Coastal Geohazards—Storm Surges

Increased sea level and storm surge impact roads and trails at Golden Gate National Recreation Area.

NPS photo.

A storm surge may occur during large coastal storms. Storm surge is the water that advances toward the shore by the force of storm generated winds. Most (>85%) of the storm surge is caused by winds pushing the ocean surface ahead of the storm. The sudden rise in water level can cause severe flooding in coastal areas, particularly when the storm surge coincides with periods of high tides. In areas where there is a significant difference between low tide and high tide, storm surges can be particularly damaging when they occur at the time of a high tide (National Hurricane Center 2009).

Near historic water levels caused by Hurricane Irma in 2017 flooded Fort Pulaski for the second time in less than a year. The fort was flooded by Hurricane Matthew in 2016.

National Park Foundation photo.

In addition to the storm’s meteorology (e.g., wind speed and size), the level of surge in a particular area is determined by the slope of the continental shelf and the shape of the coast. A shallow slope off the coast will allow a greater surge to inundate coastal areas. Areas with a steeper continental shelf will not have as much surge inundation, although large breaking waves can still present major problems (National Hurricane Center 2009).

Hurricane Katrina 2005
Storm surge is a hazard at many coastal National Parks. During Hurricane Katrina in 2005, storm surge along the Mississippi coast reached 30 feet (9.1 m) in height. Gulf Islands National Seashore experienced high levels of coastal erosion around Fort Massachusetts on West Ship Island, a significant historical structure and park resource. Since then, mitigation efforts to guide restoration of the barrier islands and renourish the beaches surrounding the fort may reduce future storm surge and hurricane damage.

Since at least the early 1900s, the area between Bush and Garden keys has fluctuated from a land bridge to a channel of varying depths. This photo taken in 2011 shows the presence of this white sand bridge adjacent to Fort Jefferson.  NPS Photo.

Low-elevation Islands
Dry Tortugas National Park’s low elevation at the end of the Florida Keys makes it susceptible to impacts of storm surge. Nearly all of the land area at Dry Tortugas National Park is at or within 3 m (10 ft) of sea level. Additionally, the very nature of the landscape at Dry Tortugas is one of change. Shifting sands continually alter the shape and profile of the islands. Sand erodes from one beach and is deposited elsewhere in the course of a single storm event. The navigation channel between Bush and Garden Keys exemplifies this active morphology. Storms have caused temporary land bridges to form up to 29m (95ft) wide between the islands. Accretion can close channels thus affecting the circulation and flow of water, movement of marine life, and navigability around the islands. Subsequent accretion near the supply pier and dock is expected to increase because the land bridge will prohibit sediment transport. This can prevent boat access to areas of the park. However, due to its unstable nature, the land bridge may wash away and re-open as a navigable channel just as easily as it formed. Increased storm surge will only enhance dynamic process such as this and place the future of nineteenth century Fort Jefferson at a potential risk. Read the case study to learn more about the challenges, needs, and responsive actions of Dry Tortugas National Park

Related Links

• NOAA—National Hurricane Center

• Fort Matanzas National Monument, Georgia [Geodiversity Atlas] [Park Home]

• Golden Gate National Recreation Area, California [Geodiversity Atlas] [Park Home]

• Gulf Islands National Seashore, Florida and Mississippi [Geodiversity Atlas] [Park Home]

• NPS—Coastal Geology

• NPS—Geohazards

• NPS—Geology

• NPS—Oceans