The Role of Large Woody Debris in Beach-Dune Interaction

Coastal foredune evolution involves complex processes and controls that result from the interaction of aeolian and nearshore dynamics. No studies to date have quantified and examined the role of large woody debris (LWD) as a modulator of sediment delivery across the backshore and as a control on foredune development and maintenance. Results from a 4-year research initiative on a high-energy, macrotidal beach, and foredune system show that storm events lead to wave-induced erosion of the backshore and consequent reworking of the LWD matrix. The exposed LWD matrix subsequently traps wind-blown sand on the upper beach, reducing sediment delivery to the foredune by 99% in some cases. In turn, deposition within the LWD matrix leads to rapid burial of the LWD, at least until the next reworking or dune erosion event occurs. Interannual observations at this site indicate that infilling of the accommodation space within the LWD matrix can be rapid, so sediment starvation of the foredune is typically a relatively short-lived phase. This suggests that that the LWD matrix is a highly effective, yet ephemeral, sand-trapping reservoir. Critical to these interactions is the frequency and magnitude of nearshore events that erode the beach periodically and reorganize the LWD matrix, which directly impacts the ability of LWD to modulate onshore sand transport to the foredune, store sediment in the backshore, and act as a buffer against erosive events. An empirically derived conceptual model explaining these relationships is presented.