Overnight formation of a bouldery alluvial fan by a torrential rain in a granitic mountain (Mt. Seoraksan, Republic of Korea)

Abstract

Mt. Seoraksan, Korea, is a rugged granitic mountain where extremely steep slopes and strongly seasonal rainfallhave facilitated bedrock exposure and geomorphic changes mainly by rockfalls and streamows. Although the environmentwas not suitable for alluvial fan formation, a bouldery alluvial fan, 170 m long and 330 m wide, formed overnight by a heavysummer rain in 2006. The fan consists of several meter-high boulder mounds and gently undulating cobble bars/sheets thatare arranged in a uvial longitudinal bar-like pattern. They are interpreted to have formed by highly competent and turbulentsheetoods, which temporarily had the properties of hyperconcentrated ood ows. Formation of the whole alluvial fan bya single, casual hydro-meteorological event is inferred to have been possible because a threshold condition was reached inthe source area. A rainfall event, which would have had no extreme effects before reaching the threshold, could probablytrigger massive remobilization of bouldery sediments on the valley oors. The Seoraksan alluvial fan thus demonstrates therole of a geomorphic threshold in causing drastic changes in the hydrologic performance of the watershed. The morphologyand sedimentology of the Seoraksan alluvial fan suggest that the fan is a modern example of a sheetood-dominated alluvialfan, which has largely been ignored in spite of their potential diversity and abundance in glacial to periglacial, tropical, andtemperate environments.