QUANTIFICATION OF SHORELINE RHYTHMICITY

Abstract

The study of beach morphology, for example, its changes with wave and tide conditions, is facilitated by the development of simple numerical values which characterize the morphology. Multivariate (EOF) analysis of topographic contour data is a means for determining important morphologic components which vary independently. If these components correspond to familiar shoreline features the researcher considers important, then the development of each component can be quantified by its significance, or weighting, in each sample. Alternatively, the components may be complicated and not useful in quantifying beach morphology. A study of these morphologic components, however, can provide insights into the dynamics of the beach system. If multivariate analysis produces complicated components, an alternative approach, of subjectively identifying shoreline characteristics of interest, can be taken. The characteristics may be the same as those frequently used in past studies, such as beach slope or sand volume. It is likely, though, that EOF analysis of topographic data will suggest more sophisticated characteristics which should be used. Some of these, for example, mean shoreline position or amplitude of a rhythmic shoreline, may be easily quantified, whereas, others such as longshore position of rhythmic features or cusp width relative to embayment width, may be more difficult to quantify. Both of these analysis approaches were applied to beach survey data obtained over a period of ten months (including the El Nifto winter of 1982/83) on Siletz Spit, Oregon. The shoreline was rhythmic with an 800-850 m wavelength throughout the duration of the study. Rhythmic topography has been associated with significant past beach and dune erosion at this site. Hence, it is of interest to describe the beach morphology quantitatively, and relate three dimensional beach changes to wave and tide conditions.