How coastal flooding is measured

A tide gauge turns sea-level rise into calendar days on a street.

The question this tool answers

Coastal flooding is not only a hurricane story. As mean sea level rises, ordinary high tides need less help from wind and pressure to reach roads, seawalls, storm drains, and basements. This dashboard asks a local question first: at the NOAA tide gauge nearest you, how often is high tide crossing the minor-flood line, and what do the next seven days of predicted tides look like?

The Battery record

The Battery tide gauge in lower Manhattan has measured New York Harbor since 1856, making it one of the longest continuous sea-level records in the United States. The instrument has changed over time, but the point of the record is continuity: the same harbour, the same tidal reference, year after year. That is why a New York flood-day chart is more than a local curiosity. It is a century-and-a-half story of a rising baseline.

How we know

NOAA CO-OPS, the Center for Operational Oceanographic Products and Services, operates the tide-gauge network behind Tides & Currents. The gauges record water level at frequent intervals. NOAA also publishes tide predictions, flood thresholds, and annual high-tide-flooding counts. This dashboard compiles thirteen US stations into a daily snapshot.

The long-term charts use NOAA's High Tide Flooding product: annual counts of days when water reached the local minor-flood threshold. “Minor” does not mean harmless. It often means water in low roads, parking lots, storm drains, waterfront paths, and basements. The key metric is cadence: ten flood days a year is not an emergency every day, but it changes maintenance, commuting, insurance, and expectations.

The choices we made

The hero leads with cadence because coordinates do not tell a resident what matters. “Seven flood days last year” is understandable; “station 8518750 at 40.7004, -74.0141” is not. The station picker is still in the hero so readers can override geolocation and compare ports.

The seven-day tide band is promoted because it is the actionable part of the page. Astronomical high tides are predictable. If the next high tides are approaching the minor-flood threshold, a city can check pumps, move equipment, warn residents, or simply know why sunny streets may flood.

Datum caveats

Tide data live in datums: reference surfaces such as NAVD88 and MLLW. Flood thresholds and tide predictions may be published against one datum while real-time levels are requested in another. This dashboard uses a deliberately coarse “near / above threshold” screen and explains the mismatch. It is situational awareness, not an official flood warning.

Storm surge is different

High-tide flooding is chronic. Storm surge is acute. A hurricane can pile water far above predicted tide, especially when wind, pressure, coastline shape, river flow, and timing align. This page does not run NHC storm-surge models, SLOSH, evacuation zones, or local flood warnings. During storms, use the National Hurricane Center and local emergency management.

Further reading

• NOAA Tides & Currents — tide gauges, water levels, and predictions. tidesandcurrents.noaa.gov
• NOAA High Tide Flooding Outlook — annual nuisance-flood counts and outlooks. NOAA HTF Outlook
• NOAA CO-OPS API — public water-level and prediction endpoints used by this dashboard. api.tidesandcurrents.noaa.gov
• IPCC AR6 Sea Level Projection Tool — mid-century local relative sea-level projections. NASA Sea Level Portal
• National Hurricane Center storm surge — official life-safety products during storms. nhc.noaa.gov/surge

Credits

This dashboard depends on NOAA tide-gauge operators, CO-OPS data systems, high-tide-flooding analysts, and IPCC sea-level projection teams. It summarizes public data for context. It does not replace official flood, surge, or evacuation guidance.