Coastal changes at Kaikoura, 1942–74, determined from air photographs

Abstract

Changes in shoreline position at 100 locations along 45 km of the Kaikoura coastline have been determined for the 32-year period from 1942 to 1974 by comparison of air photograph sequences. The areas studied in detail comprise the mixed sand-pebble beaches to the north and south of the Kaikoura Peninsula and the raised beaches and shore platforms of the peninsula itself. Both pronounced erosion and progradation of sectors of the shoreline have occurred and there are large variations in the rate of change within small distances. These variations are thought to reflect local differences in the availability of sediments in the littoral zone and the longshore distribution of dominantly high energy wave action. Maximum accretion was more than 40 m (weighted mean = 12.65 m) and maximum erosion was 47.6 m (weighted mean = 8.23 m). Both mixed sand-pebble beaches are dominantly accreting whereas the major zones of erosion occur along the northern and southern flanks of Kaikoura Peninsula and along the northern margin of the Hapuku Delta. While the calculated weighted mean accretion rate (0-39 m/year) has been much higher than the weighted mean rate of erosion (-0,26 m/year ), more than half the total measured changes were erosional and this proportion rises to 77% for the peninsula. The principal zones of accretion occur south of the Hapuku River mouth near the center of the northern beach and immediately north of each of the two major river mouths along the southern beach. South of Kaikoura the small pocket beaches at Goose Bay and Oaro appear to be stable or growing only slowly.