The impact of storm-triggered landslides on sediment dynamics and catchment-wide denudation rates in the southern Central Range of Taiwan following the extreme rainfall event of Typhoon Morakot

Extreme erosion events can produce large short-term sediment fluxes. Such events complicate erosion rates estimated from cosmogenic nuclide concentrations in river sediment by providing sediment with a concentration different from the long-term basin average. We present a detrital Be-10 study in southern Taiwan, with multiple samples obtained in a time sequence bracketing the 2009 Typhoon Morakot, to assess the impact of landslide sediment on Be-10 concentrations ((NBe)-Be-10) in river sediment. Sediment samples were collected from 13 major basins, two or three times over the last decade, to observe the temporal variation of (NBe)-Be-10. Landslide inventories with time intervals of 5-6 years were used to quantify sediment flux changes. A negative correlation between (NBe)-Be-10 and landslide areal density indicates dilution of (NBe)-Be-10 by landslide sediment. Denudation rates estimated from the diluted (NBe)-Be-10 can be up to three times higher than the lowest rate derived from the same basins. Observed increases imply that, 3 years after the passage of Typhoon Morakot, fluvial channels still contain a considerable amount of sediment provided by hillslope landslides during the event. However, higher (NBe)-Be-10 in 2016 samples indicate that the contribution from landslide sediment at the sampled grain size has decreased with time. The correlation between changes in (NBe)-Be-10 and landslide area and volume is not strong, likely resulting from the stochastic and complex nature of sediment transport. To simultaneously evaluate the volume of landslide-derived sediment and estimate the background denudation rate, associated with less impulsive sediment supply, we constructed a sediment-mixing model with the time series of (NBe)-Be-10 and landslide inventories. The spatial pattern of background erosion rate in southern Taiwan is consistent with the regional tectonic framework, indicating that the landscape is evolving mainly in response to the tectonic forcing, and this signal is modified, but not obscured by impulsive sediment supply. (c) 2019 John Wiley & Sons, Ltd.