Geomorphic responses of uplifted mixed sand and gravel beaches: combining short-term observations from Kaikōura, New Zealand with longer-term evidence

ABSTRACT Along the northeast coast of New Zealand’s South Island, a 7.8 Mw earthquake on the 14th of November 2016 induced instantaneous uplift of approximately 1 m around the Kaikōura Peninsula. This paper documents a unique field study examining short-term responses in the Mixed Sand and Gravel (MSG) beaches of Kaikōura to a relative sea level fall as a result of this event. We present two conceptual response pathway models which consider the pre-earthquake trends of 18 coastal sites, and two surveys taken over a 22 month period following the earthquake to define the short-term response of MSG beaches to instantaneous relative sea level fall. This study found MSG beach responses vary strongly according to their pre-quake sediment supply, source connectivity and open coast exposure characteristics. Overall, there were two predominant trends in which the beaches were responding in the short-term post-earthquake environment: accretionary profiles and erosional profiles. The difference between these two profile response types can be summarised as pre-earthquake erosional profiles were found to be retreating to pre-earthquake extents 22 months post-earthquake, whilst pre-earthquake accretional and dynamic beaches had an increased or stable trend which promoted stability or growth in beach volume post relative sea level fall.