National assessment of shoreline change: Historical shoreline change in the Hawaiian Islands (2012)

Beach erosion is a chronic problem along most open-ocean shores of the United States. As coastal populations expand and community infrastructure comes under increasing threat from erosion, there is a demand for accurate information about trends and rates of shoreline movement, as well as a need for a comprehensive analysis of shoreline movement that is consistent from one coastal region to another. To meet these national needs, the U.S. Geological Survey began an analysis to document historical shoreline change along open-ocean sandy shores of the conterminous United States and parts of Hawaii and Alaska. An additional purpose of this work is to develop systematic methodology for mapping and analyzing shoreline movement so that consistent periodic updates regarding coastal erosion can be made nationally. This report on shoreline change on three of the eight main Hawaii islands (Kauai, Oahu, and Maui) is one in a series of reports on shoreline change in coastal regions of the United States that currently include California, the Gulf of Mexico region, the Southeast Atlantic Coast, and the Northeast Atlantic Coast. The report summarizes the methods of analysis, documents and interprets the results, explains historical trends and rates of change, and describes the response of various communities to coastal erosion. Shoreline change in Hawaii was evaluated by comparing historical shorelines derived from topographic surveys and processed vertical aerial photography over time. The historical shorelines generally represent the past century (early 1900s–2000s). Linear regression was used to calculate rates of change with the single-transect method: long-term rates were calculated from all shorelines (from the early 1900s to the most recent), whereas short-term rates were calculated from post-World War II shorelines only. Beach erosion is the dominant trend of shoreline change in Hawaii. However, shoreline change is highly variable along Hawaii beaches with cells of erosion and accretion typically separated by only a few hundred meters on continuous beaches or by short headlands that divide the coast into many small embayments. The beaches of Kauai, Oahu, and Maui are eroding at an average long-term rate for all transects (shoreline measurement locations) of -0.11 ± 0.01 m/yr (meters per year) and an average short-term rate of -0.06 ± 0.01 m/yr. The majority, or 70 percent, of transects on the three islands indicate a trend of erosion in the long term and 63 percent indicate a trend of erosion in the short term. A total of 22 kilometers of beach, or 9 percent of the total length of beach studied, was completely lost to erosion over the past century. Annual erosion is greatest on Maui with an average long-term shoreline change rate of -0.17 ± 0.01 m/yr and erosion at 85 percent of transects. Short-term analysis for Maui indicates a similar erosional trend with an average rate of -0.15 ± 0.01 m/yr and erosion at 76 percent of transects. Nearly 7 kilometers (11 percent) of beach was completely lost to erosion in the analysis period on Maui. Annual erosion for all transects on Kauai is intermediate in the long term, with an average rate of -0.11 ± 0.01 m/yr and erosion at 71 percent of transects. The short-term average rate for Kauai (0.02 ± 0.02 m/yr) suggests stable or accreting beaches; though, the majority (57 percent) of transects indicate a trend of erosion. Six kilometers or 8 percent of Kauai beaches were completely lost to erosion in the analysis period. Oahu beaches are the least erosional of the three islands in the long term; though, erosion is still the dominant trend of shoreline change with an average long-term rate of -0.06 ± 0.01 m/yr and erosion at 60 percent of transects. Shoreline change trends on Oahu beaches are roughly the same in the short term as in the long term with an average rate of -0.05 ± 0.01 m/yr and erosion at 58 percent of transects. The single-transect method of rate calculation indicates that erosion rates are statistically significant (95-percent confidence interval) at 30 percent of transects in the long term and 22 percent of transects in the short term.