Modelling storm behaviour and relative sea level change on mixed sand and gravel barrier beaches in southern Hawkes Bay, New Zealand

Abstract

The mixed sand and gravel (MSG) barrier beaches of southern Hawkes Bay are subject to ongoing storm-driven erosion and inundation concerns, prompting calls for predictive stormimpact modelling tools. XBeach-G is a newly-developed process-based model designed to simulate the cross-shore storm-driven evolution of pure gravel beaches. This study presents the results of an investigation into the potential application of XBeach-G at the Hawkes Bay MSG coastline, with focus on the storm-forced morphological behaviour exhibited by the model under relative sea level change. Model simulations were conducted on three barrier profiles impacted by relative sea level changes during the 1931 Napier earthquake: one tectonically uplifted profile (relative sea level fall), one subsided profile (relative sea level rise) and one ‘hinge line’ profile that remained geologically stable. XBeach-G was calibrated against site-specific measurement data, and model performance critically assessed. The model was shown to be a useful tool for capturing Hawkes Bay MSG barrier dynamics despite being formulated for pure gravel beaches. XBeach-G was then used to simulate barrier behaviour immediately following the earthquake and barrier behaviour under modern (2016) conditions. Simulations reveal mechanisms of response unique to each profile. In the absence of sea level change the hinge line profile demonstrated increasing overtoppingdriven resilience between 1931 and 2016. Relative sea level fall of 1.5 m prompted swash zone erosion and ridge building at the uplifted profile, which was stable (i.e. no inundation) under all 1931 and 2016 input conditions, a result of the high barrier elevation preventing cross-shore flows. In contrast, model results and historical profile data demonstrate that instantaneous earthquake-driven sea level rise of 0.78 m at the subsided profile led to sudden barrier overtopping and narrowing. Increases in barrier crest elevation have occurred since the earthquake at this profile, reducing overwashing impacts, but driving further barrier narrowing over time. Results draw attention to the possibility that the modern beach-barrier may be on a trajectory toward catastrophic barrier destruction under extreme waves. This study describes the morphological linkages between historic and modern barrier forms, giving essential historical context to the interpretation of modern storm vulnerability and the resilience of different profiles.