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Increased High Tide Flooding Threatens Coastal Security in the US

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Soldiers with the Texas Army National Guard move floodwaters from Hurricane Harvey (U.S. Army photo by 1st Lt. Zachary West)

By Dr. Marc Kodack

The National Oceanic and Atmospheric Administration recently published its latest outlook for high tide flooding in the United States, which covers the period May 2020-to-April 2021. According to the report, the increases in coastal flooding described in last year’s report will continue with even greater frequency as sea levels rise. U.S. Department of Defense coastal installations will continue to be affected, such as those in San Diego and Norfolk. High tide flooding is non-linearly accelerating along the east and Gulf coasts, whereas it is linearly increasing elsewhere. For all these coastal locations the extent and frequency of high tide flooding is projected to continue to increase over the coming decades.

High tide flooding usually occurs when coastal water levels rise above 0.5-to-0.65 meters (1.6-2.1 feet). Local conditions may affect this range because of topography and the extent of development. Infrastructure that can be negatively affected includes roadways, buildings and other facilities, homes, storm and/or wastewater systems, and levels of groundwater. Compared to the year 2000, high tide flooding is twice as likely to occur today. Land subsidence exacerbates the flooding. These worsening conditions occur in the absence of any changes or increases in storms that may occur on the coast.

Looking out to April 2021, Norfolk may experience between 9-13 days of high tide flooding, whereas San Diego may experience 4-7 days. Elsewhere in the U.S., the northeast coast may experience 6-11 days, the western Gulf coast 5-11 days, and the southeast Atlantic, eastern Gulf, and northwest Pacific coasts projected to have 3-6, 2-5, and 0-7 days respectively. However, the 2020 Atlantic hurricane season (June-November) is anticipated to be busy, so the projected number of high tide flooding days may increase anywhere along the Gulf or Atlantic coasts.

While high tide flooding affects many coastal communities, its low-level chronic affects for coastal military installations could cumulatively decrease the ability of those installations to perform necessary critical missions, particularly when increasing amounts of financial resources are needed to adapt to rising sea levels and the high tide flooding that it causes. With worsening high tide flooding continuing indefinitely, at some point the number of days may become so common that the high tide flooding spatial extent transitions to forming a new high tide boundary. The Department of Defense will need to consider if managed retreat is an option, instead of increasing the size of sea walls or other constructed features to defend against the unrelenting rising sea.

Dr. Marc Kodack is Senior Fellow at the Center for Climate and Security and former Sustainability and Water Program Manager in the Office of the Deputy Assistant Secretary of the Army for Energy and Sustainability.

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