Rates of coastal erosion and accretion in New Zealand

Abstract

Rates of coastal erosion and accretion for New Zealand are calculated for the period since early European colonization. Methods used for calculating rates from cadastral plans, vertical aerial photographs and field measurements are described, evaluated, and illustrated with examples. The most natural reference line for measurements of shoreline changes and for defining the seaward boundary of land is the seaward limit of land vegetation. Measurements made from air photographs and plans at scales larger than 1:4000 have errors less than .+-.1 m. As scales become small, errors increase proportionately. Along depositional shorelines, erosion and accretion generally occur at 0.5-4.0 m .cntdot. yr-1. Maximum erosion and accretion rates are 25.4 m .cntdot. yr-1 at North Kaipara Head and 68.9 m .cntdot. yr-1 at Farewell Spit, respectively. Cliff recession generally occurs at 0.25-1.0 m .cntdot. yr-1 with maximum rates of 2.25 m .cntdot. yr-1 for mudstone cliffs at Cape Turnagain and 3.46 m .cntdot. yr-1 for conglomerate cliffs at Ngapotiki.