By the USGS Pacific Coastal and Marine Science Center
Natural and engineered sand dunes serve as barriers to protect against storms. Engineered sand dunes, called “Nature-based Solutions“, are increasingly being used for coastal protection, yet their effectiveness remains understudied. New research finds that dunes’ flood protection benefits depend not only on the size of the storm, but also on how the dunes themselves change during the event.
Using a modeling framework incorporating both hydrodynamic and morphodynamic feedbacks, scientists evaluated how dunes at two beaches in Santa Cruz County respond to rising sea levels and increasingly powerful storms. The study found that storm-driven erosion can dramatically reduce the flood protection dunes provide, highlighting the importance of accounting for changing coastal landscapes when planning for climate resilience.
Looking Beyond Static Coastlines
Many coastal flood assessments assume beaches and dunes remain unchanged during storms. But powerful waves can rapidly erode dunes, reshape beaches, and move large volumes of sand offshore over the course of a storm event.
To capture these dynamic processes, the study authors developed a process-based modeling framework that simulated both flooding and changes in beach and dune morphology.
The team examined three future time periods—2025, 2055, and 2085—and modeled storms with return periods of 5, 20, 50, and 100 years at two contrasting locations: Santa Cruz Beach and Capitola Beach.
Natural Sand Movement Can Reduce Flooding
At Santa Cruz Beach, the models demonstrated that beaches naturally adjust during storms in ways that can reduce flooding.
As waves reshape the shoreline, sand is redistributed and nearshore sandbars can form, absorbing wave energy before it reaches the beach.
These storm-driven adjustments reduced predicted flood extent by as much as 32.4 percent compared with simulations that did not account for these morphodynamic processes.
The findings suggest that allowing beaches to evolve naturally during storms can, in some cases, enhance coastal resilience.
Dunes Work—Until They Don’t
The study found that dunes can substantially reduce flooding during moderate storm conditions.
When waves collide with dunes but do not overtop them—a condition known as the collision regime—the dunes reduced flooding by up to 60 percent.
However, as sea levels rise and storms become more intense, those benefits decline rapidly.
Once waves begin overtopping dunes, entering overwash and eventually inundation regimes, erosion accelerates and the dunes lose much of their protective capacity.
At Capitola Beach, limited space for the beach and dunes to migrate left little opportunity for natural adjustment. Models indicate that the site is already close to its flooding threshold under present-day conditions. Across nearly every future scenario, dunes experienced near-complete collapse and provided little additional flood protection.
These findings demonstrate that the success of dune-based adaptation depends heavily on local coastal setting.
RESEARCH ARTICLE: Morphodynamic controls on the performance of dune-based coastal flood mitigation under sea-level rise
By Zhengtong Yang, Natalia Herrán, David Gutiérrez-Barceló, Li H. Erikson, Sean Vitousek, Borja G. Reguero
ABSTRACT: Dune-based adaptation is increasingly used as a nature-based solution for coastal flood mitigation, yet many assessments rely on hydrodynamic-only approaches that treat beach and dune morphology as static, neglecting storm-driven erosion that can degrade dune performance during extreme events.
This study develops a process-based modeling framework coupling extreme value analysis of regional wave climate with XBeach surfbeat simulations to evaluate coastal flooding, erosion, and dune performance across three time horizons (2025, 2055, 2085) and four storm return periods (5, 20, 50, and 100 years) at two contrasting sites in Santa Cruz County, California.
At Santa Cruz Beach, flood extent increases at an accelerating rate with sea-level rise (SLR) for a given return period, while higher return period storms exhibit diminishing incremental increases under fixed SLR conditions. Storm-scale morphodynamic feedbacks reduce predicted flood extent by up to 32.4% through profile adjustment and nearshore bar formation. Dune effectiveness is strongly regime-dependent, reducing flooding by up to 60% under collision-regime conditions but declining rapidly as the system transitions toward overwash and inundation under stronger storms and late-century SLR. Comparison of static and erodible dune representations shows that flood estimates are not consistently conservative, with differences reaching up to 54.3% depending on storm intensity and time horizon.
At Capitola Beach, limited accommodation space and water-level-dominated dynamics collectively constrain morphodynamic adjustment, render the site near its inundation capacity under present-day conditions, and result in near-complete dune collapse and limited flood mitigation across all scenarios. These findings highlight the importance of site-specific regime evaluation, process-based morphodynamic modeling, and adaptive management when assessing dune-based adaptation under rising sea levels.
