to exceed local drainage or elevation thresholds.
3. Persistent Onshore Winds
Winds blowing toward shore can pile water into bays, lagoons, estuaries and shallow coastal basins.
4. Wave Setup and Runup
Breaking waves increase water levels near shore and can send water above still-water elevations onto dunes, roads and coastal structures.
5. River Backwater Effects
Elevated coastal water can slow or prevent rivers from draining normally into the sea.
6. Heavy Rainfall
Rain falling over a coastal city can become much more damaging if storm drains, canals or rivers are unable to discharge because ocean levels are already elevated.
7. Rising Relative Sea Level
A higher baseline means a smaller tide, surge or wave event is required to reach the same land elevation.
8. Land Subsidence
Sinking land effectively raises local sea level relative to streets, buildings and coastal defenses.
9. Compound Flooding
Several flood drivers may arrive together and produce inundation greater than one driver acting alone.
Storm Surge, Storm Tide and Coastal Flooding
These terms are related but should not be treated as synonyms.
Storm Surge
Storm surge is the abnormal increase in coastal water level associated with a storm, above the water level that would otherwise be expected from the astronomical tide alone.
Storm Tide
Storm tide refers to the actual storm-elevated water level resulting from the combination of astronomical tide and storm-related water-level effects.
Coastal Flooding
Coastal flooding is what happens when those elevated water levels inundate normally dry land.
| Term | Meaning |
|---|---|
| Storm surge | Storm-driven abnormal water-level component. |
| Storm tide | Storm-elevated coastal water level including the astronomical tide. |
| Coastal flooding | Inundation of normally dry coastal land. |
High Tides, King Tides and Tidal Flooding
A destructive hurricane is not required for a coast to flood.
Low-lying roads, harbors, wetlands and waterfront neighborhoods can flood during unusually high tidal conditions.
High-Tide Flooding
High-tide flooding occurs when coastal water levels exceed locally important thresholds during high tide.
It is also sometimes described as:
- tidal flooding;
- nuisance flooding;
- sunny-day flooding.
King Tides
King tide is an informal term commonly used for especially high astronomical tides.
They occur predictably because of astronomical geometry, but their actual coastal impact depends on local weather, ocean conditions and the elevation of the land.
Why a Clear Day Can Still Flood
Coastal flooding is fundamentally about water level, not rainfall.
If the tide and background sea level are sufficiently high, seawater can enter streets and storm drains beneath completely clear skies.
Wave Setup, Wave Runup and Coastal Flooding
Waves can increase flood levels beyond those produced by tide and surge alone.
Wave Setup
As waves break in shallow water, they transfer momentum toward shore.
This process can raise the average nearshore water surface, producing wave setup.
Wave Runup
Individual waves then rush up beaches, dunes, seawalls and other coastal slopes.
The vertical extent of this uprush is called wave runup.
Overtopping
Runup can carry water over seawalls, dunes, revetments and other barriers even if the underlying still-water level remains below their crest.
Overwash and Barrier-Island Flooding
Barrier islands and low dunes can be crossed by storm-driven water and waves.
Overwash occurs when water carries sediment landward across a beach, dune or barrier island.
During severe events, seawater may cross roads, bury infrastructure in sand and connect the ocean temporarily with back-barrier lagoons or marshes.
Overwash vs Coastal Erosion
Overwash moves water and sediment across the coastal barrier.
Coastal erosion describes the removal or redistribution of shoreline material.
Both processes can occur simultaneously during the same storm.
Why Estuaries, Deltas and River Mouths Flood So Easily
Estuaries and deltas occupy one of the most complicated flood environments on Earth because inland and ocean water meet there.
Floodwater can arrive from:
- the ocean;
- the river;
- local rainfall;
- storm drains;
- shallow groundwater;
- or several sources simultaneously.
River Meets Sea
High river discharge normally needs somewhere to go.
If sea level at the river mouth is already elevated by tide or surge, the downstream hydraulic gradient may weaken and drainage can slow.
Low Elevation
Many deltas are extremely flat, allowing relatively small changes in water level to inundate broad areas.
