Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Environmental geology of the Avon-Heathcote Estuary Thesis

MacPherson, J.M.
Journal / Source
PhD Thesis
Publisher / Organisation
Department of Geology, University of Canterbury
Acaena, Rosaceae, taxonomy, new species, Acaena juvenca, Acaena emittens, New Zealand flora, forest flora, shrubland flora
The Avon-Heathcote is a small, microtidal, predominantly intertidal, weather-dominated estuary. It has experienced large alterations to its physical environment as a result of the establishment and growth of the adjacent Christchurch City, on what was previously a swampy, dune-bordered coastal plain. During the period 1850-1920 the tidal compartment decreased by 30%, then returned rapidly to its original volume. It has since increased to about 30% more than its pre-European volume. The inlet area progressively changed its configuration to accommodate these alterations in volume. An analysis of the energetics of waves and tidal currents shows that wave-induced shear stresses predominate in this estuary, particularly above the MTL, and are only exceeded by tidal current stresses in and adjacent to subtidal channels, where tidal velocities exceed 60cm/s. Because of depth limitations the largest waves in this estuary normally have periods of 1.4 s, amplitudes of 30cm and lengths of 3.5m. Wave energy gradients are due to downfetch variations in wave frequency, not variations in wave size. The muddiest sediment occurs close to the entrances of the Avon and Heathcote rivers, and patterns of mud deposition are regulated both by rates of sediment supply and by wave energy. The sand fraction of active surface sediment can be divided into two groups