Variability of the modern sand cover on a tide and storm driven inner shelf, south Wellington, New Zealand

Abstract

Abstract The embayed coast, south of Wellington, is notorious for the severity of its storms and tides. Time‐series side‐scan sonar and sediment surveys, made of the bays and inner shelf over an 11–16 year period, show that the general locations of four main seabed types—(1) a modern, fine‐medium sand cover, (2) a generally megarippled, coarse sand ‐ fine gravel substrate, (3) a basal cobble‐boulder deposit, and (4) greywacke basement—remain fairly stable despite considerable mobility at the edges of each deposit. The sand cover is best developed (1.5 m max. thickness) in the larger bays centred on Wellington Harbour entrance. Here it has undergone bouts of accretion (indicated by advances of cover edges) that have ended in erosion (with retreat of edges). Mobility of sand is induced on a daily to annual basis by tides reinforced by southerly swell and storm‐driven currents. These high‐frequency events are superimposed on annular to decadal variations that probably relate to the frequency of gales and storms, and to variations in sediment supply caused by local earthquakes, changes in land use, and climatic cycles. Outside the bays, on the open shelf, the combination of a more vigorous tidal flow and reduced sediment supply mean that the sand cover is much less extensive and more mobile. The seabed is predominantly coarse sand ‐ fine gravel with a megarippled surface formed by southerly swell. Tides become progressively stronger to the west, and, in the Narrows of Cook Strait, currents are sufficiently powerful to keep sand in near‐constant motion and erode the fine gravel down to underlying boulders and rock.