Paper No. 26-3
Presentation Time: 9:15 AM
CONTRASTING BEACH DYNAMICS IN ABUTTING LITTORAL CELLS WITH A REEF BOUNDARY, KAANAPALI BEACH, MAUI, HAWAII
An offshore reef separates the continuously sandy shoreline of Kaanapali Beach, Maui, Hawaii into two distinct littoral cells: the Hanakaoo Littoral Cell (HLC) and the Kaanapali Littoral Cell (KLC). Although the two littoral cells are not separated by a subaerial feature, such as a rocky headland, the cells respond to oceanographic changes with distinct and contrasting dynamics. A shallow fringing reef is located offshore of the HLC, while the KLC to the north is not protected by an offshore reef. The north end of the reef, Hanakaoo Point, acts as a boundary between the two cells. Sand transport within the cell changes with the wave climate. Waves move approximately 25,000 cubic yards of sand each season, resulting in 200 feet of landward or seaward migration of the shoreline. During the summer months, sand builds up adjacent to Puu Kekaa, a rocky headland and the northern boundary of the KLC. During the winter months, sand builds up to the south at the shared boundary between the two cells, Hanakaoo Point. The formation of this sand point is dependent on the wave refraction pattern across the fringing reef, which begins just to the north of the point. In contrast to the KLC, beach dynamics within the HLC are driven by changes in water level, with larger changes correlated to higher water levels and concurrent wave events, regardless of the season. Alongshore sediment transport is negligible during most seasons, as large waves become shore-parallel near the coastline due to refraction by the fringing reef. Long-term trends for both cells tend to be strongly influenced by infrequent, extreme erosion events. Extreme erosion events within the KLC occur during extended or above average seasons of winter swell, such as during El-Nino years, or summer swells. Extreme erosion events within the HLC occur during periods of high water levels, such as during spring tides and mesoscale eddies, when more wave energy is able to traverse the reef flat. Hanakaoo Point extreme erosion events are a combination of both seasonal impacts, at the end of summer, and higher water levels concurrent with waves or swell from a southerly direction. The contrasting dynamics that characterize Kaanapali Beach are common on tropical island shorelines and must be considered as part of development and conservation efforts in these environments.