The efficacy of green and gray coastal structures in storm surge mitigation

Over the past decades, gray coastal structures have played a pivotal role in shoreline protection. However, in light of climate change, nature-based solutions (NbS) have gained prominence due to their added ecological benefits. Although coastal vegetation such as mangroves is known to dissipate wave energy effectively, limited studies have quantitatively compared the wave-attenuation efficacy (i.e., wave damping per unit area) of green and gray coastal structures under storm surge conditions. This study addresses this gap through full-scale numerical simulations using OpenFOAM to model the interactions between storm surge and coastal structures. The model is validated against laboratory experiments, showing good agreement. Simulations are then conducted to assess the wave-attenuation efficacy of mangrove forests and submerged breakwaters across a range of storm surge scenarios. Results show that within the mangrove zone, additional wave height reduction occurs due to drag forces from roots and canopies, an effect that is particularly strong in younger, denser forests. The wave attenuation efficacy of mangroves varies with growth age, forest width, and surge level: at a 3 m surge level, a 50 m-wide 15-year-old mangrove reduces wave heights by ∼40%, compared to 20%–30% for older stands. A 100 m-wide forest can reduce wave heights by up to about 40%–65% for three mangrove groups. At a higher 5 m surge level, the canopy effect becomes significant, and a 100 m-wide young mangrove forest can reduce wave heights by 75%–82% for moderate to extreme waves, while older stands reduce 50%–68%. The 25-meter submerged breakwaters with crest heights equivalent to mangrove roots generally provide effective attenuation of 30%–50% under moderate waves, while mangroves require at least twice the breakwater width to reach comparable performance. Under extreme waves, however, large waves readily transmit over submerged breakwaters, reducing their effectiveness to only 15–20%. In such cases, young mangroves may remain efficient, although their survival under extreme storms remains uncertain. Finally, new empirical formulations are proposed to estimate the wave transmission coefficient for both mangrove forests and submerged breakwaters under storm surge conditions. These findings provide practical insights into the spatial planning and design of hybrid coastal defense strategies. • The wave attenuation efficacy of submerged breakwaters and mangroves are compared by using full-scale numerical simulations. • Younger mangroves (i.e., 15-year-old mangroves), with lower porosity and greater wave-canopy interaction, provide more effective wave dissipation than mature stands. • A forest width of approximately 100 m is required to achieve more than 40% wave height reduction under tested conditions. • Empirical formulas are proposed to estimate wave damping by mangroves and submerged breakwaters under storm surge conditions.