GEOMORPHIC EVOLUTION OF A PROGRADATIONAL POCKET BEACH AND ESTUARY SYSTEM, OKAINS BAY, BANKS PENINSULA, NEW ZEALAND

Coastal hydrosystems, including features such as estuaries, bays, and lagoons, are important both economically and ecologically, but are also particularly fragile. The morphological evolution of these systems is thought to be dependent on changes to sediment supply in the long term and punctuated events like earthquakes, storms, and tsunamis in the short term. Many workers believe that systems like estuaries, in the absence of punctuated events, exist in a state of 'dynamic equilibrium,' self-regulating towards a standard morphology. Aerial photogrammetry has proven to be a cheap, effective, and intuitive way to analyze morphological changes over time. Using this method, an analysis of a “pocket beach” at Okains Bay, Banks Peninsula, New Zealand, was undertaken to assess broad changes to its morphology from 1941 to 2017. Representative features (channel and sand bars, dune line, berm line, spit line, wedge line) were chosen and their movements over time were measured to assess progradation patterns within the bay. This analysis revealed several trends. Although channel morphology and the overall shape of the bay change very little, there are consistent patterns in progradation rates across the beach. Progradation rates vary in magnitude and direction depending on both distance from the main estuary channel (along the beach) and distance from the water within the bay (across the beach). A list of punctuated events that affected or may have affected Okains Bay was collated and compared to the results of the photographic analysis. A 2D model was also drafted to illustrate sediment movement within the bay.

The findings of this study suggest that punctuated events like earthquakes, storm surges, and tsunamis alter progradation rates within the bay. Understanding how distinct types of punctuated events affected the bay in different ways was complex because of difficulty distinguishing between the effects of local storm events and far-field tsunamis. However, it can be tentatively concluded that far-field events lead to greater progradation whereas near-field events lead to either slower progradation or active retrogradation. Going forward, it will be important to continue to monitor Okains Bay to see how it adjusts to recent punctuated events and to consider developing management strategies for progradation trends.