Storm Station 247

The Field Book/Vol. VI, The Events/FB-EVT-105

Coastal flood

A coastal flood is the sea standing higher than the land it borders. The tide sets the level, a storm adds to it by pressing the water in with its wind and lifting it with its low pressure, and waves ride on top. The flood comes at high tide.

Plate FB-EVT-105Water, flood, coast and marineRevision 1, 2026-09-24Status draftAlso called tidal flooding, high tide flooding, king tide, storm tide, nuisance flooding

Section across a beach, a dune and the street behind it during a coastal storm at high tide, heights to scale: from the chart datum, mean lower low water, up through mean sea level and mean higher high water; then the storm's parts stacked on the tide: the rise under its low pressure, the wind setup, and the wave setup; the still water level they make together standing above the street; and the waves running up the dune face.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-EVT-105-ASECTION ACROSS A BEACH AND THE STREET BEHIND IT, DURING A STORM AT HIGH TIDEFIG. A THE WATER THAT FLOODS, STACKED ON THE TIDEMEAN LOWER LOW WATER, 0 m, THE CHART DATUMMEAN SEA LEVELMEAN HIGHER HIGH WATERTIDE 1.70 mLOW PRESSURE 0.25 mWIND SETUP 0.55 mWAVE SETUP 0.30 mSTREET 2.1 m0.7 m (2.3 ft) OF WATERSTILL WATER 2.8 m, 1.3 m ABOVE MHHWWIND, ONSHORE0123404812mftSEADUNEKEYSea waterGround, cut1234567891011TITLECoastal flood, the water that floodsVOL. VI THE EVENTS · WATER, FLOOD, COAST AND MARINETYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-EVT-105-ADRAWN AS linework on paperSOURCES NWS, National Hurricane Center, NOAA National Ocean Service
Fig. A The water that floods. Section across a beach and the street behind it, during a storm at high tide HEIGHT TO SCALE, WIDTH NTS The sheet, SVG, 11 by 17
  1. Mean lower low water, the chart datum
  2. Mean sea level
  3. Mean higher high water
  4. The astronomical tide, a spring high tide
  5. The inverted barometer rise
  6. The wind setup
  7. The wave setup
  8. The still water level, the storm tide plus wave setup
  9. Wave runup on the dune face
  10. The street, below the water
  11. The wind, onshore

What it is

A coastal flood is sea water standing on land that is normally dry. The Weather Service warns of it along the ocean coasts and the bays and sounds joined to them; along the Great Lakes the same event is a lakeshore flood.

It is a sum. The tide sets the base: twice a day, near most of the coast, the sea rises and falls. A storm adds to it. Its wind pushes water toward the shore and piles it against the land. Its low pressure lets the sea surface rise beneath it. Its waves break and set the water up higher still at the shore, and run up the beach above that. When the sum rises above the land, the sea comes in.

Because the tide is part of the sum, a coastal flood keeps time with it. The water comes in at high tide, goes out at low tide, and comes back at the next high tide if the storm is still there. A storm that lasts three days can flood the same street five or six times.

What it is not

It is not the storm surge. The surge is the part the storm adds: the water level observed, less the tide predicted for that minute. The flood is the storm tide, the surge on top of the tide. A surge of 1 m (3.3 ft) at low tide may flood nothing; the same surge at a spring high tide may flood a city. Storm tide, storm surge and wave runup FB-EVT-112 draws the three terms apart.

It is not rain. Rain can flood a coastal city at the same time, and it makes the coastal flood worse when the drains that should carry it out are held shut by the sea, but it is a different flood with a different warning.

It is not a tsunami, which is made by the sea floor moving, not by the weather.

It is not always a storm. On a coast that sits close to the high tide line, a spring tide alone, or a perigean spring tide (the king tide, when the moon is also at its closest), can flood low streets on a sunny day. As sea level rises, those days are becoming more frequent.

Lookalikes

Storm surge
The part of the rise the storm adds. The surge is not the flood: the flood is the storm tide, the surge on top of the tide. That is Storm tide, storm surge and wave runup FB-EVT-112.
Tsunami
Waves made by the sea floor moving, not by the weather. It has its own warnings. That is Tsunami FB-EVT-117.
Rip current
A current flowing seaward through the surf. It drowns swimmers, and floods nothing. That is Rip current FB-EVT-108.

The machine

Fig. A stacks the terms on a section across a beach.

