Disproportionately Magnified Sediment Export in Response to Increased Event Runoff: Analysis of Long-term Data from Mountainous Rivers in Taiwan

Abstract:

Fluvial sediment export from small mountainous rivers in Oceania dominates the global land-to-ocean sediment delivery, which might be speeding up at the recognized conditions of increased rainfall intensity. In this study, the historical runoff and sediment export from 16 major rivers in Taiwan are investigated and separated into an early stage (1970-1989) and a recent stage (1990-2010) to illustrate the changes of both runoff and sediment export. The mean daily sediment export from Taiwan Island in the recent stage significantly increased by >80% with subtle increase in daily runoff, indicating more sediment being delivered to the ocean per unit of runoff in the recent stage. In the early stage, 1 km³ runoff could transport sediment of 1.10 Mt. However, the 2x larger runoff now transported 3.38 Mt sediment which is ~3x larger than in the early stage. The medians of the runoff depth and sediment yield extremes (99.0-99.9 percentiles) among the 16 rivers increased by 6.5%-37% and 62%-94%, respectively, reflecting the disproportionately magnified response of sediment export to the increased runoff. Taiwan is facing increasing event rainfall intensity which has resulted in chain reactions on magnified runoff and sediment export responses. As the globe is warming, rainfall extremes, which are proved to be temperature-dependent, very likely intensify runoff and trigger more sediment associated hazards. This seems the Taiwan’s destiny facing high rainfall intensity and having erodible lithology. We are afraid that so far engineering still could not resist the power of nature.