Headland bypassing timescales: Processes and driving forces

In this paper, a natural headland bypassing is investigated in terms of its short (months to years) and long-term (years to decades) variability and its relationship with wave conditions, climate drivers and anthropogenic interventions. The research is focused on Fingal Head (New South Wales, Australia) where nine detailed topo-bathymetric surveys were undertaken between June 2018 and January 2020. To extend the analysis in time, over 30 years of satellite and aerial images were used to describe the headland bypassing variability based on the shoreline and sandbar position changes. Shoreline and sandbar positions presented moderate to strong correlation between updrift and immediate downdrift of the headland highlighting the influence of the bypassing process for the longshore transport on the study area. Results indicate that the headland bypassing around Fingal Head is governed by two distinct processes and their dominance is controlled by waves and sediment availability. The sandbar-driven bypassing scenario requires a storm wave event to trigger the sandbar system and over seven months to complete the full bypassing cycle. A quicker bypassing cycle (i.e. few months) happens when sediment leaks around the headland following a persistent low energy wave condition that largely accretes the updrift upper beach. Headland bypassing cycles occur in multiple timescales, including seasonal variability of the wave climate to interannual and decadal cycles of shoreline progradation and retreat. Shifts in the large-scale climate drivers such as El Niño-Southern Oscillation, Pacific Decadal Oscillation and Interdecadal Pacific Oscillation were observed to influence on changes to the low frequency of variability of the headland bypassing in the study area by affecting the predominant wave direction and updrift beach sand availability. The understanding of this intermittent nature of the headland bypassing process and particularly considering its periodicity and the related driving forces is crucial to predict future coastal hazards and develop management strategies.