Shoreline recovery on wave-dominated sandy coastlines: the role of sandbar morphodynamics and nearshore wave parameters

Abstract

This study quantifies and characterises the temporal variability of shoreline recovery on a high-energy sandy coastline using a 10-year dataset of daily shoreline and sandbar positions from a Coastal Imaging station at Narrabeen-Collaroy Beach, Australia. Following a total of 82 individual storm events, rates of the cross-shore return of the shoreline to its pre-storm position were analysed. Observed rates during shoreline recovery were characterised by an overall mean of ~0.2m/day. Temporal variability in rates was most evident at shorter timescales of 1–2weeks and included rates most frequently between 0 and 0.3m/day, less frequent more rapid rates of up to 2m/day and also minor landward movements. This temporal variability was significantly correlated with nearshore forcing parameters describing the ratio of wave height to wave period, the cross-shore proximity (and attachment) of the sandbar to the shoreline and the rate of cross-shore sandbar migration. These findings are summarised in a new conceptual model that characterises temporal phases and rates of shoreline recovery corresponding to stages of onshore sandbar migration following a storm, from fully detached storm-deposited sandbar morphology through to complete sandbar welding with the shoreline. More gradual shoreline recovery rates are observed with fully detached and semi-attached sandbar conditions. In contrast, more rapid rates of shoreline recovery occur when sandbars are closer and attached to the shoreline, observed to be on average 3–4 times greater than for detached sandbars. In conditions with attached and semi-attached sandbars, shoreline recovery rates are negatively correlated to the forcing of nearshore wave steepness and dimensionless fall velocity, and coupled with concurrent rates of onshore sandbar migration. The findings provide insight into key parameters influencing shoreline recovery following storms.