Extreme coastal erosion enhanced by anomalous extratropical storm wave direction

Abstract

Extratropical cyclones (ETCs) are the primary driver of large-scale episodic beach erosion along coastlines in temperate regions. However, key drivers of the magnitude and regional variability in rapid morphological changes caused by ETCs at the coast remain poorly understood. Here we analyze an unprecedented dataset of high-resolution regional-scale morphological response to an ETC that impacted southeast Australia, and evaluate the new observations within the context of an existing long-term coastal monitoring program. This ETC was characterized by moderate intensity (for this regional setting) deepwater wave heights, but an anomalous wave direction approximately 45 degrees more counter-clockwise than average. The magnitude of measured beach volume change was the largest in four decades at the long-term monitoring site and, at the regional scale, commensurate with that observed due to extreme North Atlantic hurricanes. Spatial variability in morphological response across the study region was predominantly controlled by alongshore gradients in storm wave energy flux and local coastline alignment relative to storm wave direction. We attribute the severity of coastal erosion observed due to this ETC primarily to its anomalous wave direction, and call for greater research on the impacts of changing storm wave directionality in addition to projected future changes in wave heights.

Second-Pass Assessment of Potential Exposure to Shoreline Change in New South Wales, Australia, Using a Sediment Compartments Framework

“…An unusual storm featuring a coupled ETC and anticyclonic intensification impacted the entire southeast Australian coast in June 2016, generating high waves with ENE to E directions, and causing severe beach erosion along the NSW coast. The storm coincided with a spring high tide, and the unusual easterly storm-wave direction resulted in minimal wave transformation prior to entering coastal embayments and impacting beaches [1,53]. Considerable damage to properties and infrastructure occurred at several NSW beaches (e.g., Figure 2).…”

“…As the greatest beach erosion volumes are typically achieved when the pre-storm beach state is fully accreted [75], we derive the 25-m alongshore-spaced beach-dune profiles from two LiDAR surveys, which together capture a fully accreted beach state along the northern and southern parts of Wamberal Beach. The 2011 LiDAR survey, from which the alongshore-averaged profile was derived (Figure 9), captured an accreted state along the southern two-thirds of the beach, while a 2016 LiDAR survey [1] captured an accreted state along the northern Figure 9B compares the alongshore-averaged terrain profile for Wamberal Beach, with a profile from the narrow point of the sand barrier (Figure 10), as an example of terrain variability relative to the alongshore-averaged profile. As the greatest beach erosion volumes are typically achieved when the pre-storm beach state is fully accreted [75], we derive the 25-m alongshore-spaced beach-dune profiles from two LiDAR surveys, which together capture a fully accreted beach state along the northern and southern parts of Wamberal Beach.…”

“…As the greatest beach erosion volumes are typically achieved when the pre-storm beach state is fully accreted [75], we derive the 25-m alongshore-spaced beach-dune profiles from two LiDAR surveys, which together capture a fully accreted beach state along the northern and southern parts of Wamberal Beach. The 2011 LiDAR survey, from which the alongshore-averaged profile was derived (Figure 9), captured an accreted state along the southern two-thirds of the beach, while a 2016 LiDAR survey [1] captured an accreted state along the northern third of the beach. Similarly, the baseline (2 m AHD elevation contour) from which we measure modelled fluctuating shoreline change (R F ), is a hybrid beach berm position based on both LiDAR surveys, representing a fully accreted beach state along the length of Wamberal Beach (Figure 10).…”

“…Historically, Wamberal Beach has been impacted by erosion that is caused by coastal storms, resulting in significant damage and loss of properties [52]. Many beachfront properties were damaged by severe erosion (Figure 2) that was caused by a storm that impacted the entire NSW coast in June 2016 [1]. However, the barrier morphostratigraphy (stationary-receded) suggests that from a geohistorical perspective, Wamberal beach has been relatively stable or very slowly receding during the mid-late Holocene.…”

“…Using aerial LiDAR surveys, Harley et al [1] found that 11.5 million m 3 of sand was temporarily lost to the sea from 177 km of shores in southeast Australia during a single storm in June 2016, with an average of 65 m 3 (and maximum of 228 m 3 ) being lost from each metre of beach alongshore. While the loss of sand offshore is usually temporary, full beach recovery to the pre-storm state may take several months to several years, depending on the post-storm wave climate and frequency of storms [2][3][4].…”