D.W. Poppema1*, S. de Vries1 , A.J.H.M. Reniers1, A. Antonini1
1 Delft University of Technology, The Netherlands
* Corresponding author: d.w.poppema@tudelft.nl
Introduction
Globally, coastal defense strategies are increasingly integrating hybrid dunes, which meld sandy dunes with hard structures. These hybrid formations often intentionally merge traditional erosion-resistant materials like rocks and concrete with natural dunes. This approach aims to harness the benefits of both worlds: hard structures occupy less space and offer predictable performance, while sandy dunes provide recreational space, support biodiversity, and exhibit greater resilience to rising sea levels. Occasionally, hard structures like buildings or outfall pipelines are constructed in dune environments for different objectives, yet inadvertently alter erosion during storms.
When combining hard and soft elements, their stability and hydraulic response are no longer only a function of the classical behaviour of either individual hard or soft parts under wave action, but also of the complex feedback processes between dune morphology, wave attack and structural stability. These interactions, though poorly understood, are pivotal for designing new hybrid dunes and evaluating the effectiveness of existing hybrid defenses.
Objective and Methods
To better understand hybrid dunes, we aim to provide a structured overview of international hybrid dune cases, where sandy dunes provide coastal protection together with, or affected by, hard structures. This expands the definition of hybrid dunes beyond dunes with hard defensive constructions to include coastal dunes influenced by any hard structure, to consider both potential adverse and beneficial effects of such elements. The inventory categorizes hybrid dune cases based on the type of hard elements (e.g., revetment, geotube, building, road), the functions provided, synergies or drawbacks of the hybrid composition, positioning of the hard elements (e.g., behind, underneath, on top of the dune), and the sequence of development (e.g., constructed as a hybrid dune, hard structure added later, hard structure coincidentally present).
Characteristic or particularly notable types of hybrid dunes in the inventory will be examined to identify knowledge gaps and lessons learned on the design and safety assessment of hybrid dunes. This will be valuable for engineering practice in general, and also help us with the design of a unique field experiment that we are planning, in which an artificial full-size hybrid dune is to be constructed at the high-water line of the Sand Motor.
Results
An inventory of hybrid dunes around the world was collected (see figure). In Bay Head (New Jersey, USA), a relic rock seawall was covered by sand dunes. During hurricane Sandy, this forgotten seawall saved the hinterland from flooding (Walling et al., 2016). Similar hybrid constructions exist abroad. In Australia, the Gold Coast A-line seawall is likewise covered by dunes, but as part of the original design, to facilitate recreation while offering solid protection. The same principle is used at the Hondsbossche sea dike (NL), which was upgraded by constructing a dune landscape in front. Additional hybrid dune cases of various types can be found in the rest of the world, with the Netherlands and United States showing especially numerous examples.
Some valuable lessons are drawn from this overview. Hard structures built for coastal defense are often relatively large and alongshore uniform. Hereby, they tend to affect cross-shore sediment transport over a larger area. Other structures (buildings, outfall pipelines) usually have more local effects such as concentrated scour, potentially causing local weak spots in the dunes. During the NCK days, we will expand further on example cases and lesson learned.
Left: a map of the hybrid dune cases in the inventory. Blue dot: the Hondsbossche Sea Dike, shown on the right
References
Walling, K., Herrington, T. O., & Miller, J. K. (2016). Hurricane Sandy damage comparison: Oceanfront houses protected by a beach and dune system with vs. without a rock seawall. Shore Beach, 84(3), 35-41.
