Evaluating the Influence of Constructed Subtidal Reefs on Marsh Shoreline Erosion, Sediment Deposition, and Wave Energy

Abstract Salt marshes play a critical role in providing economic and ecological benefits but are susceptible to shoreline erosion. Natural and nature-based features (NNBF), such as breakwater reefs, are often used to reduce shoreline exposure to wave action and provide biogenic benefits. However, waves and water level are also responsible for the sediment supply necessary for marsh accretion, a critical component of marsh resilience to sea level rise. The goal of this study was to evaluate the effects of two subtidal breakwater reefs on wave energy, marsh shoreline erosion, and sediment deposition onto the marsh platform. As a restoration intervention, oyster shell and limestone gravel reefs were constructed within the nearshore zone of a high-energy shoreline where active shoreline erosion is causing marsh habitat loss. Although both sediment deposition and shoreline erosion were reduced after reef installation at all sites, the reefs demonstrated a statistically significant reduction in sediment deposition, whereas its effect on decreasing shoreline erosion was less pronounced. This variability in erosion reduction may be partly influenced by the physical dimensions of the reefs, affecting wave attenuation and leeward circulation. Wave measurements indicate that the reef reduced wave energy, particularly during south and southeast winds that could lead to the largest onshore waves. Given that these strong onshore winds are seasonal, extending the duration of data collection could provide deeper insights into the reef's influence on marsh shoreline erosion. This study is novel in that there are limited experimental or observational studies quantifying the wave reduction capacity and effects of subtidal reefs on marsh shoreline erosion and sediment dynamics. Studies such as these are critical to evaluate the capacity of subtidal reefs to protect marsh shorelines from erosion, but also to measure their impact on accretion processes necessary for the marsh to maintain elevation under future sea level rise.