Managing retreat for sandy beach areas under sea level rise.

Sea level rise (SLR) is projected to impact approximately one billion people by 2100. For many coastal communities, retreat is the most viable long-term option due to exposure risk under SLR and increased coastal hazards. Our research analyzes the costs of retreating coastal development at an iconic beach in Hawai'i that is experiencing severe erosion. We assess three retreat approaches: all-at-once, threshold-based, and reactive. Utilizing detailed SLR modeling projected to the year 2100, we estimate the public and private costs of retreat approaches and the amount of increased beach area. We find an all-at-once approach is most costly but maintains the largest beach area over time. In contrast, a reactive approach has the lowest direct costs but offers the least beach area gained over time and incurs the greatest public safety and environmental risk. The threshold-based approach largely mitigates public safety and environmental risks while providing more beach area over time than the reactive approach with similar direct costs. We find that a threshold-based approach should be further explored as a SLR response for coastal communities to maintain their sandy beach areas. Our study informs coastal adaptation research and identifies a new framework to explore the financial costs alongside social and ecological values.

Risk of shoreline hardening and associated beach loss peaks before mid-century: O'ahu, Hawai'i.

Shoreline hardening, which causes beach loss globally, will accelerate with sea level rise (SLR), causing more beach loss if management practices are not changed. To improve beach conservation efforts, current and future shoreline hardening patterns on sandy beaches need deeper analysis. A shoreline change model driven by incremental SLR (0.25, 0.46, 0.74 m) is used to simulate future changes in the position of an administrative hazard zone, as a proxy for risk of hardening at all sandy beaches on the island of O'ahu, Hawai 'i. In Hawai 'i, hardening can be triggered when evidence of erosion is within 6.1 m ("20 ft") of certain structures, allowing an applicant to request emergency protection. Results show an increase in shoreline vulnerability to hardening with SLR governed by backshore land use patterns. The largest increase (+ 7.6%) occurred between modern-day and 0.25 m of SLR (very likely by year 2050) with half of all beachfront shoreline at risk by 0.74 m of SLR. Maximum risk of shoreline hardening and beach loss is projected to occur from modern-day and near-term hardening because of the heavily developed aspect of some shoreline segments. Adaptation to SLR should be considered an immediate need-not solely a future issue.