Empirical Projection of Long-Term Coastal Erosion Hazards in Hawaii Under Rising Sea Levels: Preliminary Findings

Chronic erosion dominates sandy beaches of Hawaii causing loss and beach narrowing; and damaging homes, infrastructure, and critical habitat. Increased rates of sea level rise (SLR) will likely exacerbate these problems. Shoreline managers and other stakeholders need guidance to support long-range planning and adaptation efforts. Despite recent advances in sophisticated numerical models, there remains a need for simple approaches to estimating land areas that are threatened by erosion on decadal-to-century time scales due to SLR. While not as detailed as numerical models, empirical approaches can provide a first-order approximation to shoreline change that may be useful for coastal management and planning. Shoreline managers in Hawaii commonly work with historical data to provide information on coastal erosion. Simple linear regression methods have been especially attractive in Hawaii, where complex reef topography can cause high spatial variability in sediment transport patterns. Yet, facing projected future increases in the rate of SLR, extrapolating historical trends is insufficient. Predictions of shoreline change with SLR commonly employ controversial geometric models (e.g., the Bruun Model) that do not account for sediment availability and alongshore variability captured in historical data. Furthermore, these two projections often produce conflicting results. We report here on the early results of mapping probability-based erosion hazard areas, determined by combining the extrapolated historical shoreline change model with a geometric model of shoreline response (Davidson-Arnott, 2005) to strictly accelerated SLR. A geographic information system is used to explore the intersection between potential erosion hazards, coastal geology, and development patterns. This approach is attractive because it is simple and utilizes existing datasets. Yet, its simplicity implies broad assumptions of the coastal system and leads to large uncertainty in projections. To investigate the applicability of such an approach and identify management needs, erosion hazard areas are projected at various time intervals and confidence levels for ten Hawaii beaches. Ref.: Davidson-Arnott, R. (2005) Conceptual Model of the Effects of Sea Level Rise on Sandy Coasts, J. Coast. Res. 21:1166-1172.