HR: 0800h
AN: H51C-1128    [Abstracts]
TI: Are They Beach Cusps or Beach Horns? A Kauai Observation
AU: * Pederson, D T
EM: dpederson2@unl.edu
AF: University of Nebraska-Lincoln, 304 Bessey Hall Department of Geosciences, Lincoln, NE 68588-0340 United States
AB: Beach cusps on a southwest Kauai shoreline exhibit departures from the current schools of thought on cusp dynamics. Beach sediment is either carbonate sand or volcanic sand. The horns of the beach cusps are depositional features, reflecting the migration of sediment along the beach during the waning storm-wave (energy) phase and represent a building of the beach face. This phase may be comparable to the development of distinct sediment-transport bedforms in a unidirectional flow environment that develop at distinct ranges of Froude number. Heterogeneities such as groundwater-sapping patterns after storms, protruding bedrock, and variation of sediment size along the initial beach front (perhaps as a result of previous cusp development) provide the initial irregularities to initiate cusp morphology with evolution to a more rhythmic pattern because of the major feedback processes in the beach cusp environment. Although cusps may be stationary at some locations, transport of sediment along the shore is still occurring. The term beach horns, instead of beach cusps, should be used because the morphology is developed as a consequence of horn development. At the start of the field observation, long-period (LP) waves from south-southwest of Kauai dominated the shorter period Trade-Wind (TW) waves which are refracted around the island. The LP waves generated high-surf warnings. At this time, the cusps had an asymmetric pattern with the main backwash close to the horn located on the side of the cusp that represents the direction of longshore transport. Active erosion of the horn took place on the side of the cusp where more backwash occurred. The horn crests had an acute angle towards the approaching LP wave front. As the energy of the LP waves decreased, the TW waves became more dominant in the interference pattern of the two wave forms. The beach cusps reflected this change of interference in terms of symmetry as they became more symmetrical. Significant transport of sediment within the cusp feature was limited to high-tide periods. Troughs form offshore in front of the horns and sediment buildup occurs offshore of the cusps. Backwash in the cusp interferes with incoming waves, resulting in sediment deposition. In contrast, waves impinging on the horns had no interference from backwash and eroded the offshore troughs, depositing the sediment on the surface of the horn with additional sediment being carried into the cusps. Beach cusp processes were the same on the two types of beaches along this shoreline.
DE: 4558 Sediment transport
DE: 1815 Erosion and sedimentation
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting