Taiwan's rugged, mountainous terrain, a result of active tectonics, and its monsoonal climate create conditions that produce the greatest sediment yield compared to all land surfaces on Earth. It is estimated that, on average, Taiwanese rivers discharge ~ 150 million tons of sediment annually into Taiwan Strait. The rivers discharge most of the sediment during the relatively short time periods of torrential rains often associated with typhoons, and thus the waters have a high mud fraction (fine-grained sediment, mainly composed of clay). The voluminous amount of sediment that rapidly accumulates near river mouths is dispersed effectively. On a longer time scale, there is persistent northward sediment transport. In this study, we examine the Jhoushuei River system, which has the largest average annual sediment load in Taiwan. The first survey was conducted in 2008, 40 hours after peak fluvial discharge caused by Typhoon Kalmaegi. We measured sediment discharge in the lower reach of the river channel and surveyed suspended sediment distribution in the coastal zone near the river mouth. During Kalmaegi's landfall on Taiwan, suspended-sediment concentration in the Jhoushuei River reached as high as 120 g L–1 and the estimated total sediment discharge was about 30 million tons. Since then, we have conducted surveys on four more occasions in the coastal zone near the river mouth, where we measured suspended-sediment concentrations and currents, and sampled surface sediments over several tidal cycles. The initial results shed light on the mechanism of efficient sediment dispersal during massive sediment-discharge events. Fine-grained sediments originally deposited near the river mouth following typhoon floods, generally in summer and fall, are dispersed during winter. The transport mechanism responsible for the sediment removal warrants further investigation.
Full Text Finder