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.
Highlights
Extreme coastal storms such as hurricanes and extratropical cyclones (ETCs) can rapidly mobilise and redistribute vast quantities of sediment over large (>100 km) lengths of coastline, leading to erosion of beach and coastal dune systems[1] and breaching or overtopping of coastal barriers[2, 3]
While these dependencies provide a valuable framework for predicting hurricane impacts on low-lying coastlines[10], the slower-moving and diffuse nature of Extratropical cyclones (ETCs) means that coastal impacts during these events may be more a result of prolonged wave energy over several tidal cycles than the extreme and rapid increases in water levels observed during hurricanes[11, 12]
The corresponding average shift in shoreline position was landwards by 22 m. These average volume losses are similar in magnitude to those observed along the U.S east coast in response to the 2012 Hurricane Sandy event[21], highlighting the comparable morphological change that can occur during these less documented ETCs
Summary
Extreme coastal storms such as hurricanes and extratropical cyclones (ETCs) can rapidly mobilise and redistribute vast quantities of sediment over large (>100 km) lengths of coastline, leading to erosion of beach and coastal dune systems[1] and breaching or overtopping of coastal barriers[2, 3]. Regional-scale observations of hurricane impacts along the U.S east coast indicate that spatial variability in the morphological response corresponds strongly to local maxima in storm-induced total water levels (combing astronomical tide, storm surge, wave setup and wave run-up) relative to antecedent dune morphology[9]. While these dependencies provide a valuable framework for predicting hurricane impacts on low-lying coastlines[10], the slower-moving and diffuse nature of ETCs means that coastal impacts during these events may be more a result of prolonged wave energy over several tidal cycles than the extreme and rapid increases in water levels observed during hurricanes[11, 12]. Morphological response was assessed at cross-shore transects spaced every 100 m along the survey region (1768 transects in total) by calculating, at each transect, the total subaerial sand volume change (i.e., all sand volume change above mean sea level) between the pre- and post-storm surveys, as well as the change in the shoreline position (defined by the mean high water contour)
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