A century of shoreline change along the Kihei coast of Maui, Hawaii

Increasing demands on coastal areas drive a need for accurate projections of coastal erosion hazards. However, our understanding of and ability to predict coastline migration on timescales of years and decades remains limited. Historical shoreline change is investigated for a study site encompassing 5 km of the Kihei coast on the island of Maui in the Hawaiian archipelago. Horizontal movement of the landward and seaward boundaries of the beach from aerial photographs and topographic surveys (T-sheets) is used to develop a decade to century scale high-resolution database of sediment volume changes. Over the entire period covered by the study (1900–1997) the northern portion of the site accreted 4.2 x 105 m3 of sediment, while 1.5 x 105 m3 eroded from the southern portion. The database of volume changes is used to develop a time series of net longshore sediment transport for periods between historical shorelines, which shows significant variation in the direction and magnitude of net longshore sediment transport over the past century. The pattern is consistent with, and believed to be largely a result of, variations in Kona storm activity. Occasional large Konas storms (rain-bearing stormy winds from the southwest) result in high rates of northward longshore sediment transport. The more typical trade wind-driven eolian transport is to the south, but at much lower rates. Hurricanes, tsunami, and south swell appear to be of secondary importance in changing this coastline. The Pacific Decadal Oscillation (PDO) is hypothesized to cause multi-decadal shifts in mean Kona storm activity in the vicinity of Hawaii, resulting in the majority of shoreline changes documented at the Kihei site. A possible mechanism is by altering the position of the ridge aloft relative to the islands. The hypothesis is investigated using models of wave transformation and longshore sediment transport. Statistically significant correlations between the PDO cycle, Kona storm activity, and net longshore sediment transport suggest that the PDO does exert a strong control on decadal scale changes along this coastline. Consideration of the PDO may improve our understanding of coastal sediment dynamics, enhancing existing efforts to forecast erosion hazard areas and effectively manage sandy shorelines.