Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

A "blind-folded" test of equilibrium beach profile concepts with New Zealand data Journal Paper

Dean, R.G.; Healy, T.R, Dommerholt, A.
Journal / Source
Marine Geology
Abstract Methodology for calculating equilibrium beach profiles for uniform sand size characteristics is extended to the case of an arbitrary distribution of sediment characteristics across the profile. The application of this method and comparison with actual profiles is posed as a means of interpreting whether the profile contains a deficit or excess of sediment and thus whether long-term shoreline recession or advancement can be anticipated. Various types of profile disequilibrium are reviewed and the significance discussed. The methodology is applied using measured profiles, sediment sizes and beach face slopes for ten sites on the Northern Island of New Zealand. One profile was documented in this study, whereas the data for the other nine were obtained from published sources. The number of sediment samples available for each profile varied from three to twelve. The agreement between the actual and calculated profiles differs considerably for the ten sites. The degree of disequilibrium is quantified by calculating the shoreline adjustment, Δy, required for the actual profile to equilibrate for depths less than 7 m, which represents the near-maximum depth available on all profiles. These shoreline adjustments ranged from − 105 m (recession) to + 159 m (advancement) with four of the ten sites having positive values. Three of the sites with negative shoreline adjustment have been, or are presently, sites of substantial sand extraction from the beach or in the nearshore waters. However, the differences between the actual and equilibrium profiles are not consistent with anticipated profile forms and/or volumes and it is thus concluded that sand mining is not responsible for most of the observed deficits. At this stage, it is not possible to state with confidence whether differences between actual and (calculated) equilibrium profiles are due to true disequilibriums or to limitations in the equilibrium beach profile methodology. Studies of the type reported here when applied to many different areas will advance the methodology and contribute to the confidence in the resulting interpretations.
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