Dynamics of Mixed Sand and Gravel Beaches Between Cape Palliser and Wellington, New Zealand

Abstract

Palliser Bay is situated at the southern extremity of a 20 kilometre wide block faulted depression in southern North Island, New Zealand. The beaches are of mixed sand and gravel, derived from the adjacent highlands of Mesozoic greywacke, and have been investigated to determine the important processes affecting their short and long-term stability. Beach gravel tracer experiments, each of a Year's, duration, were conducted at three sites, using up to 75 tonnes of limestone pebbles at each. The experiments show that although beach pebbles at the head of palliser Bay occasionally move rapidly alongshore, in the order of 1000 metres per day, longshore transport here is oscillatory and the nett rate of transport is very small. In contrast, pebbles on the sides of the bay are being transported steadily towards its head by the prevailing southerly waves at an average rate of around 1600 metres per year. The experiments also showed that at first larger pebbles moved more rapidly alongshore than the smaller pebbles, but this process was not sustained over a long period. More consistent size sorting trends were observed across the beach, and were produced by the concentration of coarse sediment at the landward limit of the swash. Pronounced shore-normal shape sorting was also observed, and this, rather than attrition, appears to be the primary reason for the abundance of flat pebbles commonly observed on gravel beaches. Improvement in the roundness of limestone tracer pebbles at each site during the year of observation was compared with the rate of weight loss and change in roundness of limestone and greywacke pebbles in a tumbler. These comparisons made it possible to work out the rate of pebble attrition. A weight loss of 41% each year was found for pebbles at the most exposed site but much lower values were found at the other two (15% and 7%). A sediment budget for the Bay based on currently active processes indicates that gravel is supplied, mainly by rivers at a rate of around 20,000 m3/y and lostthrough attrition at a rate of around 40,000 m3/y. Thus the 1 m/y progradation experienced along the western half of the bay head over the last 120 years must be an unusual event. It may be related to the sudden influx of rock debris to the rivers which was caused by intense earthquakes in the 1840's and 1850's, possibly compounded since then by the depredations of introduced mammals. The same events may also be the cause of sudden progradation of the nearby beaches between Turakirae Head and Eastbourne.