Subsidence
Some deltas also experience land subsidence, further increasing relative sea level and flood exposure.
How High Coastal Water Can Block Drainage
Some coastal flooding occurs without seawater visibly rushing far inland.
Instead, elevated water at the coast can prevent inland water from leaving.
Storm Drains
Urban drainage networks commonly discharge toward rivers, canals, bays or the ocean.
When the receiving water is high, gravity drainage becomes less efficient or may stop altogether.
Backflow
In some systems, seawater can even move backward through drainage infrastructure unless protected by valves or tide gates.
Rivers and Canals
Rivers and canals also drain more slowly when downstream coastal water levels rise.
Compound Coastal Flooding: When Several Flood Drivers Collide
Compound flooding describes flood events involving interacting or coincident drivers.
Along the coast, these combinations can be particularly dangerous because ocean, atmosphere, rivers, drainage networks and groundwater are hydraulically connected.
| Combination | What Happens |
|---|---|
| Storm surge + high tide | Storm-driven water arrives on top of an already elevated astronomical water level. |
| Storm surge + waves | Elevated water allows damaging waves and runup to reach farther inland. |
| Rain + high tide | Urban drainage becomes less efficient while rainfall adds water inland. |
| Rain + storm surge | Coastal water blocks drainage while intense precipitation floods streets and low areas. |
| River flood + high tide | River discharge encounters elevated downstream water and may back up. |
| River flood + storm surge | Freshwater pushes seaward while seawater pushes landward. |
| Coastal water + shallow groundwater | A high water table can reduce underground storage and contribute to emergence flooding. |
Why Compound Floods Can Be Worse
Flood-control infrastructure is often designed around individual hazards.
A storm-drain system capable of handling heavy rainfall may perform very differently if its outlet is submerged.
Likewise, a river that could normally carry runoff to sea may flood if downstream coastal water removes the gradient needed for efficient drainage.
Can Groundwater Contribute to Coastal Flooding?
Yes.
In some low-lying coastal areas, the groundwater table lies close to the surface.
Elevated coastal water, sustained rainfall and high groundwater can interact.
Groundwater Emergence
If the water table rises to the land surface, water can emerge from below rather than arriving only across the ground.
Reduced Storage
A shallow groundwater table also leaves less underground space available to absorb rainfall.
Infrastructure Impacts
Basements, buried utilities, roads and drainage systems can be affected even where direct ocean inundation is limited.
How Relative Sea-Level Rise Changes Coastal Flooding
Sea-level rise does not need to independently create a flood to increase flood risk.
Its most important effect is often to raise the starting water level.
Imagine a road that begins flooding when water reaches a particular elevation.
If average local sea level rises, tides and storms have less vertical distance to travel before reaching that road.
Same Storm, Higher Starting Point
A surge that once remained below damaging thresholds may begin crossing them as the baseline rises.
High-Tide Flooding Becomes More Frequent
Events that once required an unusually strong combination of tide and weather can become more common because the entire coastal water-level distribution has shifted upward.
Land Subsidence and Coastal Flood Risk
Coastal flood risk depends on the level of the sea relative to the land.
If the land itself sinks, the effect can resemble an additional rise in local sea level.
Why Coastal Land Can Subside
Causes vary by region and can include:
- natural sediment compaction;
- groundwater withdrawal;
- oil and gas extraction;
- drainage of organic soils;
- tectonic movement;
- loading and compaction of young delta sediments.
Deltas Are Especially Vulnerable
Many major cities occupy deltas built from unconsolidated sediment.
Where subsidence combines with rising ocean levels, relative sea-level rise can be particularly rapid.
Why Saltwater Flooding Can Cause Long-Lasting Damage
Coastal floodwater differs chemically from most freshwater river and rainfall flooding.
Salt can continue causing damage after visible water drains away.
Soils
Salt can alter soil chemistry and harm salt-sensitive crops and vegetation.
Freshwater Ecosystems
Saline water can stress freshwater plants, wetlands and aquatic habitats.
Drinking-Water Supplies
Saltwater flooding can contaminate shallow wells, surface-water systems and other freshwater sources.
