Observations of beach gravel transport, Wellington Harbour entrance, New Zealand

Abstract

Two small embayments at the entrance to Wellington Harbour, New Zealand, which are thought to have been depleted of sediment by regional tectonic uplift in 1855, have shown a sudden and rapid accumulation of gravel since 1941. A tracer experiment in this locality revealed that pebble-sized sediment moves northwards into the harbour at an average rate of 1.66 km y^-1. Further north, inside the harbour, there is a sudden change in the nature of the beach. Towards the entrance, plentiful gravel of fresh appearance occurs, while to the north scarce weathered gravel is present. This sudden change in the nature of the beach is here named a gravel front and it is moving northwards at a rate of approximately 0.42 km y^-1. Pebbles are transported during high wave energy events in a zone near low water level. Loss of pebbles offshore and by abrasion is probably small. It appears that gravel is moved alongshore during storms in discrete packages that subsequently coalesce and move landwards to form a berm during normal low wave energy conditions. It is suggested that the sudden progradation at the harbour entrance and the existence of the mobile gravel front are due to a large influx of gravel to the coast that occurred after landslides were induced by the earthquakes of the 1840's and 1850's, when the base level of Orongorongo River was raised.