How we track high-tide flooding around the world

What the dashboard shows, where the numbers come from, how current they are, and what they can’t tell you.

The short answer

As sea level rises, ordinary high tides reach further. In many coastal towns, streets and parking lots now flood on sunny days at the highest tides, not only during storms. This dashboard counts, for tide gauges around the world, how often high tide reaches flood level now compared with 2005–2014. For US gauges it adds NOAA’s live readings, the high tides of the coming week, official flood counts and projections to 2050.

How we know

Tide gauges worldwide. The University of Hawaiʻi Sea Level Center gathers hourly sea level from national tide gauge networks and publishes it as “fast-delivery” data, about one to two months behind real time. We use every gauge that is still reporting and has enough data in 2005–2014 to set its flood level: about 130 gauges in more than 40 countries and territories, alongside NOAA’s 18 US gauges. Gauge coverage is uneven: the Pacific, Japan, Australia and the Americas are well covered; Europe, Africa and mainland Asia much less, because many national networks do not share hourly data through this archive.

Flood level. There is no worldwide official flood threshold, so we use NOAA’s method. NOAA sets the minor flood level of each US gauge at the average daily high tide plus 0.50 m plus 4% of the tide range (moderate and major levels are set the same way, higher up). We apply the same formula to every gauge, using its own tides in 2005–2014. Minor flooding is disruptive: low roads and parking lots flood. A flood day is a day the highest hourly reading reaches that level. The formula was set from US gauges, most with tide ranges under 3 m; where tides are much larger (Broome in Australia, Brest in France) ordinary spring tides pass it, so we show those gauges but leave them out of the comparisons.

Checked against NOAA. Eight US gauges are in both networks. Our thresholds match NOAA’s to the millimetre at seven of them; at Honolulu NOAA uses a lower, locally set threshold. Our flood counts are lower than NOAA’s for the same gauge, because NOAA measures the average high tide over 1983–2001, when the sea was lower, while the hourly records we use mostly start around 2000. The change between the two decades agrees with NOAA’s, which is why the map and ranking compare changes.

United States. For 18 US gauges we use NOAA’s own data: live readings every six minutes, tide predictions, official thresholds and flood counts, NOAA’s outlook for the current year, and the 2022 interagency sea level rise scenarios. Along the US coast, sea level is projected to rise 10–12 inches (0.25–0.30 m) on average between 2020 and 2050. The difference between the measured level and the predicted tide is the weather effect: onshore winds and low air pressure push water up (storm surge).

How current each part is

• Worldwide gauges: fast-delivery data arrives in monthly batches, one to two months behind; we check every night.
• US water level now: preliminary 6-minute data, read every night. A gauge that has not reported for six hours shows no reading.
• US high tides next week: NOAA tide predictions, refreshed every night.
• Flood days: complete calendar years with at least 80% of data.
• US outlook and projections: the outlook is issued each summer; the scenarios come from the 2022 report.

What the dashboard can’t tell you

It is not a flood warning. For a coming storm or king tide, follow your national weather service and local officials.

A gauge describes its own harbour. Streets a few kilometres away can flood at lower or higher levels, depending on their height, drainage and sea walls. Rain and river flooding are not included. Outside the US the flood level is our application of NOAA’s formula, not a threshold set by the local authority.

Further reading

• University of Hawaiʻi Sea Level Center: the worldwide tide gauge data used here. uhslc.soest.hawaii.edu
• NOAA, High Tide Flooding: outlooks, annual reports and the flood-day data used here. tidesandcurrents.noaa.gov/high-tide-flooding
• Sweet et al. (2022), Global and Regional Sea Level Rise Scenarios for the United States (NOAA Technical Report NOS 01; record at USGS). pubs.usgs.gov
• Sweet et al. (2018), Patterns and Projections of High Tide Flooding Along the U.S. Coastline Using a Common Impact Threshold (NOAA Technical Report NOS CO-OPS 086): how the flood thresholds are set. tidesandcurrents.noaa.gov
• NOAA, Sea Level Trends: measured trends at every US gauge. tidesandcurrents.noaa.gov/trends-and-extremes

Credits

Worldwide data comes from the University of Hawaiʻi Sea Level Center and the national agencies that run the gauges and share their records. US data comes from NOAA’s National Ocean Service and its Center for Operational Oceanographic Products and Services. We only download and present their public data.