Infrastructure
Saltwater accelerates corrosion of metals, vehicles, electrical equipment and reinforced infrastructure.
Aquifers
Coastal flooding can also contribute to temporary or persistent salinization of shallow groundwater systems in vulnerable locations.
Coastal Flooding vs Storm Surge: What Is the Difference?
This is one of the most important distinctions in the entire coastal-flood topic.
| Feature | Coastal Flooding | Storm Surge |
|---|---|---|
| What is it? | Flooding of normally dry coastal land. | Storm-related abnormal rise in coastal water level. |
| Category | Flood outcome | Flood driver |
| Requires a storm? | No | Yes |
| Can tides cause it? | Yes | No — although tide combines with surge to determine total storm water level |
| Deep guide | Coastal Flooding Explained | Storm Surge Explained |
Coastal Flooding vs River Flooding
Both can inundate huge areas, but their dominant water sources differ.
| Feature | Coastal Flooding | River Flooding |
|---|---|---|
| Main source | Ocean, sea or estuary | Drainage-basin runoff |
| Main drivers | Surge, tide, waves, coastal water levels | Rainfall, snowmelt, tributary inflow |
| Typical landscape | Coasts, bays, deltas, estuaries, islands | River valleys and floodplains |
| Can interact? | Yes — coastal water can block river outflow | Yes — high river discharge can worsen estuary flooding |
| Deep guide | Coastal Flooding Explained | River Flooding Explained |
Coastal Flooding vs Urban Flooding
Coastal cities frequently experience both.
Coastal Flooding
Ocean or estuarine water crosses onto normally dry land or blocks drainage from reaching the sea.
Urban Flooding
Rain overwhelms streets, drains, sewer systems, basements and other components of the built environment.
Compound Coastal-Urban Flooding
The worst situation occurs when intense rainfall fills the city while elevated sea levels prevent that water from escaping.
Coastal Flooding vs Tsunami
A tsunami can produce devastating coastal inundation, but tsunami flooding should not be classified as ordinary meteorological or tidal coastal flooding.
Tsunami
A tsunami is a series of long waves generated by rapid displacement of a large volume of water, commonly following an undersea earthquake and sometimes triggered by landslides or volcanic processes.
Ordinary Coastal Flooding
Ordinary coastal flooding is associated primarily with tides, storm surge, winds, waves, sea-level anomalies and compound hydrological conditions.
Coastal Flooding vs Coastal Erosion
A major storm can cause flooding and erosion simultaneously, but the processes are different.
Coastal Flooding
Water temporarily covers normally dry land.
Coastal Erosion
Sediment, beach, dune or cliff material is removed or redistributed by waves, currents and storms.
A road beneath seawater is primarily a flood problem.
A road collapsing because the shoreline beneath it disappeared is primarily an erosion problem.
What Makes Coastal Flooding Worse?
Coastal-flood severity depends on much more than storm intensity.
| Factor | Why It Matters |
|---|---|
| Coastal elevation | Low-lying land requires only a small water-level increase before inundation begins. |
| Astronomical tide | A storm arriving near high tide begins from a higher baseline. |
| Storm surge | Adds abnormal storm-driven water elevation. |
| Wave climate | Large waves increase setup, runup and overtopping. |
| Coastal shape | Bays, estuaries and funnel-shaped coastlines can concentrate water. |
| Water depth offshore | Bathymetry affects waves, surge propagation and nearshore water levels. |
| River discharge | Large freshwater inflow can combine with elevated coastal water near river mouths. |
| Rainfall | Adds inland water while coastal water may block its outlet. |
| Relative sea level | A higher baseline lowers the additional water level needed to reach flood thresholds. |
| Land subsidence | Lowering the land increases effective relative sea level. |
| Dunes and wetlands | Natural coastal features can influence water movement, waves and storage. |
| Drainage capacity | Pumps, drains and canals determine how quickly inland water can escape. |
How Is Coastal Flooding Forecast?
Coastal-flood forecasting combines meteorological, oceanographic and hydrological information.
Tide Predictions
Astronomical tides can be predicted far in advance.
Storm Forecasts
Atmospheric models estimate storm position, intensity, wind fields and pressure.
