Storm Surge: How Coastal Storms Flood National Parks

Rising seas and storm surge reach roads and trails at Golden Gate National Recreation Area. NPS photo.

Storm surge is seawater driven toward the shore by a storm's winds. More than 85% of it comes from wind pushing the ocean surface ahead of the storm. The sudden rise can flood coasts badly — worst when it arrives at high tide, especially where the gap between low and high tide is large.

What makes a surge bigger

• The storm: wind speed and size.
• The seafloor: a shallow, gently sloping continental shelf lets a larger surge pile up and flood inland; a steep shelf limits the surge, though big breaking waves can still do damage.
• The shape of the coast.

Hurricane Katrina, 2005

Along the Mississippi coast, Katrina's surge reached 30 feet (9.1 m). At Gulf Islands National Seashore, it caused severe erosion around Fort Massachusetts on West Ship Island. Restoring the barrier islands and replenishing the beaches around the fort may reduce future damage.

Low islands: Dry Tortugas

Fort Jefferson on Garden Key, with a sand bridge to Bush Key, in 2011. NPS photo.

Nearly all of Dry Tortugas National Park, at the end of the Florida Keys, lies within 3 m (10 ft) of sea level, and its sands never stop moving: a single storm can strip one beach and build another.

• The channel between Bush and Garden keys has flipped between a land bridge and a channel of varying depths since at least the early 1900s; storms have built bridges up to 29 m (95 ft) wide.
• A bridge changes currents and the movement of marine life, and can trap sand near the supply pier and dock, cutting off boat access.
• The bridge can wash away as easily as it formed.

Bigger surges will speed this change and put nineteenth-century Fort Jefferson at greater risk.

Sources

Based on "Coastal Geohazards—Storm Surges," Geologic Resources Division, National Park Service, citing the National Hurricane Center (2009); a work of the United States government in the public domain. A National Park Foundation photograph of Fort Pulaski flooded by Hurricane Irma in 2017 is not reproduced.