Duck Harbor Tidal Breach

In mid-December 2020, a tidal breach was first documented at Duck Harbor, likely due to elevated water levels from an astronomically high tide. Water levels rose above the height of the dune, and water poured into the low-lying basin behind it. Since the December 2020 breach, an overwash event has occurred near monthly even in non-storm conditions, particularly during astronomically high tides. Often, the overwash occurs several days in a row, in a series of overwash events. This has radically altered the hydrology, geomorphology, and biodiversity of the system. Salt water from the overwash has killed approximately 120 acres of vegetation in the low-lying forested area behind the dune. In spring 2023, the National Park Service began to remove the dead trees and shrubs to increase flow and drainage of the salt water, as well as promote the recruitment of salt marsh vegetation. The Land-Sea Interaction Program is actively monitoring the changes in the system.

To better understand how the system at Duck Harbor has changed in the past, we want to look at what’s below the ground. Ground-Penetrating Radar (GPR) sends a pulse into the ground and records how the pulse bounces back. Different types of sediment bounce back differently, so we can see how the sediment changes several meters into the ground. We also take sediment cores to look directly at the sediment below our feet. Sediment cores can be taken back to the lab and analyzed to give us a better understanding of what they’re made of and how old they are.

To monitor changes at Duck Harbor in the present and into the future, we use flow monitoring, such as deploying current meters before overwash events to provide an estimate of current speeds during the overwash. We also take RTK-GPS elevations at established transects throughout the washover fan to document changes in the erosion and deposition of sand throughout the system.

For more information and a great overview of the history of Duck Harbor, please visit the story map created by former Scientist in the Park, Eliza Fitzgerald.