Storm-Surge Models
Hydrodynamic models estimate how storm winds and pressure may alter coastal water levels.
Wave Models
Wave forecasts help estimate offshore wave height, direction and periods that influence runup and overtopping.
River Forecasts
Estuaries require information about upstream river discharge as well as ocean conditions.
Rainfall Forecasts
Coastal cities need rainfall forecasts because pluvial flooding may compound elevated coastal water.
Tide Gauges
Real-time water-level observations show whether forecast thresholds are actually being approached or exceeded.
Topographic Models
High-resolution elevation data help estimate where water may spread once it crosses the shoreline.
Coastal Flood Maps and Inundation Models
Forecasting a coastal water level is only the first step.
Emergency planners also need to know:
- where the water may go;
- how deep it may become;
- which roads may be cut;
- which neighborhoods may become isolated;
- which critical facilities may be affected.
Digital Elevation Models
Detailed terrain elevation is fundamental because small vertical differences can strongly control flood extent on flat coastal land.
Hydrodynamic Modeling
More sophisticated models simulate how water actually moves across channels, wetlands, streets and floodplains rather than simply drawing a horizontal water line.
Uncertainty
Actual flooding can differ because of waves, erosion, barrier failure, drainage blockages, rainfall uncertainty, debris and changes in coastal terrain.
Where Is Coastal Flooding Most Dangerous?
Coastal flooding can occur almost anywhere land meets the sea, but some landscapes are particularly vulnerable.
| Coastal Setting | Typical Flood Pattern |
|---|---|
| Low-lying deltas | River discharge, tide, surge, rainfall and subsidence can combine. |
| Estuaries | Freshwater and seawater meet, allowing backwater and compound flooding. |
| Barrier islands | Storm surge, waves, overwash and inlet formation can inundate narrow low islands. |
| Coastal cities | Tidal flooding, drainage backup, rainfall and infrastructure exposure overlap. |
| Shallow bays | Wind can pile water into confined or semi-confined coastal basins. |
| Tropical-cyclone coasts | Storm surge, large waves and extreme rainfall may arrive together. |
| Subsiding coasts | Relative sea level rises faster because the land itself is lowering. |
| Small low islands | Limited elevation allows surge, high tides and waves to reach large portions of the land area. |
Historic Coastal Floods and Major Case Studies
Major disasters demonstrate that coastal flooding can result from tropical cyclones, extratropical storms and combinations of tide, surge and waves.
| Event | Region | Why It Matters |
|---|---|---|
| 1953 North Sea Flood | Netherlands, United Kingdom and North Sea coast | A landmark European storm-tide disaster that transformed coastal-flood protection. |
| 1970 Bhola Cyclone | Bay of Bengal | Devastating cyclone-driven coastal flooding across the low-lying Bengal delta. |
| 2005 Hurricane Katrina | U.S. Gulf Coast | Extreme surge, waves and infrastructure failure produced catastrophic coastal inundation. |
| 2008 Cyclone Nargis | Myanmar | Severe storm-surge flooding across the low-lying Irrawaddy Delta. |
| 2012 Hurricane Sandy | U.S. Northeast | Storm surge and tide produced major urban coastal flooding around New York Harbor and surrounding areas. |
| 2013 Typhoon Haiyan | Philippines | Extreme storm-driven coastal inundation caused devastating impacts around Tacloban and Leyte Gulf. |
Why Coastal Floods Become Major Disasters
Water Depth
Deeper inundation increases structural damage and makes evacuation more difficult.
Wave Action
Waves riding on elevated water can exert far greater forces than relatively still floodwater.
Saltwater
Saltwater damages soils, electronics, vehicles, utilities and freshwater resources.
Erosion
Flooding may coincide with beach, dune and foundation erosion.
Drainage Failure
Floodwater can enter sewer and stormwater systems while preventing those systems from discharging normally.
Transportation Isolation
A relatively shallow flooded road can isolate barrier islands and waterfront communities.
Critical Infrastructure
Ports, substations, wastewater plants, hospitals, tunnels and transport networks are often concentrated near coasts.
