Risk of shoreline hardening and associated beach loss peaks before mid-century: Oʻahu, Hawaiʻi

Abstract

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.

A global analysis of extreme coastal water levels with implications for potential coastal overtopping

“…Finally, coastlines have been modified in various ways by human activities, particularly in urbanized areas in which, for example, ports have been constructed, land has been reclaimed from the ocean 28 , seawalls built to combat coastline recession, cliffs stabilized, and groins placed in an attempt to retain a beach fringe and maintain dunes 68 . For example, in the US alone, 14% of the national coastline is estimated to be hardened with engineering structures, and this percentage is expected to increase to 33% by 2100 75 . Such human interventions (e.g., seawalls, dikes) to the natural system generally results in steepening of coastal slopes 76 , resulting in smaller wave dissipation zones compared to natural coasts, which is not accounted for in this study.…”

Metabarcoding as a tool to examine cryptic algae in the diets of two common grazing surgeonfishes, Acanthurustriostegus and A. nigrofuscus

“…They are all readily accessible for fishing and are used heavily for recreation. The north and west sides of the island of Oʻahu experience high surf in the winter months; the south shore receives summer swell, and the east side receives consistent trade winds (Tavares et al, 2020). All locations sampled are near heavily populated areas, but the sites on the southern shore of Oʻahu are adjacent to Honolulu, which is the most developed, urbanized area in Hawaiʻi and has the highest population density.…”

Managing retreat for sandy beach areas under sea level rise

“…It is estimated that 1 m of SLR exposure will cover an area of 105 km 2 of land in Hawaiʻi, which has a land and dwelling value of $19 billion ($2013) 30 , 31 . By the end of the century, half of Hawai‘i’s sandy shorelines may be at risk of beach loss, threatening native ecosystems, cultural practices, and recreational use 32 – 35 . SLR is also projected to threaten many homes and jeopardize major infrastructure such as wastewater systems and coastal highways, posing an increased public safety and health risk and disrupting community livelihoods 29 , 32 , 36 – 38 .…”

“…By the end of the century, half of Hawai‘i’s sandy shorelines may be at risk of beach loss, threatening native ecosystems, cultural practices, and recreational use 32 – 35 . SLR is also projected to threaten many homes and jeopardize major infrastructure such as wastewater systems and coastal highways, posing an increased public safety and health risk and disrupting community livelihoods 29 , 32 , 36 – 38 . Although there is little agreement on how to accomplish it, the preservation of the diverse uses and values of Hawaiʻi’s beaches in the face of SLR has been emphasized by coastal managers 39 .…”