Reliability of Geomorphic Indicators of Littoral Drift: Examples from the Bay of Plenty, New Zealand

Abstract

Healy, T. and de Lange, W., 2014. Reliability of geomorphic indicators of littoral drift: Examples from the Bay of Plenty, New Zealand.The net direction of littoral drift can be evaluated by multiple lines of evidence, including a range of geomorphic indicators and trends in sediment characteristics. In this article, the processes that influence the longshore transport of sediment are reviewed and assessed by the types of geomorphic indicators and sediment trends that can be expected. Various proxies for measured littoral drift are used to evaluate the littoral drift in the Bay of Plenty, New Zealand. Previous studies, utilising geomorphic indicators and, to a lesser extent, sediment characteristics and simple models of longshore sediment transport, developed a conceptual model of two main littoral drift systems starting at the western and eastern margins of the Bay of Plenty and converging in the vicinity of Ohiwa Harbour, an estuary enclosed by two opposing spits within the bay. Comparison of geomorphic indicators of littoral drift with the results of 30 years of monitoring and investigation demonstrate that geomorphic indicators can be misleading or misinterpreted. Overall, they do not appear to have been reliable for the Bay of Plenty, whereas sediment characteristics have performed better. The published rates of net littoral drift, and also the directions, appear to be too high and are inconsistent with observed patterns of erosion and accretion. Further, the exchange of sediment between the beach and the inner shelf has not been considered. Within the Bay of Plenty the net littoral drift magnitude and direction varies in response to changes in the wave climate associated with climatic variability, particularly the magnitude, frequency, and duration of the La Niña and El Niño extremes of the El Niño–Southern Oscillation. Decadal-scale fluctuations in the patterns of erosion and accretion along the Bay of Plenty coast have also been recognised. Finally, the influence of tectonic effects on geomorphic indicators of littoral drift needs further investigation.