Long Recovery
Salt contamination, mold, erosion and damaged electrical infrastructure can keep buildings unusable long after water levels fall.
How Can Coastal Flood Risk Be Reduced?
No single coastal defense works everywhere.
Risk reduction usually combines engineered infrastructure, land-use planning, natural systems and forecasting.
Seawalls
Vertical or sloping structures can reduce direct inundation and wave attack in selected locations.
Levees and Dikes
Embankments can protect low-lying areas from selected coastal water levels.
Storm-Surge Barriers
Movable barriers can close estuaries, rivers or harbor entrances during extreme water-level events.
Tide Gates
Tide gates help prevent seawater from moving backward through drainage channels.
Pumps
Low-lying cities may require mechanical pumping when gravity drainage is impossible.
Raised Buildings
Elevating vulnerable structures above expected flood levels reduces exposure.
Managed Retreat
In some repeatedly flooded locations, moving infrastructure away from the hazard may provide a more durable solution than continually increasing defenses.
Wetlands, Dunes, Reefs and Natural Coastal Buffers
Natural coastal features can influence waves, water storage and flood propagation.
Salt Marshes and Wetlands
Wetlands provide friction, water-storage space and room for tides and floodwater to spread.
Dunes
Dunes form elevated barriers between the beach and inland areas.
Severe storms can erode, overtop or breach them, but intact dune systems can still provide important protection during smaller events.
Mangroves
Dense coastal vegetation can reduce wave energy and influence nearshore flow in suitable tropical environments.
Reefs
Coral and rocky reef systems can cause waves to break offshore and reduce some of the energy reaching shore.
Room for Water
Preserving undeveloped floodable coastal land gives extreme water somewhere to spread without immediately encountering dense infrastructure.
Coastal-Flood Safety
- Follow evacuation orders issued by local authorities.
- Do not drive through flooded coastal roads.
- Do not assume seawater flooding is shallow because the road surface is normally familiar.
- Stay away from seawalls, jetties and exposed beaches during high-wave events.
- Do not enter water flowing across barrier islands, causeways or coastal roads.
- Remember that waves can suddenly reach far beyond the average waterline.
- Monitor official tide, surge, weather and flood forecasts.
- Expect storm drains to back up when coastal water is elevated.
- Keep away from damaged electrical equipment and submerged infrastructure.
- Treat floodwater as potentially contaminated.
- Be alert for erosion and unstable ground after water retreats.
Coastal-Flood Glossary
- Coastal flooding
- Inundation of normally dry coastal land by seawater or water strongly influenced by coastal water levels.
- Coastal inundation
- The covering of coastal land by water.
- Storm surge
- Abnormal storm-related increase in coastal water level above the level expected from astronomical tide alone.
- Storm tide
- The storm-elevated coastal water level resulting from astronomical tide combined with storm-related water-level effects.
- High-tide flooding
- Coastal inundation occurring when high tidal water levels cross locally important flood thresholds.
- King tide
- An informal term for an especially high astronomical tide.
- Nuisance flooding
- A commonly used term for recurrent, relatively shallow coastal flooding that disrupts roads, drainage or property.
- Sunny-day flooding
- Informal term for high-tide coastal flooding that can occur without a storm.
- Wave setup
- Increase in mean nearshore water level produced by breaking waves.
- Wave runup
- The maximum uprush of waves onto a beach, dune or coastal structure.
- Overtopping
- Water passing over the crest of a seawall, dune, levee or other coastal barrier.
- Overwash
- Movement of water and sediment across a beach, dune or barrier island during elevated water and wave conditions.
- Estuary
- A coastal water body where river water and seawater interact.
- Delta
- A low-lying sedimentary landform at a river mouth where river channels meet the sea.
- Backwater effect
- Water-level rise caused when elevated downstream water slows or prevents upstream drainage.
- Compound flooding
- Flooding associated with interacting or coincident drivers such as surge, rainfall, river flow and groundwater.
- Relative sea level
- Sea level measured relative to the local land surface, incorporating both ocean-level change and vertical land movement.
- Subsidence
- Downward movement of the land surface.
- Saltwater intrusion
- Movement of saline water into freshwater systems, soils or aquifers.
