Hypsometry of New Zealand estuaries

Abstract

Estuarine morphology is constantly changing because of continual sediment transport mainly driven by tidal asymmetry and wind waves. For modelling purposes, intertidal areas have often been simplified to relatively semi-circular basins. Although suitable for providing a preliminary understanding of processes, this approach does not take into account the real curvature of natural basins. One way of studying the morphology of an estuary is to use its hypsometry. This quantity, often represented as a curve, describes the area-weighted distribution of depth and therefore includes the uneven form of a basin. Overseas studies have shown that the shape of the hypsometric curve of an estuary can be used as a measure of its state of infilling. The present study focuses on the morphology of New Zealand estuaries by comparing their hypsometric curves and examining the relative contribution of various forcing factors towards the shape of these curves and consequently towards the development of the morphology. I applied the hypsometric relationship defined by Boon & Byrne (1981) in order to assess how well it describes New Zealand estuaries and what factors influence the shape of the hypsometric curve. Their equation introduces the empirical parameter which controls the concavity of the curve. Area-depth curves were computed using bathymetric data for the 22 studied estuaries. The best fitting curves relative to Boon & Byrne’s relationship were determined by minimising the error between the modelled and observed curves and provided a measure of for each site. This relationship appeared to be relatively satisfactory (error of less than 5%) for most estuaries. A database was created where the tidal range, the length of the longest fetch, the length of the fetch along the direction of most common winds, the wind speed and the significant wave height were stored for each study estuary. The impact of each parameter was estimated using statistical clustering analysis. No clear correlation could be identified. Conversely the results suggest that estuaries with the same environmental conditions could present very different values of γ. Current data, sedimentation accumulation rates and proportion of intertidal area at high water were also collected which enabled to observe that for some study sites the value of seems to be a reasonable indicator of the degree of infilling. However there were exceptions and some sites with higher γ were in an advanced state of infilling. This suggests that even though Boon & Byrne’s relationship provides a reasonable representation of the hypsometry of New Zealand estuaries, the parameter does not seem to be a satisfactory measure of infilling of New Zealand estuaries. The present study also showed that when comparing with studies conducted in the United Kingdom and in the United Stated it appeared that the values of estimated for New Zealand estuaries are in the whole higher than overseas. This could be attributed to the fact that New Zealand basins are relatively young systems but also could be due to other specificities of those basins such as the influence of the geology.