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A Dutch Approach to Coastal Protection

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Course Information

The U.S. coastline is under attack! But not in the way many would assume; our coasts are under attack by the constantly changing environment, rise in sea level and increased severe-weather events. Although storms, floods and erosion have always been hazards, they now occur on top of higher sea levels. Combined with coastal development, these hazards now threaten approximately $1 trillion in real estate along U.S. coasts. According to the “2022 Sea Level Rise Technical Report,” the average global sea level rose around 10-12 inches during the last 100 years and is predicted to rise an additional 10-12 inches during the next 30 years. With more than 60,000 miles of U.S. roads and bridges in coastal floodplains, coastal designers will need to work fast to protect our valuable infrastructure.

When it comes to battling floods and sea level, one country stands out: the Netherlands. With about a third of the country sitting below sea level, the Dutch have become masters of controlling floodwaters and the impacts of the sea. One of the greatest Dutch accomplishments is the construction of the Afsluitdijk, a 20-mile dam and causeway. The Afsluitdijk was built in 1932 to close off the Zuiderzee (South Sea) to protect against flooding that was occurring due to storm surges. With a core of glacial till and armored by natural basalt columns, the dam had a height of approximately 26 feet above mean sea level, and the 3.5H:1V slope face was designed for 8-foot-high waves. During the last 90 years, however, the basalt columns have eroded and, coupled with increasing water levels and wave heights, led to the need to strengthen the Afsluitdijk. The rehabilitation included increasing the crest level to reduce the wave overtopping and reinforce the armor layer on both sides of the dam.

To strengthen the dam, the Rijkswaterstaat (the Dutch Ministry of Infrastructure and Water Management) desired to maintain the structure’s historic value and visual appearance. The rehabilitation contract included several unique aspects, including an approach that required the design to be adaptable for wave overtopping following actual sea-level rise.

Author

Daniel Priest, P.E., and Pieter Bakker

Learning Objectives

At the conclusion of this article, the reader should be able to understand the following:

• How the stability of an armor layer is obtained and used to size armor units for coastal protection systems.

• What physical model tests are, and how they add value to the construction and design of flood defense systems attacked by wave action.

• How regularly placed armor units can be used to provide safer, more-efficient installations on breakwaters, jetties, groins and shorelines.

• Which technology can meet short-term design and construction goals while still being resilient and adaptable for future changes.

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