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Title: Field investigations of infragravity oscillations on an energy dissipative beach
Author: Davidson, M. A.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1991
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Simultaneous time-series measurements of the bidirectional, horizontal velocity field and free surface elevation were obtained using a two component electromagnetic current meter and a pressure transducer respectively. Field measurements were obtained within the inner 100m of the surfzone, on a high energy, dissipative beach, (Llangennith, Gower, S. Wales), during both storm (H$ = 2.75m) and calm (H(, = 0.75m) conditions. Of particular interest is the response of infragravity band, (0.003-0.05 Hz) oscillations to incident wave forcing. Shorewards dissipation of incident wave energy in the saturated region of the surfzone, (H,/ft = 0.44), is accompanied by a shorewards growth in infragravity energy. Shoreline levels of infragravity variance increase linearly with the squared breaker height, leading to a pronounced dominance of infragravity energy during high energy conditions, particularly within the inner surfzone. Cross-spectral analysis of shore-normal velocity components and surface elevation time-series show infragravity motions to be standing in the cross-shore direction with at least three zero crossing nodes in the offshore decay structure of the long waves. Analyses of the infragravity band were carried out in terms of a spectral transformation function, (M(x,f)), the predicted spectrum observed by an offshore located sensor, if the shoreline amplitude spectrum is white with unit spectral energy density. The spectral transformation function accurately predicts significant peaks and valleys in the onshore velocity and surface elevation spectra, but not in the longshore current spectra. These observations in combination with theoretical arguments indicate that the infragravity spectra are dominated by free, edge waves forced near resonance with significant contributions from high modes. Storm day infragravity run-up spectra forced from short crested, swell dominated incident waves, (f,-= 0.0769Hz), were essentially white. Contrastingly, calm day long wave run-up spectra resulting from high frequency, (0.106 < f,-< 0.172) wind driven waves were significantly blue. Semi-empirical models for the prediction of (a) gravity and (b) infragravity band variance are developed which, when combined, provide a means of estimating the total free-surface elevation variance in the nearshore zone with a knowledge of the beach profile and incident wave conditions. A spectral transformation function driven by Eckart's (1951) solution for high mode (n= 4) edge waves forms the basis for infragravity band variance model.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available