MASS WASTING OF COASTAL HEADLANDS IN SOUTHERN THAILAND: A COMPARISON

The landscape on the east coast of Koh Samui, Surat Thani Province is characterized by rocky granitic headlands and sandy pocket beaches. The most prominent positive relief landform on the headlands are turrets (vertically elongate blocks). The distribution of the turrets is structurally controlled by two orthogonal joint sets, which are steeply dipping (80-87 degrees). Joint spacing is typically 2 to 3 meters. The presence of leaning and collapsed turrets suggests that the dominant type of mass wasting of these granitic headlands is by rock topples and rock falls. Failure surfaces for the rock topples and falls occur both along the two near-vertical orthogonal joint sets and sub-horizontal exfoliation joints.

The landscape in Krabi Province along the southwest coast of Thailand is characterized by steep, limestone headland cliffs, sandy pocket beaches, and karst towers (stacks) offshore from the retreating headlands. The steepness of the limestone headlands and karst towers (stacks) is maintained by rock falls and rock slides. The triggering mechanism for these rock falls and slides is oversteepening of the headland and tower slopes by the formation of notches at their base. Thus, the steepness of the limestone headlands and karst towers (stacks) is maintained by the continuous erosion of their base.

In summary, mass wasting processes are structurally controlled in the granitic headlands whereas they are controlled by erosional processes in the limestone headlands and karst towers (stacks).