Sea-cliff Erosion and Retreat in Semi-enclosed Macrotidal Embayment: Hampyung Bay, West Coast of Korea

Abstract

The coastline of semi-enclosed, macrotidal Hampyung Bay, on the west coast of Korea, was monitored seasonally to investigate sea-cliff retreat processes. The long-term retreat rate of the coastline was measured by comparing aerial photographs taken in 1976 and 1990. The coastline of the bay has an irregular, saw-tooth shape and is characterized by poorly consolidated soil, sea cliffs, and steep slopes. Unusual geomorphic features (remnants of former sea cliffs and "island stacks") on the tidal flat and beach near the sea cliff"s, together with the unique, saw-tooth-like coastline configuration, indicate that the coastline has been undergoing sea-cliff erosion for hundreds of years. Quantification of sea-cliff retreat rates yields average cliff retreat rates of 1-2 m/yr between 1976 and 1990; these values agree with estimates based on short-term field measurements (0.5-2.0 m/yr). Sea-cliff erosion in the semi-enclosed bay system probably results from episodic storm/typhoon surges affecting the poorly consolidated soil cliffs. A cliff-retreat model for the study area suggests that the sea cliffs are continuously retreating through a cyclic process: erosion of the cliff base during episodic, high-energy events (mainly winter storms) forms notches; gravitational failure produces soil talus at the base of the cliffs; and talus is then removed by disintegration. Especially, high sea-level episodes by a relatively higher tidal range during summer may be a key factor in the removal of soil talus, which accelerates sea-cliff erosion.