Role of porolithon algal‐ridge growth in the development of the windward coast of Tongatapu Island, Tonga, South Pacific

Abstract

AbstractA description of the algal ridge fringing the windward coasts of the island of Tongatapu is given. Three coastal types can be distinguished according to the height of the algal ridge. Type A coasts have a low algal ridge and low wave energy. Surge channels are few and uncovered. Blowholes are poorly developed. The reef flat is wide and has numerous living corals. Many beaches occur except around promontories where a cliff‐foot notch is actively forming. Type B coasts have a medium‐height algal ridge and experience moderate wave energy. Surge channels are longer, more frequently covered and blowholes consequently better developed. The reef flat/shore platform has less coral living on its surface. Large beaches along type B coasts are the outcome of sediment becoming trapped on the shoreward side of the algal ridge following storms. Type C coasts have a high algal ridge and are characterized by a high‐energy regime. Surge channels are almost all covered, giving rise to spectacular blowholes, except where collapse has caused the algal ridge to assume a buttressed rather than linear form. The shore‐platform surface is devoid of living coral and lies above high‐tide level. The cliff‐foot notch is shielded by the algal ridge from direct wave attack. A notch is forming on the upper part of the outer slope of the shore platform: such notches were observed only along type C coasts.A model for Holocene coastline development is outlined. The reef established on the mid‐Holocene shore platform emerged abruptly by ˜ 2 m as the result of coseismic uplift around 4000 years ago. Sea level fell (around 3000 years ago): erosion concentrated on reducing emerged shore‐platform level. Sea level stabilized around 1000 years ago: the emerged shore platform continued to be lowered; around 600 years ago, algal‐ridge growth commenced thereon. Rates of vertical growth ranged from 0·5 to 2·1 mm a−1. The process regime along type C coasts changed around 362 years ago when the algal ridge reached a sufficient height to shield the shoreline from the processes which had fashioned it and a lower‐level notch began to be cut on the reef slope.