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Abstract

An in-depth study on effect of infragravity wave’s oscillations on shoreline morphodynamics was carried out to determine its implications on formation, development and destruction of beach cusps at Ibeno beach along a strand coastal plain sand in southeastern Nigeria. Detail analysis of field data acquired over a neap-spring tidal phase a strand that infragravity wave’s oscillations frequencies ranged from 0.0322 to 0.0067Hz. The effects on the horeline were classified according to the order of harmonic frequencies spectra of wave swash patterns: First, Second, Third and Fourth Harmonics Frequencies respectively. However, it was noted that wave swash of the first harmonics infragravity waves frequencies with the highest spectral energy index (SEI), which ranged from 0.5 to 1.0, formed the bays and displaced sediment from the centre of the bays to the rims of the wave swash which developed into horns. Whereas, backwash processes transported sediment seaward from the centre of the bays and formed a depression accordingly. Moreover, the second and third harmonics infragravity wave’s frequencies broadened the bays. However, interference of the fourth harmonic infragravity waves frequencies with the fifth harmonic (gravity) waves frequencies (0 - 0.033Hz) generated oscillatory and turbulent motions which sorted and winnowed sediments in the bays. While sediment displaced from two adjacent bays converged to form the horn between two bays. Averaged beach cusps wavelength increased and fluctuated between 38.50 and 44.30m. On the other hand, the first harmonic waves frequencies were also observed to be responsible for the destruction of beach cusps during flood tide. Result of beach profile surveys at the horns and bays locations within the studied transect generally showed erosion which is an indication of the fact that beach cusps formation is independent of beach volumetric change.

Keywords: Infragravity waves; Beach cusps; Horns and bays; Accretion; Erosion