Video-observations of shoreward propagating accretionary waves

A 10 year database of daily time-exposure images of the nearshore zone, collected by an Argus video system installed at Duck, NC, revealed a new phenomenon in the evolution of nearshore topography. Under some conditions, a nearshore sand bar can shed a small bar-like feature from its shoreward facing side. This pinched off daughter bar subsequently transits the trough as an intact feature and merges with the beach. The average onshore propagation rate of the feature is about 3m/day and, with an average size of about 130m by 30m and a height comparable to that of the inner bar, it represents a locally significant onshore sediment flux. Currentlyweareonlypartlyawareoftherangeofmor- phologicbehaviorthatcanoccurinthenearshorezone. Waves and currents have ample capacity to move sedi- ment around in the nearshore zone.The nonlinearities in both the sediment transport processes and the surf zone hydrodynamics carry with them a large potential forgeneratingunexpectedgradientsinsedimenttrans- portacrossthenearshoretopography,henceproducing unexpectedbathymetricchange.Documentingthenat- ural range of morphologic behavior in the nearshore zone is therefore indispensable to focus our thinking about nearshore morphodynamics. The present perception of breaker bar evolution is that bars generally reshape into linear forms in asso- ciation with storm events Under subsequent lower energy conditions, these bars are usually observed to move shoreward and to develop three-dimensionality that may be rhythmic or irregular and complex (Lippmann & Holman 1990). It bas been commonly assumed that the response of a bar to changing wave conditions occurs as an intact, albeit evolving, sand bar form. A large database of routinely collected time- exposure video images of the nearshore zone near Duck (North Carolina, USA) revealed that this is not alwaysthecase.Undersomeconditions,asandbarcan shed a small bar-like feature from its shoreward facing side.This pinched off daughter bar subsequently tran- sits the trough as an intact feature and merges with the beach (Fig. 1). Tentatively, we have named this phenomenon a Shoreward Propagating Accretionary Wave or, abbreviated, a SPAW. The term 'wave' was chosen to reflect the similarities between the observed phenomenon and a solitary wave in fluid dynamics. That is, both phenomena are single, isolated perturba- tions that maintain their shape as they propagate. In both cases, the latter involves a net displacement of material in the direction of propagation. In this paper we will present a first documenta- tion of this newly observed phenomenon and dis- cuss its relevance for understanding the nearshore morphodynamic system.