Mapping Coastal Erosion Hazard Areas in Hawaii: Observations and Errors

Coyne, M; Fletcher, C; Richmond, B

High-density shoreline change rates are calculated for the purpose of mapping 60-year coastal erosion hazard areas in Honolulu County, Hawaiian Islands. Recent (1996) and historical (1949 or 1950) aerial photographs serve as the basis from which rectified and georeferenced photomosaics are created using soft-copy photogrammetry and image processing software. Changes in the horizontal position of the beach toe, the chosen shoreline change reference feature, are measured along shore-normal transects spaced approximately 20 m (66 ft) apart along 5 non-contiguous shoreline segments. Average annual shoreline change rates are calculated at each transect using an end-point analysis of beach toe change. The boundary of the 60-year erosion hazard area is delineated by projecting the 1996 vegetation line along each transect a distance that is 60 times the average annual shoreline change rate. Analysis of beach toe movement along 873 transects reveals that statistically-significant erosion occurs on 8% of all transects, accretion is found along 9% of all transects, and 83% show long-term trends that do not fall outside of our total uncertainty range. Shoreline change rates vary from a maximum accretion rate of 0.9 ± 0.5 m/yr to a maximum erosion rate of -1.7 ± 0.2 m/yr. Estimates of the total uncertainty in the shoreline change rates include measurement errors inherent to the photogrammetric method and sampling errors associated with short-term fluctuations in the position of the toe related to beach morphodynamics but unrelated to the long-term shoreline trends we are measuring. We quantify sampling errors using field measurements of beach toe fluctuation during spring tide and observed variability in the toe position measured from beach profile data over a 42 month period. Seventeen percent of the 873 rate measurements fall outside of the total uncertainty range (sampling and measurement errors) at each transect, indicating rate significance, while 60% of all rates fall outside of the measurement uncertainty range alone. Thus, the major sources of error in calculating shoreline change rates are ambient environmental fluctuations in the shoreline change reference feature. Statistically significant negative shoreline change rates indicate the presence of erosion hotspots, the intensity and spatial boundaries of which are clearly delineated on erosion hazard maps.

Mapping Coastal Erosion Hazard Areas in Hawaii: Observations and Errors

Series