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Bay-head sand beaches of Banks Peninsula, New Zealand
Dingwall, P.R. (1974). Bay-head sand beaches of Banks Peninsula, New Zealand. NIWA Biodiversity Memoir, 15. New Zealand Oceanographic Institute: [s.l.]. 3-63 pp.
Deel van: New Zealand Oceanographic Institute Memoir. New Zealand Department of Scientific and Industrial Research: Wellington. ISSN 0083-7903
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  • Dingwall, P.R.
Abstract
    Analysis of the physical and mineralogical attributes of sands on 13 bay-head sand beaches of Banks Peninsula indicates that the beaches consist of medium sand derived from the bedrock of the peninsula and fine sand from greywacke-derived sediments mantling the adjacent continental shelf, and a contribution from the cover of loess on the peninsula. This indication is strongly supported by an evaluation of the capability of waves and currents in the vicinity of the peninsula to transport material. The beaches are arranged into three categories according to the origin of their sands: (l) those consisting predominantly of medium-grained heavy mineral sands derived largely from weathering of the peninsula's basic igneous rocks; (2) those consisting predominantly of fine, or very fine, light mineral sands derived from offshore sediments by constructive wave action; (3) those whose sands contain significant contributions from both the peninsula's bedrock and the detrital material on the shelf. On individual beaches the relative abundance of material derived from each of the two primary sources depends on the degree of exposure of the beach to wave action, which controls the strength of water movements in the bay head. The results of beach surveying and sediments sampling indicate that the interplay between the beach deposit and hydrological forces in the bay head is analogous to that on exposed coastlines. Variation in sediment characters show correlations with differences in beach morphology. Gently sloping, smooth profiles on fine sands contrast with conspicuous berms and moderately steep beach faces on medium-grained sands. Sediment properties vary slightly, both across and along the beach as a result of selective sorting by wave and tidal action. The beaches change in response to differing wave and tide regimes. For example, large winter waves smooth out summer beach profiles by cutting back berms and redepositing the sediment on the lower foreshore. Similarly spring tides destroy small neap tide berms. Measurements of sand level changes during single tidal cycles show that beach material is being continually resorted. In addition to movement of sand normal to the shore there are lateral shifts caused by changes in the direction of wave approach. On the medium sand beaches changes in the beach profile are greater and more rapid than on beaches composed of fine sand. Successive surveys of profiles over a period of seven months showed that sand levels on the medium sand beaches fluctuated through a vertical range of as much as 4 ft (1.2 m) whereas the range on the fine sand beaches was commonly no more than 2 ft (0.6 m). On some beaches short-term changes in the profile were superimposed upon long-term progradation of the beach deposit itself.
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