Storm overtopping of a marine terrace at the penultimate stage of evolution

The youngest uplifted marine terrace at Kahutara Point on Māhia Peninsula, New Zealand, is undergoing rapid retreat (backwear) despite being fronted by a wide contemporary shore platform that attenuates wave energy. In this paper, wave energy and water level were recorded across the shore platform, and these data were used to model (a) how frequently waves reach the terrace riser and potentially erode it and (b) how frequently waves overtop the terrace. Analyses of wave data across shore normal transects show that under quiescent and storm conditions >90% of the energy delivered to the back beach and terrace riser is at infragravity frequencies (i.e., <0.05 Hz). Significant wave heights are reduced in a landward direction for both gravity (H_m0H) and infragravity (H_m0L) wave frequencies, with 50%–80% of H_m0H and 20%–50% of H_m0L reduced between seaward and landward sensors. Wave energy during quiescent conditions is strongly attenuated at the seaward margin whereas under storm conditions, proportionally more energy is delivered to the marine terrace riser. The development of a simple inundation model at Māhia reveals that the northern flank of Kahutara Point is more vulnerable to wave inundation, with 17 storms overtopping the youngest marine terrace between 1980 and 2020. Furthermore, despite there being no evidence of a 1-in-100 year storm event occurring at Māhia since 1980, there has been an increase in storminess since 2013. Changes in storm frequency may have offset decreases in wave energy (from energy attenuation) with increased platform width associated with marine terrace retreat.