MAPPING SHORELINE CHANGE USING DIGITAL ORTHOPHOTOGRAMMETRY ON MAUI, HAWAII

Digital, aerial orthophotomosaics with 0.5 – 3.0 m horizontal accuracy, used with NOAA topographic maps (T-sheets), document past shoreline positions on Maui Island, Hawaii. Outliers in the shoreline position database are determined using a least median of squares regression. Least squares linear regression of the reweighted data is then used to determine a shoreline trend (AEHR – average annual erosion hazard rate or accretion). The resulting database consists of approximately 3350 separate erosion rates spaced every 20 m along 70 km of sandy shoreline. Three regions are analyzed: Kihei, West Maui, and North Shore coasts. The Kihei Coast has an average annual erosion hazard rate (AEHR) of about 0.3 m/yr, over 2 km of beach loss and 26 percent beach narrowing in the period 1949 - 1997. Over the same period the West Maui coast has an average AEHR of about 0.2 m/yr, about 3 km of beach loss and 37 percent beach narrowing. The North Shore has an average AEHR of about 0.4 m/yr, 0.8 km of beach loss and 12 percent beach narrowing. The mean, island-wide AEHR is about 0.3 m/yr. Island wide changes in beach width show a 25 percent decrease over the period 1949/1950 to 1997/2002. Island-wide, about 6 km of dry beach has been lost since 1949 (i.e., high water against hard engineering structures and natural rock substrate). Over the next 30 years over 4 km of coastal roadway is threatened by erosion.