Elsevier

Geomorphology

Volume 88, Issues 3–4, 1 August 2007, Pages 214-225
Geomorphology

Estuarine shore platforms in Whanganui Inlet, South Island, New Zealand

https://doi.org/10.1016/j.geomorph.2006.11.007Get rights and content

Abstract

Whanganui Inlet is a low mesotidal environment where wave energy at the shoreline is limited due to a small fetch, a narrow entrance and tidal flat accretion to intertidal elevations. Wave energy is therefore only an erosive force at high tide and under storm conditions. Despite this low-energy environment extensive shore platforms occur within the inlet. They are sub-horizontal and range in width from 4.1 to 185.2 m with an average of 44.9 m. All the platforms are formed in sandstone of low resistance (mean N-type Schmidt Hammer rebound value of 17 ± 8) and have their seaward edges buried by intertidal sediment flats. The majority of platforms occur at around MHWN level, corresponding to the elevation of those flats. Where wave energy is highest, opposite the inlet's entrance and at those sites with the largest fetch, platforms develop to 0.5–1.0 m below MSL. A higher platform level is also found at MHWS elevations, however it appears to be relict with active erosion of its seaward edge occurring and therefore is most likely related to a higher mid-Holocene sea level. Apart from the location of the lowest platforms little correspondence is found between platform morphology and wave energy. Platform evolution appears to be intrinsically linked to the intertidal sediment flats which determine the degree of surface saturation of the bedrock and, hence, the number of wetting and drying cycles the platforms may undergo. As the seaward edge is buried platform development is primarily through retreat of the landward cliff. This process can, however, be complicated by the migration of intertidal water channels onto the seaward edge of the platforms or relative sea level fall which may rejuvenate landward retreat of the low-tide cliff.

Introduction

Shore platforms have been referred to as neglected coastal geomorphic features, especially when compared to other coastal landforms such as beaches and dunes (Trenhaile, 1980, Stephenson, 2000). Recently there has been a renaissance of interest in the rock coast, with investigations ranging from millimetre scale changes of platform surfaces over hourly cycles (Trenhaile, 2006, Gomez-Pujol et al., 2006) to the evolution of terraces during Quaternary eustatic oscillations (Trenhaile, 2001a, Trenhaile, 2002a). Geology and tidal range exert a major influence on broad platform morphology. Lithologies with a low hardness tend to erode faster and to a lower elevation than more resistant rocks (Gill, 1972, Kirk, 1977, Trenhaile, 1987, Dickson, 2006, Thornton and Stephenson, 2006) while on a regional scale platforms are steeper in areas of higher tidal range (Trenhaile, 2002b). There is, however, still no consensus on the relative role of wave and weathering processes in determining their final shape. Interestingly nearly all investigations of shore platforms have been conducted on the open-ocean coast, which are impacted by waves developed over a significant fetch. It is the high-energy nature of these settings that has often made data collection difficult, especially at the seaward edge of the platform.

In low-energy marine environments, such as estuaries, wave energy decreases rapidly over a short distance and, in turn, the dominant process driving landform change varies markedly (Davis and Hayes, 1984). On depositional landforms such as beaches, profile shape can be determined by wave and tidal processes even in microtidal regimes, or there may be relict morphologies only activated during storms (Hegge et al., 1996, Kennedy, 2002, Goodfellow and Stephenson, 2005). Such environments provide an ideal setting for the investigation of the role of marine and subaerial processes in driving landform evolution, as the relative role of each in eroding the shoreline changes markedly over a short distance. For shore platforms very few studies have been conducted in fetch-limited environments, and those that have are focussed within tideless lakes (e.g., Lake Boverbrevatnet, Norway, Matthews et al., 1986; Storavatnet and Bergen areas, Norway, Aarseth and Fossen, 2004a, Aarseth and Fossen, 2004b; Lake Waikaremoana, New Zealand, Allan et al., 2002). Lakes Erie and Huron, Canada, (Trenhaile, 2004) also contain well-developed shore platforms although the fetch is significantly higher than the previous examples. Despite evidence that platforms can form in sheltered marine settings (Bartrum, 1916, Dionne and Brodeur, 1988, Dionne, 1993) there is still little knowledge about rocky coasts from estuarine environments, especially those with a limited fetch. This study sets out to describe the morphology of such a system investigating the morphology of the estuarine shore platforms of Whanganui Inlet, New Zealand. It aims to infer the processes driving platform formation, and to develop a model for platform evolution within this estuary.

Section snippets

Regional setting

Whanganui Inlet is a narrow estuary 13 km long and up to 3 km wide located near the north-west tip of the South Island of New Zealand (Fig. 1). The majority of the inlet's shoreline is rocky, composed of quartzo-feldspathic shallow marine sandstones and localised conglomerates of Tertiary to Late Cretaceous age (Pakawau and Kapanui Groups respectively) (Rattenbury et al., 1998). The catchment is steep rising to an elevation of 506 m at Knuckle Hill, where Early Cretaceous granites outcrop (Fig.

Materials and methods

A total of 53 profiles was surveyed from the cliff platform junction to the seaward edge of the shore platform, using an electronic distance meter (Sokkia SET 4010). At each main study area sites were surveyed at regularly spaced intervals to allow for the parameterization of platform characteristics at that particular location. The distance between surveys was therefore determined by the occurrence of platforms along that section of shoreline. Where the seaward edge of the platform was buried

Platform morphology

Extensive platform surfaces are found within Whanganui Inlet ranging in width from 4.1 to 185.2 m with an average of 44.9 m. Almost all (83%) have a steep (> 60°) outer edge, while the remainder have little change in slope as they descend below low-tide level. In general the profiles tend to be widest on the tips of the promontories, however there is little correlation between platform width and fetch distance (r2 < 0.1) with both wide and narrow platforms occurring in the most protected and

Discussion

The shore platforms within Whanganui Inlet are all semi-horizontal, range in width up to 185 m, with the majority (77%) of surface gradients being < 1.5°, ranging up to 3.4°. In all cases the seaward portion of the platform is buried by intertidal muddy-sand flats to depths of up to 1.5 m. This means that unlike platforms found on the open-coast the seaward edge is not exposed to either waves or subaerial processes during tidal cycles. Therefore, although they are morphologically similar to

Conclusions

The extensive shore platform development found within Whanganui Inlet indicates that significant rocky coast landforms can develop in relatively low-energy marine environments. Little relation occurs between platform morphology and wave exposure, with platforms most commonly forming at or very close to the level of intertidal sediment flats that occur throughout the inlet. This corresponds to the level of surface saturation of the rocky surfaces at around mean high water neap levels. These

Acknowledgements

The authors would like to thank Rhea Dasent, Mike Henry and Hamish McKoy for assistance in the field. Victoria University of Wellington provided funding for this project. Critical reviews by Alan Trenhaile and an anonymous reviewer were appreciated.

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    Present address: Northland Regional Council, Private Bag 9021, Whangarei, New Zealand.

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