ABSTRACT: Nearshore morphodynamics and the relative role of low frequency wave motion

CGRG Bibliography of Canadian Geomorphology
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Abstract

A deterministic theory explaining the development of many types of rhythmic and irregular nearshore features as a function of specific modes of wave motion in coastal geomorphic systems was proposed by Holman and Bowen (1982). The model incorporates secondary drift velocity fields associated with various combinations of edge waves which produce equilibrium forms that resemble those commonly found along many non-cohesive shorelines. The viability of this theory has remained in question because of difficulties in substantiating the theoretical constructs with empirical evidence. Detailed measurement of the fluid and sediment motions at Wymbolwood Beach, Ontario, Canada, demonstrated the validity of the deterministic theory. One shore-parallel array and two shore-normal arrays of instruments comprising eighteen continuous-resistance, surface-piercing waves staffs, and fifteen electromagnetic biaxial current meters were deployed across an experimental area spanning 130 m alongshore and 156 m offshore. These instruments were cable-linked to a high-speed data acquisition and mass storage system, and the data were transferred to a mainframe computer and analysed using spectral methods. Discrimination of wave modes was accomplished on the basis of velocity phase relationships, of spatially-lagged cross-spectral analysis, and of the nodal frequency structure of spatially-distributed wave energy spectra. Morphological changes were measured using standard profiling techniques, depth of disturbance rods, and a boat-mounted fathometer. During an intense storm, a mode 3, standing edge wave at 0.035 Hz dominated the low frequency nearshore motions. An examination of the associated morphological changes, showed clearly that the nearshore bar-trough systems adjusted so as to attain a bathymetry that approached an 'equilibrium' state with the persistent long wave forcing. The bar forms migrated landward slightly to conform more closely with the offshore position of the wave antinodes, and an alongshore pattern of scour and accretion developed along an offshore bar with length scales coincident with the standing wave form. A subsequent storm caused a markedly different response in the morphology, producing strongly rhythmic features close to shore and linear features farther alongshore. Both configurations are consistent with theory, and the lack of alternative mechanisms of sediment rearrangement leads to the conclusion, that edge waves played a dominant role in governing the nature of the morphological changes during the study period.

Bibliography of Canadian Geomorphology