RISK OF SHORELINE HARDENING AND ASSOCIATED BEACH LOSS PEAKS BEFORE MID-CENTURY

Beach loss due to shoreline hardening continues to threaten beaches globally and is expected to accelerate with sea level rise (SLR). Modeling risk of hardening on future beaches provides important data for resource managers and other stakeholders; however, few comprehensive studies exist for this issue. The capitol island of Oʻahu, Hawaiʻi, provides a unique opportunity to model risk of shoreline hardening with SLR in order to identify ongoing and future challenges to beach conservation. We model modern (2015) and future (m of SLR) erosion hazard zones for all sandy beaches on the island. We, then, identify the intersection of coastal development and erosion hazard zones to define areas vulnerable to hardening. Our results show half of the beachfront shoreline will be at-risk of hardening at 0.74 m of SLR. Today, hardening is present on more than 28% of the shoreline. As a whole, island shorelines become increasingly at-risk of hardening throughout all SLR scenarios, with the largest increase (+7% of all sandy shoreline) occurring between the modern-day and 0.25 m of SLR. Modern day and near term hardening under 0.25 m of SLR (by year 2050), pose maximum risk of beach loss. Therefore, the greatest priority for planning beach conservation exists now. We conclude from these results that coastal resource managers and stakeholders in other settings may be facing significant modern-day and near-term threats to beach resources that have not been identified.