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Hybrid approaches to shoreline protection, where natural (“green”) features are combined with hardened (“gray”) infrastructure, are increasingly used to protect coastlines from erosion and flood-based hazards. Our understanding of hybrid systems is limited, and it is unknown whether the components of these systems interact in any meaningful sense to provide flood reduction benefits that are greater or less than “the sum of the parts.” In this study, a large-scale physical model was used to investigate the overtopping of a vertical wall protected by a hybrid system where an idealized Rhizophora mangrove forest of moderate cross-shore width fronted a rubble-mound revetment. Configurations included the wall alone, the wall with a low- or intermediate-density mangrove forest without the revetment, the wall with the revetment, and the wall with an intermediate- or high-density mangrove forest and the revetment. The study isolated the reduction in overtopping of the wall by the revetment component, the mangrove forest component, and the interaction between the components of the hybrid system. The total reduction by the hybrid system was estimated within 5% accuracy as the sum of the reduction by each component minus the product of the component reductions. Comparison of the proportional reduction in overtopping by the mangrove forest on the wall alone and the wall with the revetment indicated that the mangrove forest reduced the overtopping of the revetment by approximately the same proportion that the forest reduced the overtopping of the wall. Therefore, (1) total overtopping reduction by the hybrid system was modeled as the reduction expected from the green and gray components in series. Additional analysis showed that (2) for the same wave conditions, a mangrove forest of moderate cross-shore width can have equal or greater protective benefits than a coastal revetment, (3) there is an exponential relationship between the discharge rate and the forest density, and (4) the mangrove forest, the revetment, and the hybrid system all provided greater reduction in overtopping as wave steepness increased. The tests in this study were conducted without wave breaking, with constant freeboard and water depth, with a specific revetment geometry, and without a mangrove canopy. Therefore, these results should be interpreted with caution if used for engineering design. 

A prototype-scale physical model was used to study wave height attenuation through an idealized mangrove forest and the resulting reduction of wave forces and pressures on a vertical wall. An 18 m transect of a Rhizophora forest was constructed using artificial trees, considering a baseline and two mangrove stem density configurations. Wave heights seaward, throughout, and shoreward of the forest and pressures on a vertical wall landward of the forest were measured. Mangroves reduced wave-induced forces by 4%–43% for random waves and 2%–38% for regular waves. For nonbreaking wave cases, the shape of the pressure distribution was consistent, implying that the presence of the forest did not change wave-structure interaction processes. Analytical methods for determining nonbreaking wave-induced loads provided good estimations of measured values when attenuated wave heights were used in equations. The ratio of negative to positive force ranged between 0.14 and 1.04 for regular waves and 0.31 to 1.19 for random waves, indicating that seaward forces can be significant and may contribute to destabilization of seawalls during large storms. These results improve the understanding of wave-vegetation-structure interaction and inform future engineering guidelines for calculating expected design load reductions on structures sheltered by emergent vegetation. 

We constructed a hybrid system consisting of a 19.6-m mangrove forest and a rubble-mound revetment seaward of a vertical wall. We investigated the mangrove forest and revetment features separately and in combination to compare the mitigating effects of the features on the overtopping of the vertical wall. We considered 3 different forest densities and tested regular, single- and double peaked spectra, and transient (tsunami-like) wave regimes. Water surface elevations and flow velocities were measured along the test section, and overtopping volumes were measured shoreward of the vertical wall. 

A 1:16 scaled physical model was constructed to investigate the effectiveness of a seawall, a submerged breakwater, and mangrove forests to mitigate overland flooding and forces on structures in an idealized urban coastal environment. The experiment was performed using tsunami-like waves at different water levels, wave amplitudes, and time scales to simulate long-wave dynamics. The baseline condition (no mitigation), seawall, submerged breakwater, and mangrove forest were tested individually, and the seawall and submerged breakwater were also tested in combination. Wave gauges, acoustic Doppler velocimeters, loadcells, and pressure gauges were used to measure wave elevations, velocities, forces, and pressures on coastal structures, respectively. The performance of these hard structures and mangroves was compared through their effects on wave elevation, particle velocity, and force reduction. Experimental results showed that each protecting structure reduced the horizontal wave forces and inland flow hydrodynamics in the low-water-level case, with a similar performance by the individual seawall, submerged breakwater, and four rows of mangroves. The combined configuration, when the seawall and submerged breakwater were installed simultaneously, caused the most significant maximum force percent reduction by approximately 50%, while mangrove forests arranged in eight rows resulted in a force reduction of 46% in the first building array. However, in the high-water-level cases, the impulsive force measured with the presence of the submerged breakwater was larger than in the baseline case; thus, the submerged breakwater may amplify the impulsive force on the vertical building rows for certain incident wave conditions. Generally, the combined hard structures induced the lowest force reduction factor measured in almost every building row compared to the seawall, submerged breakwater, and mangroves considered separately for all wave conditions and water levels. That means this multi-tiered configuration showed better performance than individual alternatives in reducing horizontal forces inland than the individual alternatives considered separately. 

Natural and Nature-Based Features (NNBF) are promoted as alternatives to structural flood protection measures. Progress has been made in understanding the physics and engineering of these systems; however, engineering, ecological, and social barriers to implementation remain. This paper identifies these barriers using the results of a literature review and summary of expert opinion; contrasts the state of the practice of NNBF with traditional structures; and details the main engineering challenges to NNBF implementation, including the uncertainty in current calculation techniques and lack of engineering design guidelines. We suggest that emergent vegetation systems can be designed with the current body of information, and an example framework is proposed for assessing these systems for their wave attenuation performance. The framework is discussed in the context of risk, and future research priorities are presented.