  1. The datum. Every height at the coast is measured from a tidal datum. Mean lower low water is the chart datum, the level ships' soundings count from. Mean higher high water, the average of each day's higher high tide, is the line flood thresholds count from.
  2. The astronomical tide. The moon and the sun raise the tide, and the ocean basins shape it. Near new and full moon, when the two pull in line, the range is largest: the spring tides. At the quarters it is smallest: the neaps. The tide can be predicted years ahead for any gauge, to the minute. The machine behind it is Moon and tides FB-CLK-003.
  3. The inverted barometer. Where the air pressure is low, the sea surface rises: about 1 cm for each hPa the pressure falls. A deep coastal low 30 hPa below its surroundings lifts the sea about 30 cm (1 ft). In a hurricane it is more.
  4. The wind setup. A steady wind blowing toward the shore drags on the sea surface and pushes water shoreward faster than it can return beneath. The water piles up until its slope balances the wind's push. The equation for that slope is below: the setup grows with the square of the wind speed and inversely with the depth, so a long, shallow shelf, like much of the Gulf and the mid Atlantic, sets up far more water than a deep one. In a nor'easter it is usually the largest term.
  5. The wave setup and the runup. Waves breaking in the surf zone push the mean water level up at the shore, by tenths of a metre in large surf. Each wave then runs up the beach above that. Runup erodes dunes and overtops them; once a dune is breached the still water flows through.

The flood itself is the still water level, the tide and the three storm terms together, standing above the land. Fig. B draws how that level moves through three days at one gauge: the tide rising and falling twice a day, the surge building and falling as the storm passes, and the flooding only at the high tides near the surge's peak.

Why the threshold is counted from the tide

A water level of 2 m means nothing by itself; the same level floods one harbour and is ordinary in the next. So every threshold is set against the local tide, above mean higher high water. NOAA's common thresholds put minor flooding about 0.5 m (1.6 ft) above it, moderate about 0.8 m (2.6 ft) and major about 1.2 m (3.9 ft), each a little higher where the tide range is large. The Weather Service offices set their own thresholds for many gauges, from what floods at each.

Time strip of three days at a tide gauge as a coastal storm passes: the predicted tide rising and falling twice a day; the surge building to about one metre and falling away; the observed water level, their sum, crossing the minor flood threshold only at high tides near the surge's peak and the moderate threshold at one of them, while the low tides between stay below flood.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-EVT-105-BWATER LEVEL AT ONE GAUGE OVER THREE DAYS AS A STORM PASSES, ABOVE THE CHART DATUMFIG. B THREE DAYS AT A TIDE GAUGE0 h12 h24 h36 h48 h60 h72 h0 m1 m2 m3 m4 mABOVE MLLWMHHW 1.5 mMINOR FLOOD, MHHW + 0.5 mMODERATE, MHHW + 0.8 mSURGEKEYFlooding, above minor123456TITLECoastal flood, three days at a tide gaugeVOL. VI THE EVENTS · WATER, FLOOD, COAST AND MARINETYPETIME STRIPSCALETIME TO SCALE, HEIGHT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-EVT-105-BDRAWN AS linework on paperSOURCES NWS, National Hurricane Center, NOAA National Ocean Service
Fig. B Three days at a tide gauge. Water level at one gauge over three days as a storm passes, above the chart datum TIME TO SCALE, HEIGHT TO SCALE The sheet, SVG, 11 by 17
  1. The predicted tide
  2. The surge, observed less predicted
  3. The observed water level
  4. Minor flood threshold
  5. Moderate flood threshold
  6. Flooding, at high tide

Ingredients

  • The tide: a spring tide near new or full moon, or a perigean spring tide, starts higher
  • Wind blowing toward the shore for hours over a long fetch and a shallow shelf, which piles the water up
  • Low pressure, which lets the sea surface rise under the storm
  • Waves, which set the water up further at the shore and run up the beach
  • Low land: a coast already near the high tide line, and sea level that is rising

Scales

time
hours, at each high tide, over one to several days
horizontal
tens to hundreds of kilometres of coast (tens to hundreds of miles)
vertical
a few tenths of a metre above the highest tides for minor flooding, to several metres in a hurricane's surge
orlanski
meso-alpha to synoptic

Equations

Wind setup over a shelf

∂η∂x=τsρw g h,τs=ρaCDU2\frac{\partial \eta}{\partial x} = \frac{\tau_s}{\rho_w\, g\, h}, \qquad \tau_s = \rho_a C_D U^2
η\eta
the rise of the sea surface above its level without wind, m
xx
distance toward the shore, m
τs\tau_s
the wind's stress on the sea surface, N m⁻²
ρw,ρa\rho_w, \rho_a
the density of sea water (about 1,025 kg m⁻³) and of air (about 1.2 kg m⁻³)
gg
gravity, 9.81 m s⁻²
hh
the depth of the water, m
CDC_D
the drag coefficient of the sea surface, about 0.001 to 0.003
UU
the wind speed at 10 m, m s⁻¹

Assumes A steady wind blowing straight onshore over a shelf of depth h, in one dimension, with the friction of the sea bed neglected. The setup is largest where the water is shallowest, which is why a wide, shallow shelf floods worst.

