Coral reefs offer crucial coastal protection by dissipating wave energy and reducing erosion and flooding, especially in the western Atlantic where broad reef flats are less common. Historically, *Acropora palmata*, with its robust structure and rapid growth, was the dominant reef builder in this region. However, coral disease and thermal stress have decimated *A. palmata* populations over the past half-century, leading to flatter, shallower reef crests and reduced wave-energy dissipation capacity. This decline threatens coastal protection as rising sea levels allow larger waves to reach the shore. Understanding the changing reef-building process, particularly in reef-crest habitats, is vital for evaluating future coastal hazard risks. While many coral restoration efforts are underway, their effectiveness in restoring ecosystem services like shoreline protection remains largely unproven. This study aims to quantify the potential of coral restoration, specifically *A. palmata* restoration, to mitigate coastal flooding at Buck Island Reef National Monument (BIRNM) in the U.S. Virgin Islands, a location experiencing significant shoreline erosion alongside reef degradation. The research combines carbonate budget models to assess reef accretion potential with hydrodynamic modeling to project future coastal flooding under various scenarios.

Existing literature highlights the crucial role of coral reefs in coastal protection (Ferrario et al., 2014; Beck et al., 2018; Reguero et al., 2021). Studies have shown that coral reef degradation, coupled with sea-level rise, leads to increased wave energy reaching the coast (Sheppard et al., 2005; Quataert et al., 2015; Harris et al., 2018). The loss of *Acropora palmata*, a key reef-building species, has been linked to reduced reef accretion and increased erosion (Kuffner & Toth, 2016; Yates et al., 2017; Alvarez-Filip et al., 2009; Perry et al., 2018). Carbonate budget studies provide valuable insights into reef accretion-erosion balance (Hubbard, 1988; Toth et al., 2022), but incorporating the high-energy reef crest, crucial for wave dissipation, remains challenging. Hydrodynamic models have demonstrated the theoretical impact of increased water depth on coastal flooding (Storlazzi et al., 2011; Grady et al., 2013; Pearson et al., 2017; Reguero et al., 2019). However, few studies have integrated carbonate budget assessments and hydrodynamic modeling to evaluate the combined impacts of reef degradation and restoration on coastal protection, particularly in the context of sea-level rise. This research seeks to address this gap by applying a combined approach at BIRNM.

The study employed a multi-faceted approach combining field surveys, carbonate budget models, coral growth experiments, and hydrodynamic modeling. **Carbonate Budgets:** Carbonate budgets were calculated for 54 sites around BIRNM, categorized into fore-reef, reef-crest, and back-reef habitats, across windward and leeward sectors. Reef census data from July 2016 were analyzed using a modified ReefBudget v1 methodology (Hubbard et al., 1988; Perry et al., 2018). This involved quantifying the percent cover of calcifying taxa (corals and coralline algae) using point-count analysis of photographs, and estimating gross carbonate production using taxon-specific calcification rates. Bioerosion rates were estimated by analyzing the abundance, size, and life phase of bioeroding parrotfish and sea urchins. Endolithic macro- and microbioerosion were estimated based on available substratum and published rates for nearby regions. Net carbonate production was calculated by subtracting bioerosion from gross production, and then converted to estimates of reef accretion potential using local reef framework density and porosity data from Holocene reef cores (Hubbard et al., 2005). The researchers acknowledge that reef-accretion potential is an optimistic maximum estimate, as it doesn’t include physical erosion or chemical dissolution. **Coral Growth Experiments:** Vertical growth and calcification rates of *A. palmata* and *Pseudoporia strigosa* were measured at three locations around BIRNM from June 2019 to July 2021. Colonies were transplanted onto PVC discs and monitored for height, planar dimensions, and weight over two years. **Population Model:** A simple population model was developed to project changes in *A. palmata* cover under various restoration and mortality scenarios. The model uses the average *A. palmata* growth rate, literature-derived mortality and fragmentation rates, and three scenarios for mortality (fixed, increasing, and decreasing) to project cover changes to 2100. **Hydrodynamic Modeling:** A two-dimensional XBeach surfbeat model (Roelvink et al., 2009; Van Dongeren et al., 2013; Lashley et al., 2018) of BIRNM was used to simulate wave and water-level transformation across the reef. The model used lidar-derived bathymetry, spatially varying hydrodynamic roughness, and was validated against observed wave data. Simulations were run with a range of sea-level-rise scenarios (+0.0, +0.2, +0.5, +1.2, and +2.0 m), wave conditions (10- and 50-year storm events), and bathymetries (present-day and those projected under reef degradation and restoration). Maximum total water levels along the shoreline were extracted to assess flooding potential.

