Impact of Coral Reef Mining Pits on Nearshore Hydrodynamics and Wave Runup During Extreme Wave Events

Small island developing states are among the most vulnerable areas to the impact of natural hazards and climate change. Flooding due to storm surges and extreme waves, coastal erosion, and salinization of freshwater lenses are already a serious threat and could lead to irreversible consequences in the coming decades. Reef flat mining is one of the most common practices to source the required material for the implementation of coastal protection measures, but concerns remain that partial removal of the protective reef could increase wave loading on the islands. However, the available data and knowledge on the effects of these mining pits are currently very limited. This study provides new insights on the effects that pits may have on nearshore hydrodynamics and wave runup. Results are based on a large numerical data set of fringing reefs, derived using the validated XBeach nonhydrostatic+ process-based model. Model results indicate that excavation pits cause a decrease in infragravity wave energy around the fundamental mode of the reef, which is partly caused by reduced wave transmission. Additionally, changes in sea and swell wave energy are attributed to reduced transmission, a decrease in wave dissipation, and (triad) wave-wave interaction. Furthermore, in 13% of all modeled cases, an increase in wave runup is observed, mainly due to more sea and swell wave energy reaching the shoreline. This probability is lowest for narrow pits relative to the reef flat width or pits located further from shore. Plain Language Summary Many small island states are among the most vulnerable areas to the impacts of climate change. Additionally, flooding due to storms, coastal erosion, and loss of freshwater supply already form a serious threat. Mining of dead coral sediments from the reef flat is a common practice to source material for coastal protection works. This could potentially lead to higher waves at the coastline through the partial removal of the reef, which breaks large waves. However, the knowledge on the effects of these mining pits is currently very limited. The present study provides improved explanations on the effects that a pit may have on waves and assesses the resulting risk of flooding it poses on these islands. The results are obtained by simulating a large set of different reefs and wave conditions using a numerical model named XBeach. These show that the effect of a pit can vary significantly, but in general, a pit causes lower waves near the shoreline. At some cases, larger waves at the shoreline were found, thereby increasing the risk of flooding. This risk is the lowest for pits that are located further from the coastline, or those that have narrow width relative to the reef width.