Working form The pressure part, the inverted barometer, is simpler: the sea rises about 1 cm for each hPa the pressure falls below its surroundings (about 0.4 in for each 0.03 inHg).

Signatures

gauge
the observed water level running above the predicted tide by the surge; and the gap growing through a day or more of onshore wind
surface
a long fetch of strong onshore wind at coastal stations; and falling pressure
satellite
a large cyclone offshore; its wind field turned toward the coast
radar
not the radar's to see; it sees the rain that may also be falling

The numbers

QuantityValue, and the kind of number it is
Storm tideThe water level during a storm: the storm surge added to the astronomical tideStandard, Storm Surge Overview
Inverted barometerThe sea rises about 1 cm for each 1 hPa fall in pressureTextbook, Pugh 2014
Semidiurnal tideTwo highs and two lows a lunar day, 24 h 50 min; one cycle every 12 h 25 minTextbook, Pugh 2014
Spring and neapSpring tides, the largest range, come a day or two after new and full moon; neap tides after the quartersTextbook, Pugh 2014
Minor flood thresholdAbout 0.5 m (1.6 ft) above mean higher high water, a little more where the tide range is largeStandard, Patterns and Projections of High Tide Flooding Along the U.S. Coastline Using a Common Impact Threshold
Moderate flood thresholdAbout 0.8 m (2.6 ft) above mean higher high waterStandard, Patterns and Projections of High Tide Flooding Along the U.S. Coastline Using a Common Impact Threshold
Major flood thresholdAbout 1.2 m (3.9 ft) above mean higher high waterStandard, Patterns and Projections of High Tide Flooding Along the U.S. Coastline Using a Common Impact Threshold
Tide gauge intervalA water level every 6 minutesStandard, Tidal Datums

How the station sees it

The tide gauge is the instrument for this hazard. Each measures the water level every 6 minutes, and each has its own predicted tide. The difference between the two, observed less predicted, is the surge, and a forecaster watching a storm reads the flood from it: the surge now, plus the tide still to come at the next high water.

The weather stations on the coast in the instrument atlas see the storm's part: the strength and direction of the wind, and how long it has blown onshore, and the fall in pressure. A day of wind from the northeast at a mid Atlantic airport is how a nor'easter's coastal flood begins.

How it is warned

A Coastal Flood Watch is issued a day or more ahead when flooding is possible. A Coastal Flood Warning is issued when moderate or major flooding is expected, and a Coastal Flood Advisory for minor flooding, the kind that closes low roads and floods low lots. A Coastal Flood Statement covers lesser or follow up information. Each gives the times of the high tides when the water will be highest, and often the expected water level at a named gauge.

For a tropical storm or hurricane the surge is its own product: the Storm Surge Watch and Storm Surge Warning, issued for the danger of life threatening inundation, with the expected depth of water above the ground.

Salt water on a road hides the road as fresh water does, and it ruins what it reaches. Move vehicles to higher ground before the high tide, not during it.

VTECPhenomenonThe alerts that carry it
CFcoastal floodCoastal Flood Advisory; Coastal Flood Statement; Coastal Flood Warning; Coastal Flood Watch

Weather radio (SAME) codes: CFA CFW

Every alert in force now

See also

  • Moon and tides FB-CLK-003
  • Storm tide, storm surge and wave runup FB-EVT-112
  • Storm surge FB-EVT-047
  • Nor'easter FB-EVT-066
  • Lakeshore flood FB-EVT-106
  • Tide gauge FB-INS-008
  • Tide as a water machine FB-WAT-042
  • How to read a warning FB-STN-001

Sources

  1. National Weather Service. Directive 10-320, Surf Zone Forecast and Coastal/Lakeshore Hazard Services.
  2. National Hurricane Center. Storm Surge Overview.
  3. NOAA National Ocean Service. Tidal Datums.
  4. Sweet, W. V. and others, NOAA National Ocean Service. Patterns and Projections of High Tide Flooding Along the U.S. Coastline Using a Common Impact Threshold, NOAA Technical Report NOS CO-OPS 086 (2018).
  5. Pugh, D. and P. Woodworth. Sea-Level Science (2014).
  6. American Meteorological Society. Glossary of Meteorology.

Definition after the Directive 10-320, Surf Zone Forecast and Coastal/Lakeshore Hazard Services. Plate FB-EVT-105, revision 1, 2026-09-24. The number is permanent; cite it.