The carbonate budget analysis revealed that reef accretion potential varied significantly across BIRNM, with an overall average of −1.56 mm y⁻¹. Only five sites showed positive accretion, mainly on the southern reef crest due to higher coral cover (dominated by *A. palmata* and *Ps. strigosa*). Bioerosion, primarily driven by parrotfish (*Sparisoma viride*), was the dominant process across most of the reef. The study projected significant reef erosion in most areas by 2100, while the southern reef crest showed minimal elevation change. The coral growth experiment demonstrated rapid growth of both *A. palmata* (average height increase of 6.99 cm y⁻¹) and *Ps. strigosa*. The population model projected that a large-scale *A. palmata* outplanting effort could increase *A. palmata* cover on the reef crest for several decades, but long-term maintenance would require additional restoration or reduced climate-related mortality. Hydrodynamic modeling showed that sea-level rise will be the primary driver of coastal flooding. Reef erosion will exacerbate flooding, particularly in the northern sector. However, successful large-scale restoration (+30% *A. palmata* cover), by increasing reef accretion and narrowing the gap between reef elevation and sea level, could significantly reduce total water levels during storms and mitigate the impacts of higher sea-level rise. A 30% increase in *A. palmata* cover could allow the reef to keep pace with the lower range of projected sea-level rise by 2100, effectively reducing the flooding potential of a major hurricane to that of a tropical storm.

This study provides strong evidence for the potential of coral restoration to mitigate coastal flooding driven by sea-level rise. The findings demonstrate that even under the most pessimistic sea-level rise projections, successful *A. palmata* restoration could allow the reef to maintain its protective function. The relatively high growth rates of *A. palmata* show that restoration can effectively counteract near-term impacts, at least for the more optimistic scenarios of climate-related mortality. However, long-term success depends not just on growth rates, but also on mitigating mortality from various stressors, including thermal stress and disease. While the model simplifies complex ecological interactions, it highlights the importance of balancing both sides of the carbonate budget equation (accretion and erosion) through restoration. The specific findings are directly applicable to BIRNM; however, the overall concept of using restoration to enhance reef accretion and maintain coastal protection is relevant to other western Atlantic reefs, with adaptations for specific local conditions. The study emphasizes the need for location-specific restoration strategies and the value of carbonate budget models as tools for planning and assessing functional impacts.

This research demonstrates the significant potential of large-scale *Acropora palmata* restoration to mitigate the effects of sea-level rise on coastal flooding at Buck Island Reef. Successful restoration, achieving a substantial increase in *A. palmata* cover, could enable the reef to keep pace with projected sea-level rise under moderate emission reduction scenarios. Long-term success requires not only substantial near-term investment but also addressing ongoing coral mortality from various stressors. Future research should focus on improving the accuracy and realism of population models and investigating hybrid restoration approaches that combine biological restoration with engineered solutions to maximize resilience in the face of climate change. The high upfront cost of restoration must be balanced against the substantial economic and social benefits derived from maintaining the reef's coastal protection function.

The study acknowledges several limitations. The carbonate budget models do not fully account for event-driven physical erosion (e.g., from hurricanes) or chemical dissolution, potentially leading to an overestimation of reef-accretion potential. The population model simplifies complex ecological dynamics, such as the influence of storm-generated fragmentation on *A. palmata* populations. The hydrodynamic modeling considers only wave-driven flooding and does not factor in other relevant factors such as rainfall, groundwater, or sea-level rise variability. The high cost of restoration and uncertainty around future mortality rates should also be acknowledged. Despite these limitations, the study provides valuable insights into the potential of coral restoration to enhance coastal resilience.