HISTORICAL SHORELINE CHANGE AND COASTAL LANDFORM MAPPING AT KALOKO-HONOKAHAU NHP, KONA COAST, HAWAII

A collaborative project between the U.S. Geological Survey and the National Park Service has been established to create an inventory of geologic resources for National Park Service lands on the Big Island of Hawai'i. This involves creating coastal-landform maps and conducting a long-term shoreline change analysis. At Kaloko-Honokahau National Historic Park, the parks' shoreline consists of a low-lying basalt terrace or bench overlain by carbonate sand and gravel beaches. Two types of beach are present: a normal intertidal to supertidal accumulation of beach sediment, and perched, storm beaches that are typically active only during large-wave events.

The long-term shoreline change analysis is based on a time series of rectified aerial photographs extending as far back as 1950. A consistent shoreline reference feature (SRF) is digitized from the photos in a GIS to quantify the migration of the SFR over time. The SRF used in this analysis is the seaward contact between the light-colored beach and the underlying basalt terrace. The landward extent of overwash sand deposits were also digitized.

The position of the SRF varies over time and is oscillatory in nature. For example, north of the Kaloko fishpond wall, there is a long-term net erosion of the SRF of as much as 20m. However, the time series analysis shows the SRF has shifted both landward and seaward throughout the 52-year analysis period. Between 1950 and 1970, the SRF in this area accreted 13 m. The overwash extent shows a very similar pattern, shifting in the same direction as the SRF. Additional evidence for this shoreline oscillation exists in the form of beachrock deposits exposed on the basalt platform. These deposits consist of both detrital carbonate and basaltic sediment and are thought to have formed within a beach that was further seaward than at present. The beachrocks are lightly cemented and very friable indicating a young age, and are possibly remnants of the 1950-70 accreted beach.