- Coastal erosion
- Removal or redistribution of shoreline sediment, beaches, dunes or cliffs by waves, currents and storms.
Coastal Flooding FAQ
What is coastal flooding?
Coastal flooding occurs when seawater or water strongly influenced by coastal water levels inundates normally dry land along coasts, estuaries, deltas, islands or waterfront communities.
What causes coastal flooding?
Coastal flooding can result from storm surge, high tides, persistent onshore winds, storm tide, wave setup and runup, river backwater effects, heavy rainfall, rising relative sea level or several drivers occurring together.
Is coastal flooding the same as storm surge?
No. Storm surge is an abnormal storm-driven increase in coastal water level. Coastal flooding is the resulting inundation when sufficiently high water reaches normally dry land.
What is storm tide?
Storm tide is the elevated coastal water level produced by the combination of astronomical tide and storm-related water-level effects.
Can coastal flooding happen without a storm?
Yes. High astronomical tides can produce tidal or high-tide flooding in low-lying coastal areas even when no major storm is present.
What is king-tide flooding?
King-tide flooding occurs when especially high astronomical tides reach land or infrastructure that normally remains dry. Wind, ocean conditions and elevated relative sea level can increase its severity.
What is sunny-day flooding?
Sunny-day flooding is an informal term for coastal high-tide flooding that can occur without heavy rain or a major storm.
What is wave setup?
Wave setup is a rise in average nearshore water level caused as breaking waves transfer momentum toward the coast.
What is wave runup?
Wave runup is the uprush of individual waves above the average water level onto beaches, dunes, seawalls and other coastal surfaces.
What is compound coastal flooding?
Compound coastal flooding occurs when multiple flood drivers interact, such as storm surge and rainfall, high tide and river flooding, or coastal water combined with rainfall and shallow groundwater.
Why do coastal cities flood during heavy rain and high tide?
High coastal water can reduce the ability of storm drains, rivers and canals to discharge into the sea. Heavy rainfall then adds water inland while the normal drainage outlet is restricted.
How is coastal flooding different from river flooding?
Coastal flooding is driven primarily by ocean, sea or estuarine water levels. River flooding is driven primarily by runoff moving through a drainage basin and exceeding river-channel capacity.
Is coastal flooding the same as a tsunami?
No. Tsunamis are long waves generated by sudden displacement of water, most commonly from undersea earthquakes. Ordinary coastal flooding is generally associated with tides, storm surge, waves and hydrological interactions.
Is coastal erosion the same as coastal flooding?
No. Coastal flooding is inundation of land by water. Coastal erosion is removal or redistribution of beaches, dunes, cliffs and shoreline sediment.
How does sea-level rise increase coastal flooding?
Rising relative sea level raises the baseline from which tides, waves and storm surge operate, meaning less additional water-level rise is required to cross local flood thresholds.
Can land subsidence increase coastal flooding?
Yes. When coastal land sinks, local sea level rises relative to roads, buildings and flood defenses, increasing inundation risk.
Why is saltwater flooding especially damaging?
Saltwater can corrode infrastructure, contaminate soils and freshwater supplies, damage vegetation and leave ecological and chemical impacts after floodwater drains away.
How is coastal flooding forecast?
Forecasting combines tide predictions, storm and wind forecasts, storm-surge and wave models, tide-gauge observations, rainfall and river forecasts, and high-resolution terrain data used to estimate inundation.
Coastal Flood Science & Reference Sources
Coastal-flood terminology and mechanisms used in this guide follow established oceanographic, hydrological and hazard science from agencies including NOAA, the U.S. Geological Survey and FEMA.
Coastal Flooding Is a Total-Water Problem
The most damaging coastal floods rarely come from one number alone. Tide, storm surge, waves, rainfall, rivers, drainage systems, groundwater and local elevation can all determine whether seawater stays offshore or spreads through streets, wetlands, ports and neighborhoods.
Start with the
Floods Explained master guide
for the complete flood hierarchy, or continue into
Storm Surge Explained
for the atmospheric and oceanographic mechanism behind some of the world’s most destructive coastal floods.
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