The compound nature of coastal flooding
Coastal flooding is rarely caused by a single factor. It results from the combination of storm surge (wind-driven water pushed ashore), astronomical tides (the gravitational pull of the moon and sun), wave action (breaking waves that push water further inland), and the long-term baseline of sea level. When multiple factors align, the resulting water levels can far exceed what any single component would produce.
Emergency managers and coastal planners think in terms of total water level — the sum of all contributing factors at a specific location and time. A moderate storm surge arriving at high tide can produce the same water level as a major storm surge at low tide. Understanding this compound nature is essential for accurate flood risk assessment.
Storm surge mechanics
Storm surge is generated when strong onshore winds push ocean water toward the coast. The depth of surge depends on wind speed, storm size, approach angle relative to the coast, forward speed, and the shape of the continental shelf. Shallow, gently sloping shelves amplify surge because the displaced water has nowhere to go but up.
The highest storm surges in recorded history have exceeded 9 meters. Hurricane Katrina (2005) produced a peak surge of approximately 8.5 meters along the Mississippi coast. Super Typhoon Haiyan (2013) produced 5–7 meter surges in Tacloban, Philippines. These extreme water levels are survivable only for reinforced structures or elevated terrain.
King tides and sunny-day flooding
King tides are the highest predicted astronomical tides of the year, occurring when the Earth, moon, and sun align (syzygy) and the moon is at its closest approach to Earth (perigee). During king tides, low-lying coastal areas that normally stay dry may experience flooding even without any storm — a phenomenon called sunny-day flooding or nuisance flooding.
Sunny-day flooding has increased dramatically in US coastal cities over the past two decades. NOAA data shows that high-tide flooding days have more than doubled since 2000 in many Atlantic and Gulf coast tide gauge locations. This increase is directly attributable to sea level rise: the higher baseline means that normal high tides now reach levels that previously required storm assistance.
Sea level rise: the escalating baseline
Global mean sea level has risen approximately 21–24 cm since 1880, with the rate of rise accelerating in recent decades. Current estimates indicate a rise of about 3.5 mm per year, driven by thermal expansion of warming ocean water and melting of land-based ice sheets and glaciers.
What makes sea level rise particularly consequential for flooding is its multiplicative effect on extreme events. A 30 cm rise in baseline sea level means that storm surge events that previously occurred once in 100 years may begin occurring once in 10 years. The infrastructure, property, and ecosystems that were designed for historical flood frequencies face fundamentally changed risk profiles.
Monitoring networks: tide gauges and satellite altimetry
Tide gauges have measured coastal water levels for over a century at hundreds of stations worldwide. The longest continuous records — in cities like Amsterdam, Stockholm, and San Francisco — extend back to the 1800s. These records provide the historical baseline against which current sea level rise is measured and are essential for calibrating satellite observations.
Since 1992, satellite radar altimeters have measured global sea surface height with millimeter-level precision. Missions including TOPEX/Poseidon, Jason-1/2/3, and Sentinel-6 provide continuous, global coverage that tide gauges cannot match. The combination of long tide gauge records for historical context and satellite altimetry for global spatial coverage gives scientists the most complete picture of how sea levels are changing.
Coastal flood forecasting
Operational coastal flood forecasting combines storm surge models (driven by weather forecast data), tide predictions (based on astronomical calculations), wave models, and sea level observations. NOAA's Extratropical Surge and Tide Operational Forecast System provides 4-day water level forecasts for US coastal areas, accounting for both tropical and extratropical storm systems.
For tropical cyclones, the NHC issues storm surge watches and warnings with specific inundation heights mapped for individual coastal segments. These forecasts translate the abstract concept of surge depth into concrete, actionable information: 'water levels 4–6 feet above ground level expected in this zone.' This locality-specific approach helps residents understand their personal risk rather than relying on a single headline number.
Adaptation and the monitoring imperative
As sea levels continue to rise, the need for continuous, high-quality water level monitoring intensifies. Planning decisions about coastal infrastructure, building codes, evacuation zones, and insurance pricing all depend on accurate projections of future flood risk. Those projections, in turn, depend on the quality and continuity of the observational record.
PlanetSentry displays coastal and flood events that originate from authoritative sources including GDACS flood alerts and EONET storm events. Users monitoring coastal areas benefit from understanding that the events they see on the globe are occurring against a backdrop of rising baseline water levels, making each successive storm season incrementally more dangerous than the last.