Arcuate duneline embayments, infragravity signals, rip currents and wave refraction at Waihi Beach, New Zealand

Arcuate duneline embayments (ADEs) are uneven rhythmic features cut into th e frontal dune and beach, ranging between 800m to 2500m long in the longsho re direction. Investigation of the causative processes of ADEs included: th e perusal of historical aerial photograph records; rip current observations ; identification of infragravity wave motions; and numerical simulation of wave refraction and focusing. ADEs were found to be erosional features that shift in location, but do not migrate in a consistent longshore direction. Infragravity wave signals ide ntified in the swash run-up distance time-series and in nearshore orbital v elocities were not maintained for sufficient duration to form rhythmic feat ures of a similar length scale to the ADEs. Rip current erosion over the st udy period was observed to be localised and on much smaller spatial scale t o the ADEs and several individual rip currents would tit into a single ADE. The focusing of waves at the beach due to refraction over offshore topogra phic irregularities was investigated by numerical simulation and provides a likely mechanism for the formation of ADEs at Waihi Beach with substantial agreement between peaks in longshore wave energy at the beach and the hist orical location of ADEs. We discern between the mechanism of "rip-current-e mbayment" formation (KOMAR, 1971,1983a,b; KOMAR and HOLMAN, 1986) and that of direct wave attack on the back beach and dune due to wave refraction and focusing, and suggest that the application of the latter mechanism is resp onsible for their formation.