Field Observations and Geotechnical Modeling of Dune Face Erosion During a Storm (Narrabeen Beach, Australia)

Quantifying dune face failure during storm wave conditions is critical for improving the predictive capability of coastal erosion models. However, most high‐resolution dune erosion observations used to validate these models were obtained from reduced‐scale laboratory experiments, raising questions about their applicability to field‐scale dunes. This study combines continuous LiDAR observations with geotechnical modeling to investigate dune face stability during a storm on a constructed dune at Narrabeen Beach, Australia. The dune face, impacted but not overtopped by wave runup, was monitored for ∼2.5 hr, failing and slumping on five occasions, causing 2.1 m of dune crest recession. Morphological changes included reduction in foreshore slope from 13 to 4° and lowering of the dune toe by 1.0 m, increasing dune face height from 2.5 to 3.3 m. The slope of slumped sediment at the toe followed a recurring pattern: it gradually steepened following each failure as sediment was transported offshore from the dune base, leading to destabilization of the dune face and triggering further slumping and recession. Geotechnical modeling, in which slope stability is coupled to groundwater flow, reproduced these failures, showing erosion resulted from morphodynamic interaction between the vertical extent of wave runup and offshore transport of slumped sediment. Importantly, collation of these and other published observations revealed a negative linear relationship ( r 2 = 0.82) between dune height and failure slope: larger dunes failed at milder slopes than smaller dunes. In models such as XBeach, this relationship can provide a quantitative method to better estimate the critical avalanching slope ( dryslp ).