Geomorphic change of an ebb-tidal delta: Mair Bank, Whangarei Harbour, New Zealand

The morphology and volume of Mair Bank, Whangarei Harbour, was examined at decadal and inter-annual time scales in order to assess the geomorphic stability of the ebb-tidal delta. Digitised aerial photography of Mair Bank over 56 years was analysed to determine multi-decadal changes in the position and planform configuration of major morphological units. Bathymetric survey data of the area were used to assess spatial and temporal elevation changes and to construct digital elevation models (DEMs) for quantitative comparisons. Results show that the footprint of Mair Bank has remained constant but significant changes in surface morphology have occurred. The western end of the seaward shell swashbar has migrated landward at an average rate of 10 m/year between 1950 and 2006. The largest subaerial change in sand storage volume occurred between 2003 and 2006 when volume increased from 1.107×10 5 m 3 to 1.690×10 5 m 3 . In terms of material accumulation over the extent of the ebb delta, this equates to a bedlevel rise of 10 cm across Mair Bank, or a 1.3% increase in total volume. In summary, Mair Bank exhibits dynamic sediment reworking, but the gross delta feature is relatively stable in volume and position.