Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

The geomorphology of Auckland and Northland Book

Ballance, P.F.; Williams, P.W.
Journal / Source
Soons & Selby (Eds), Landforms of New Zealand
intertidal sand flat, Farewell Spit, Golden Bay, New Zealand, mixed sand populations, sediment delivery by floating trees, sand partitioning
Extensive intertidal sand flats (c.10000 ha, c. 100 km2 ) on the sheltered southern side of Farewell Spit, a 30 km long sand spit located at the northern tip of South Island, New Zealand, extend for up to 8 km into Golden Bay. They consist primarily of fine sand with an upper size limit of 0.36 mm, blown from the spit during northerly storms. In parts of the flats fine sand is supplemented by significant but highly variable amounts of coarse sand, with rare stones up to 40 cm long. Tree trunks with tangled root masses, stranded on the flats, suggest that the coarse sediment is being delivered in the root masses of trees which are washed out of rivers discharging into Golden Bay from the mountainous southern hinterland. The greatest concentrations of coarse sediment are located northeast of the Aorere and Takaka Rivers, the two largest rivers discharging into Golden Bay. We propose that some trees are blown by prevailing southwesterly winds from the river mouths to the sand flat, and that a clockwise tidal current gyre carries trees from all rivers onto the flats in the northwestern corner of the bay. The patchy distribution of coarse sand on the intertidal flats indicates that redistribution of sand across the flats is partitioned: coarse sand (>0.5 mm) is not widely mixed with fine sand by surface processes, while fine sand is widely distributed. The sand flat extends subtidally to the 10 m bathymetric contour, giving the system a total area of c. 200 km2 . An estimated volume of Holocene age sand in the spit and the sand flats of c. 5.7 km3 represents only c. 10% of the sand delivered to the northern tip of the South Island by longshore movement up the West Coast. The bulk of the sand may be accumulating on the continental shelf.
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