What is high tide flooding?

Adding in Sea Level Rise

NOAA’s Office for Coastal Management offers an online tool to visualize how different local factors, such as strong tides and rainfall, combine to create coastal flooding. You can add sea level rise to the equation and see how much more frequent and severe flooding becomes.

As relative sea level rises, it no longer takes a strong storm or a hurricane to cause coastal flooding. High tide flooding occurs when sea level rise combines with local factors to push water levels above the normal high tide mark. Changes in prevailing winds, shifts in ocean currents, and strong tidal forces (which occur during full or new moon) can all cause high tide flooding, inundating streets even on sunny days.

High tide flooding falls into three levels of severity: minor, moderate, and major. The classifications measure how much water levels exceed average high tide for that location.

• Minor high tide flooding is when water levels reach 0.55 meters (1.8 feet) above average high tide. This minor flooding is mostly disruptive, causing stormwater backups and road closures.
• Moderate high tide flooding is 0.85 meters (2.8 feet) above average high tide. This can cause more disruption and can damage homes and businesses.
• Major flooding is flooding 1.20 meters (3.9 feet) above average high tide. Floods of this severity are quite destructive, may lead to evacuations, and often require repairs to infrastructure and property.

Because of rising seas, land subsidence, and the loss of natural barriers, high tide flooding is now twice as frequent in U.S. coastal communities as it was 20 years ago. Predictions from the latest interagency Sea Level Rise Technical Report show that high tide flooding will become more common and more severe over the coming decades. As sea levels continue to rise, conditions that cause minor and moderate high tide flooding today will cause moderate and major high tide flooding by 2050.