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

Abstract

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 10Be 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 10Be concentrations (N10Be) 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 N10Be. Landslide inventories with time intervals of 5–6 years were used to quantify sediment flux changes. A negative correlation between N10Be and landslide areal density indicates dilution of N10Be by landslide sediment. Denudation rates estimated from the diluted N10Be 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 N10Be in 2016 samples indicate that the contribution from landslide sediment at the sampled grain size has decreased with time. The correlation between changes in N10Be 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 N10Be 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. © 2019 John Wiley & Sons, Ltd.

30-year record of Himalaya mass-wasting reveals landscape perturbations by extreme events

“…Background rates of mass-wasting are driven by tectonic uplift 1,6 and climate [7][8][9] , though their relative contributions over geological timescales are difficult to unravel 10 . At shorter timescales, mass-wasting background rates are perturbed by a variety of low-frequency, high-magnitude drivers including extreme rainfall, earthquakes and floods [11][12][13][14] . Quantifying and unravelling mass-wasting caused by such diverse sets of drivers is fundamental in efforts to forecast and mitigate mass-wasting hazards in response to environmental change.…”

“…Compilation and comparison of erosion rates measured over different timescales can isolate the roles of different mass-wasting drivers. Such approaches typically utilize proxies including cosmogenic nuclides or suspended sediment flux to establish long-term background erosion rates against which shorter term perturbations captured by field sampling or remote sensing can be measured 2,12,13,15 . However, these approaches are inherently uncertain, with different methods over different timescales producing significantly different results 16 .…”

Hurricanes alter 10Be concentrations in tropical river sediment but do not change regional erosion rate estimates

“…The effect of single storms on sediment sourcing (Clark et al, 2017) and the impact of such major storms on the concentration of in‐situ and meteoric cosmogenic 10 Be (Chen et al, 2019; Foster & Anderson, 2016), both of which are used to estimate catchment‐averaged erosion rates when measured in detrital river sediment, are not well characterized. Detailed studies of sediment sourcing and transport pathways before and after storms suggest that there can be significant changes to river channel sediment sources during and following extreme events (Chen et al, 2019; Foster & Anderson, 2016; Grande et al, 2021; Karwan et al, 2018; Lizaga et al, 2019). These changes have the potential to affect calculated long‐term erosion rates, and by association, the basin‐wide process inferences made from such rates.…”

The effect of lithology on the relationship between denudation rate and chemical weathering pathways

“…Although this integration time varies between catchments with different denudation rates, it reflects the timescale of mineral supply to the weathering zone, and is therefore appropriate for comparison to weathering proxies. Where landslides dominate, their stochastic occurrence may disrupt the relationship between soil formation and chemical weathering (Emberson et al, 2016b;Emberson et al, 2016a) and may bias the denudation rate estimates (Niemi et al, 2005;Tofelde et al, 2018;Chen et al, 2020;Yanites et al, 2009). We assume that any such bias is within uncertainty of the denudation rate measurement.…”