Rising Seas and High-Tide Flooding at Biscayne National Park

National parks along the South Atlantic and Gulf coasts protect tidal wetlands, mangrove forests, and historic buildings and archeological sites on low-lying ground — all vulnerable to faster sea-level rise and more frequent high-tide flooding. In a project funded by the Natural Resources Preservation Program from 2021 to 2023, the U.S. Geological Survey and the National Park Service estimated how likely it is that land in Biscayne National Park and several other parks will be flooded under different scenarios.

Biscayne lies south of Miami, Florida, and covers 32.33 square kilometers of land (not counting water and submerged beds). Within its boundaries is one of the longest continuous stretches of mangroves left on Florida's east coast. Its coastal habitats are threatened unless they can build upward in place or shift to higher ground, and rising water also puts historic, cultural and recreational sites at risk.

Two kinds of rising water

Relative sea-level rise. Driven by global climate change, sea-level rise is an ongoing and intensifying problem for low-lying coasts worldwide. Its pace varies from place to place with ocean circulation and density, the rising or sinking of land, and where and how much ice melts from ice sheets and glaciers; the local rate is called relative sea-level rise. A multiagency report set out five global scenarios — low, intermediate-low, intermediate, intermediate-high and high — depending on greenhouse gas emissions and ice loss. This study used the regional and local projections for those scenarios.

The five sea-level rise scenarios for the park to 2100: each line is the median projection, the shading its likely range. Chart: U.S. Geological Survey.

High-tide flooding happens when spring tides (the highest tides, at full and new moon), strong winds or storms push water to extreme levels. NOAA sets minor, moderate and major thresholds for each location. Minor floods can back up stormwater and close roads; moderate floods cause more disruption and possibly damage; major floods can destroy property and force evacuations.

Today, minor high-tide floods typically happen only a few days a year along the South Atlantic coast, and moderate and major ones less than once a year. But sea-level rise is making them worse, and their frequency is expected to climb quickly in the coming decades. By 2100, under the intermediate-low scenario, minor flooding is projected to happen more often than every other day — so that today's flood level could become tomorrow's ordinary high tide. Even minor flooding, repeated often, is no longer a nuisance but a serious and cumulatively destructive problem. Biscayne has already installed infrastructure that can adjust to changing water levels.

Floating docks installed after Hurricane Irma struck southern Florida in 2017, built to ride out extreme spring tides, high tide flooding and storm surge. Photo: National Park Service, August 2023.

What the maps show

Sea-level rise. Using 1992 as the base year, the team mapped two scenarios for 2040 and 2080:

"Likely" means a probability greater than 0.66; the likely range of rise runs from the 17th to the 83rd percentile of projections.

Where the park is likely to be inundated under the intermediate-low and intermediate-high scenarios in 2040 and 2080. Maps: U.S. Geological Survey.

High-tide flooding. NOAA's thresholds for the park are 52 cm (±19) above mean higher high water for minor flooding, 81 cm (±25) for moderate and 119 cm (±39) for major. Under all three, most of the park's land was likely to be inundated — from about 81% at the minor threshold to about 90% at the major threshold.

Flooding probability at the minor, moderate and major high-tide flooding thresholds. Maps: U.S. Geological Survey.

How the maps can be used

The maps can help park managers assess current and future effects of rising water on habitats, historic and cultural resources, and infrastructure, and support decisions such as applying the resist–accept–direct (RAD) framework. The same approach can also show how habitats will shift — for example, how tidal saltwater wetlands may migrate as the sea rises. The data and methods are published in a USGS data release (Thurman and others, 2024).

Sources

• U.S. Geological Survey: "Projected sea-level rise and high tide flooding at Biscayne National Park, Florida," USGS Fact Sheet, produced with the National Park Service; a work of the United States government in the public domain. The maps, charts and photographs are reproduced from the fact sheet.