Historical shoreline changes on beaches of the Hawaiian Islands with relation to human impacts, sea level, and other influences on beach dynamics

Historical changes in shoreline position are measured along the beaches of Kauai, Oahu, and Maui (Hawaii) using shorelines digitized from aerial photographs and survey charts dating back to the early 1900s. Over the past century, erosion (recession) was the dominant trend of shoreline change in Hawaii with an overall average shoreline change rate of -0.11 m/yr and 70% of beaches eroding, including 21km of beach (9% of beaches studied) that completely disappeared. Maui beaches were the most erosional of the three islands (85% erosional, island-wide average rate of -0.17 m/yr). Seventy-one percent of Kauai beaches eroded (average rate -0.11 m/yr), including 8% that completely disappeared. Sixty percent of Oahu beaches eroded (average rate -0.06 m/yr), including 8% that completely disappeared. Coastal armoring (e.g., seawalls) contributes to beach narrowing and loss by limiting the ability of an eroding beach to migrate landward. On Oahu 72% of beaches that narrowed were fronting coastal armoring, including 8.6 km of beach that completely disappeared. This is in comparison to unarmored beaches where beach widths remained relatively stable (53% of beaches narrowed). Island-wide shoreline trends are recalculated for Oahu and Maui after optimizing the data to control for human impacts and applying a series of consistency checks on the results. Differing rates of relative sea-level rise around Oahu and Maui (~65% faster around Maui) remain as the best explanation for the difference in island-wide shoreline trends after examining other influences on shoreline change including waves, sediment supply and littoral processes, and anthropogenic changes. Patterns of historical shoreline change along the northeast coast and other regions of the island of Oahu are examined for spatial relationships to coastal geomorphology and temporal relationships to late-Holocene sea-level changes. Multiple lines of geologic evidence indicate that headlands, comprised largely of unconsolidated carbonate beach and eolian sediments, were formed during late- Holocene sea-level fall. We infer that a change from headland beach accretion to the observed modern pattern of headland beach erosion is related to the initiation of sea-level rise around Oahu following an earlier period of falling sea level over the past few thousand years.