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Built infrastructure, such as seawalls and levees, has long been used to reduce shoreline erosion and protect coastal properties from flood impacts. In contrast, natural and nature-based features (NNBF), including marshes, mangroves, oyster reefs, coral reefs, and seagrasses, offer not only coastal protection but also a range of valuable ecosystem services. There is no clear understanding of the capacity of either natural habitats or NNBF integrated with traditional engineered infrastructure to withstand extreme events, nor are there well-defined breakpoints at which these habitats fail to provide coastal protection. Evaluating existing NNBF strategies using a standardized set of metrics can help to assess their effectiveness to better inform design criteria. This review identifies a selection of NNBF projects with long-term monitoring programs and synthesizes the monitoring data to provide a literature-based performance assessment. It also explores the integration of NNBF with existing gray infrastructure to enhance overall effectiveness. 

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 systematic review of 20 years of studies was conducted to understand wave dissipation trends of hybrid and natural (soft) coastal features, collectively referred to as nature-based solutions (NbS). Of 13,451 studies identified and 470 studies reviewed; only 50 studies consistently reported the basic parameters required to compare wave height dissipation. These studies were used to create a basic understanding of wave dissipation across soft and hybrid features along different cross-shore widths. More specific implementation guidance for NbS is limited due to the lack of consistent monitoring practices and protocol within and between soft and hybrid features. This disparity is greatest between soft and hybrid NbS. To fully understand best practices for the wide variety of soft and hybrid NbS, more uniform monitoring data is needed to assess and more fully define wave dissipation performance. Based on the findings of this review, eight parameters to measure the wave dissipation effectiveness of NbS features are proposed. These findings will inform the development and application of evaluation protocols for future NbS projects. 

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. 

Wave overtopping of shoreline infrastructure can lead to significant flooding and consequent loss of life, impairment of transportation systems, and ecological damage. Coastal defenses against overtopping traditionally include hard structures, such as seawalls and revetments, and design guidelines for these structures, e.g., the EurOtop manual (Van der Meer et al., 2018), have been developed from empirical studies of overtopping. Recently, natural and nature-based features (NNBF) including mangroves, wetlands, reefs, and other systems have gained attention as alternatives to conventional engineered coastal protection systems. Field observations have identified the potential of emergent vegetation, particularly mangrove forests, to mitigate damage during extreme coastal flood events (Alongi, 2008; Tomiczek et al., 2020). However, there is a lack of research on engineering NNBF systems to achieve specific design requirements for overtopping protection. Hybrid or multi-tiered approaches to shoreline protection have also been proposed, where natural (“green”) features are combined with hardened (“gray”) infrastructure to protect coastlines and near-coast assets from erosion and/or flood-based hazards. For overtopping mitigation, hybrid designs can add the performance provided by emergent vegetation to the services of a revetment or a wall. It is unknown whether the green and gray features in a hybrid system perform independently and can be considered as separate design elements, or if the inclusion of one feature affects the performance of the other such that the hybrid system must be considered as a single, complex design element. This study constructed a large-scale physical model to investigate the overtopping performance of a hybrid system with an idealized Rhizophora mangrove forest seaward of a revetment abutting a vertical wall compared to that performance of the wall fronted by the revetment only, the wall fronted by vegetation only, and the wall alone. 

Mangroves and other natural coastal defenses have the potential to augment or replace traditional engineered coastal structures in preventing adverse events such as wave overtopping. Natural, or “green” systems may reduce maintenance costs, reduce sediment erosion, and increase biodiversity compared to traditional “gray” infrastructure built from stone and concrete. To effectively inform the design of hybrid green-gray infrastructure, experimental results must be reliable, but testing at 1:1 scale is time-consuming, expensive, and available at only a few facilities worldwide. This study addresses a knowledge gap in defining the nature of the interactions between green and gray coastal defenses with a focus on overtopping and scaling experimental results. This study will compare data from mangrove-related experiments conducted at scales including 1:2 and 1:8 as part of a collaborative effort between Oregon State University (OSU) and the United States Naval Academy (USNA). The study aims to analyze this data and contribute to the joint compilation of a methodology for designing prototype-scale tests from small-scale experiments to identify the relative importance of friction and scaling effects between prototype and small-scale experiments. Testing conducted at USNA as part of this study included a 1:8 scale, 0.61m-wide (2ft.) flume that replicates the conditions of 1:2 scale experiments at Oregon State University. The experimental setup includes a model Rhizophora mangrove forest placed in front of a seawall, behind which overtopping is measured as volume per unit length either computed from overtopped water weight or directly measured by overtopped volume. Mangroves are modeled as central trunks with stilt roots, as this study focuses on the effects of the root structures on overtopping. Waves generated for the 1:8 experiments include regular waves with heights between 5cm and 10cm and periods between 1 and 2 seconds, scaled according to Froude similitude. Implications of scaled-up measurements of overtopping are also discussed. 

This paper presents results of a reduced (1:8) scale experiment investigating the performance of hybrid structural (gray) and natural-based (green) infrastructure for wave overtopping reduction. Experiments were scaled to a 1:8 geometric scale based on 1:2-scale experiments conducted during the Summer of 2023 at Oregon State University. Seven wave conditions were tested, with (model-scale) wave periods ranging from 1 to 2 seconds and wave heights ranging from 6.0 to 7.5 cm. These wave conditions were conducted throughout two configurations: a seawall-only (baseline) configuration and a configuration with the seawall in combination with a mangrove forest installed seaward of the wall. The total volume of overtopped water was measured for each wave condition. Results indicated that adding mangroves reduced overtopping for all wave conditions, with an average of 32.1% reduction in overtopped volume compared to the baseline configurations. This reduction falls within the range of preexisting overtopping rates. Results from these experiments can assist engineers in understanding the performance of hybrid coastal infrastructure to design effective and sustainable shoreline protection.   

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.