Failure to protect beaches under slowly rising sea level

Abstract

Coastal land use in the USA is regulated by a chain of integrated federal to local policies that emphasize preservation of open space, public access, and the environment. Are these policies prepared to achieve lawfully stated objectives under accelerating sea level rise? To test the efficacy of these policies during the past century of slowly rising sea level, and thus their potential effectiveness under future accelerated sea level rise, we quantify land use on a section of windward Oʻahu coast, 1928–2015. Data show a shift from stable/accreting shorelines and wide beaches, to expanding erosion and beach loss concurrent with increasing development and seawall construction; trends at odds with policy objectives. Shoreline hardening increased 63%. Net shoreline change shifted from stable/accreting to erosional on 74% of the coast. More than 45% of this shift was due to flanking (erosion triggered by nearby hardening). Five geomorphic settings were analyzed. Prior to local coastal policy, from 1928 to 1975, headland beaches were mildly erosional with average change rates − 0.07 ± 0.1 m/yr. Beaches in all other geomorphic categories accreted. Between 1975 and 2015, following enactment of coastal zone management policy, average change in all geomorphologies shifted to erosional, coastal development and seawall construction expanded, average beach width declined. Today, nearly 20% of beach length has been lost; 55% of beaches have narrowed. Failure to achieve policy goals under slow, historic sea level rise (1.2 to 1.4 mm/yr) implies preserving beaches, open space, and public access will require new policies, or more effective ways for implementing existing policies, in a future characterized by accelerated sea level rise.