Observations of rip channels, sandbar-shoreline coupling and beach rotation at Tairua beach, New Zealand

Abstract

Interannual wave climate variations along the north east coast of New Zealand have caused significant changes to the erosional state of many of the popular tourist beaches. These changes threaten property, ruin expensive and labour-intensive dune plantings and destabilise walkways. The alongshore spatial and temporal variation of erosion can, however, not always be directly dependant on the variability in the wave climate. The response appears to be dependant on the alongshore extent of the beach and the existence of headlands. This work takes a 3.4 year data-base of rip channel location, sandbar position and shoreline position to quantify the patterns that occur on a headland-controlled embayed beach. Findings show that when wave events had a strong alongshore component, the alongshore bar and shoreline rotated and three dominant, persistent, rip channels developed on the end of the beach at which both the shoreline and barline were more seaward. This pattern where the beach rotated in one direction occurred on three occasions: at the beginning of the dataset (early 1999), during the persistent small storms during the winter of 2000, and in the other direction during the large storm in mid 2001. The rotation events were clearly related to the dominance of northward versus southward alongshore flux events over 3500 J/m2. These results show that on embayed beaches, the alongshore wave energy flux is pivotal in controlling shoreline erosion and the severity and location of rip channels