HAWAII SHORELINE MANAGEMENT: TURNING FROM ARMORING TO AVOIDANCE THROUGH EROSION MAPPING

Before 1998 coastal erosion in Hawaii was managed by seawall construction. Now authorities emphasize beach preservation with increased static setbacks, short-term revocable permits for sandbag armoring and sand replenishment. However, these measures lack long-term sustainability and new emphasis is placed on avoiding erosion in the earliest stages of planning. Authorities are evaluating a new setback regime defined by construction prohibition at a distance from the state shoreline (typically the vegetation line) calculated as the annual erosion rate times the defined life expectancy of the adjacent building plus 60 feet. This setback can only be implemented with high-resolution erosion rate data currently under development at the University of Hawaii in collaboration with the U.S. Geological Survey, the State of Hawaii and the County of Maui. Erosion rates are determined every 20 m using orthorectified aerial photogrammetry. This process, known as conflation, yields a total measurement uncertainty that is the root sum of squares of rectification error, digitizing error, error due to incorrect identification of the shoreline reference feature (toe of the beach), and pixel accuracy (0.5 m). Tidal fluctuation is the maximum horizontal excursion of the toe (3.0 m) and seasonal fluctuation is the difference between mean summer and winter positions of the toe in a time series of beach profiles (+3.7 m). Beach profiles allow calculation of the 3-dimensional sediment volume flux associated with historical erosion and accretion trends. Least median of squares linear fits to erosion trends are used to determine the 30 yr erosion hazard zone as an additional planning guideline. Planners access these data at http://www.soest.hawaii.edu/coasts/cgg_main.html.