Coastal Erosion (Line): Hawaii: 3.2-ft Sea Level Rise Scenario

Studies of historical shoreline change using aerial photographs and survey maps show that 70% of beaches on Kauai, Oahu, and Maui are eroding (receding landward) (Fletcher et al. 2012). Beaches exist in a delicate balance between existing water levels, wave energy, and sand supply.

Coastal erosion modeling was conducted for sandy shorelines of Kauai, Oahu, and Maui by the University of Hawaii Coastal Geology Group. The methods are described in Anderson et al. (2015) and combine historical shoreline change data with a model of beach profile response to sea level rise from Davidson-Arnott (2005) in order to estimate probabilities of future exposure to erosion at transects (shore-perpendicular measurement locations) spaced approximately 20 meters apart along the shoreline. The model accounts for localized alongshore variability in shoreline change by incorporating trends from the historical erosion mapping studies.

Historical data used to model coastal erosion consisted of: (1) historical shoreline positions and erosion rates measured from high-resolution (0.5 meters) ortho-rectified aerial photographs and NOAA topographic charts dating back to the early 1900s (Fletcher et al. 2012, Romine et al. 2013), and (2) beach profile field survey data (Gibbs et al. 2001, Fletcher et al. 2012). The vegetation line was identified in the most recent aerial photography dating from 2006 to 2008.

The output of the modeling is the estimated exposure zone to future erosion hazards. Based on the model and IPCC AR5 RCP8.5 sea level rise scenario, there is an 80% probability that land impacted by erosion would be confined within the exposure zone at that particular time. The exposure zones extend landward from the current-day shoreline (vegetation line) up to the 80% cumulative probability contour from sea level rise at 0.5, 1.1, 2.0, and 3.2 feet, which incorporates the uncertainty (upper and lower bounds) of the IPCC RCP8.5 sea level rise projection. This particular layer depicts coastal erosion using the 3.2-ft (0.9767-m) sea level rise scenario. While the RCP8.5 predicts that this scenario would be reached by the year 2100, questions remain around the exact timing of sea level rise and recent observations and projections suggest a sooner arrival.

Assumptions and Limitations: Historical shoreline change data and beach profiles needed to model coastal erosion are available only for sandy shores of Kauai, Oahu, and Maui. Exposure was not modeled for less-erodible rocky coasts and bluffs, though the latter can be prone to sudden failure in some areas. In addition, modeling did not account for: (1) existing seawalls or other coastal armoring in the backshore; (2) increasing wave energy across the fringing reef with sea level rise; (3) possible changes in reef accretion and nearshore sediment processes with sea level rise; and (4) possible changes to sediment supply from future shoreline development and engineering, such as construction or removal of coastal armoring or other coastal engineering.

Users of these data should cite the following publication:

Anderson, T.R., Fletcher, C.H., Barbee, M.M., Frazer, L.N., and B.M. Romine (2015). Doubling of Coastal Erosion Under Rising Sea Level by Mid-Century in Hawaii, Natural Hazards, doi:10.1007/s11069-015-1698-6.

For further information, please see the Hawaii Sea Level Rise Vulnerability and Adaptation Report: http://climateadaptation.hawaii.gov/wp-content/uploads/2017/12/SLR-Report_Dec2017.pdf

Data and Resources

Additional Info

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Last Updated September 19, 2019, 14:54
Created September 19, 2019, 14:54
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