Characteristics of beach erosion in headland bays due to wave action: Taking the Narrabeen beach in Australia as an example

Abstract

The special cape-shaped topography of a headland sandy bay is the reason its shoreline varies differently than that of a straight-line coast. The Narrabeen embayment is located on the southeastern coastline of Australia approximately 20 km north of Sydney. The enclosed sandy beach is 3.6 km in length and is composed of fine to medium quartz sand overlying sandstone bedrock, with a median sand grain size D50 of 0.3 mm. Due to the prevalence of moderate to high wave energy conditions and the exposure of the central and northern portions of the beach to the prevailing south-southeastern swell waves. The morphodynamic responses of the Narrabeen beach are highly variable and rapid alongshore. The beach state was predominantly influenced by episodic storm events throughout the year when erosion and accretion can occur. This study analyzed measurements of cross-shore profiles of the Narrabeen headland sandy bay from 1997-1998. With reference to an atypical huge-wave event, this paper describes in detail the beach erosion caused by huge-waves and the beach restoration caused by small waves after the huge-wave event. It also details the relationship between shoreline variation and wave parameters (wave height and wave angle). To discuss the regulations and causes of beach rotation, the study selected six huge wave events that occurred in 1990–2000, all of which were followed by over eight months of small waves, and analyzed the alongshore variation. The results show that the morphodynamic responses of the Narrabeen beach are highly variable and rapid alongshore. During the huge wave events, the beach was eroded and the shoreline retreated. In the aftermath, the small waves restored the beach. The difference in the shoreline variation due to small waves exhibits a rotational pattern. Wave height is related to sand volume above mean sea level and sand accumulation was rapid and high after the big nearshore wave events. The Southern Oscillation Index (SOI) is closely related to the variability in the shoreline. Time-lagged cross-correlations between monthly beach shoreline positions and the SOI were calculated to investigate the presence of stronger cross-correlations between the beach shoreline and the SOI; the central profile (profile 4) is the most sensitive to the feedback of SOI change. The small waves after a huge-wave event make beach rotation periodic. Analysis of directional wave data and the SOI suggests that beach change is driven by changes in wave height (power) and a subtle change in wave direction, both of which are positively related to the SOl. The change in the SOI induces changes in the wave angles, which leads to sediment transport. This is the most important factor in beach rotation. These results reconfirm huge-wave and small waves subaerial morphology, in particular, these results confirm that the wave height and angle, which are induced by the SOI after a huge-wave event, are key drivers of alongshore variation in headland sandy bay coastlines. Under small wave conditions, the northern and mid beach profiles have negative correlations with SOI index, while the southern beach profiles have positive correlations with SOI index. This lead to sediment erosion in the north while sediment deposition in the south. And the shoreline thereby shows a rotational pattern. With the variation of the SOI index, the sediment movement changes in each beach profile, and the shorelines rotational pattern shows periodicity.