Sciences marines : recherche et développement

Abstrait

Analysis of Short-Term Wave Spectra and its Effects on Sandy Shoreline at a Strand Coastal Plain in Southeastern Nigeria

Saviour P Udo-Akuaibit

An intensive field study on near-shore waves swash oscillations was carried out to ascertain its oscillation patterns and effects on shoreline morph dynamics at the shoreline, east of Qua-Iboe River estuary, Southeastern coastal plain of Nigeria. Time-series measurements of wave swash frequencies, wave- breaker heights and water depths, long shore current velocities over a near-spring tidal cycle and daily beach profile surveys were made at the shoreline. Detailed analysis of result of the wave swash spectra revealed a dominance of gravity waves over infragravity waves oscillations within spectral frequencies intervals of 1 to 0.0333Hz and 0.0322 to 0.0067Hz respectively with average spectral frequency ercentages of occurrences of 52.28 and 47.72 accordingly. The infragravity wave’s frequency domains are further filtered based on oscillation patterns, into First, Second, Third, and Fourth Harmonic waves groups. However, Fifth Harmonic waves group is the gravity wave spectrum with frequency domains from 1to 0.0333Hz. Spectral energy indices of the wave spectra revealed that the First Harmonic Waves Spectrum recorded the highest values which fluctuated between 0.5 and 1.0 while the Fifth Harmonic Waves Spectrum recorded the lowest values which ranged from 0.01 to 0.02. The First and Second Harmonic groups of infragravity wave’s frequencies were noted as the principal agents of shoreline morphodynamics. In addition, the First Harmonic group scours sediment, causes erosion and sediment transports, generates higher long shore currents velocities, and increases rip currents energetics in the surf zone. Whereas, gravity waves transport, spread, winnow and sort sediments accordingly. The Third, Fourth and Fifth Harmonic Waves frequencies facilitate the roles of the First and Second Harmonic Waves at the shoreline. Moreover, the gravity and infragravity waves oscillations at the shoreline complete a wavelength at every 2- 3 minutes interval, independent of tidal stage, and play significant role in self-cleansing potential of the surf zone. Essentially, knowledge of energy radiation and transmission by the First Harmonic group of infragravity waves oscillations is considered crucial in engineering designs and management of shoreline protection structures, constructions of marinas, harbors, seaports, etc.