Late Holocene episodic river aggradation along the Lao-nong River (southwestern Taiwan): An application to the Tseng-wen Reservoir Transbasin Diversion Project

The Tseng-wen Reservoir Transbasin Diversion Project (the Tseng-wen Project), currently the largest water-resources scheme in Taiwan, aims at constructing a tunnel system across the mountains to divert the water of the Lao-nong River to the Tseng-wen Reservoir, the largest reservoir in the island. This project was suspended, however, because the constructed tunnel entrance on the side of the Lao-nong River has been buried by the 20-30 m aggradation of the river during 2009 Typhoon Morakot, which triggered extensive landslides/debris flows and killed > 400 people in southwestern Taiwan. To understand the river history and help assess the risk to the project, the fluvial/debris-flow sequences exposed along the 5 km-long river reach upstream of the buried tunnel entrance were examined, and a total of 38 radiocarbon dates were obtained for chronological constraints. The study is limited by the intrinsic uncertainties of the dates and by the likelihood that much of the stratigraphic evidence has been eroded away or remains unexposed. Even so, this study identified 9 pre-historical aggradation events in the past 2 ka, each of which had a magnitude comparable to, or greater than, the 2009 event. The most recent event occurred < 200 BP. The event of 0.8 ka (or younger) aggraded the river to 120 m above the 2008 level. These ancient events were sourced mainly from the alternate growth of fan systems from 3 tributaries, two of which contributed to the 2009 event. There are still 3 other such fan-building tributaries joining the studied river reach, and 8 more farther upstream, that must have supplied vast amounts of sediment to the trunk river. In sum, although dominated by incision over long term in response to rapid tectonic uplift, the studied river is subject to short-term, large-scale aggradation, far beyond the coping capacity of the Tseng-wen Project as originally designed. For this and other mountain rivers in Taiwan, future engineering plans should consider the risk of significant river aggradation.