17 Jul 2023
 | 17 Jul 2023

Decadal-scale decay of landslide-derived fluvial suspended sediment after Typhoon Morakot

Gregory Ruetenik, Ken Ferrier, and Odin Marc

Abstract. Landslides influence fluvial suspended sediment transport by changing sediment supply and grain size, which alter suspended sediment concentrations and fluxes for a period of time after landsliding. To investigate the duration and scale of altered suspended sediment transport due to landsliding, we analyzed suspended sediment concentration and water discharge measurements at 87 gauging stations across Taiwan over an 11-year period after Typhoon Morakot, which generated nearly 20,000 landslides in 2009. At each gauging station, we computed annual rating curves to quantify changes over time in the sensitivity of suspended sediment concentrations to water discharge. Among the 40 stations in basins that were impacted by landsliding, the discharge-normalized rating curve coefficient ã was higher than that before Morakot by a factor of 5.1 ± 1.1 (mean ± standard error) the first year after Morakot (2010). The rating curve exponent b did not decrease at most stations until a year later (2011), when the average b value was lower than that before Morakot by 0.25 ± 0.05. Across the compilation of gauging stations, post-Morakot changes in ã were positively correlated with landslide intensity for seven years after Morakot, while post-Morakot changes in b were negatively correlated with landslide intensity from 2011 to 2014, reflecting a tendency for larger changes in ã and b to occur in basins with more intense landsliding. At 26 of these 40 stations, elevated values of ã declined after the initial post-Morakot peak, consistent with a gradual return to pre-Morakot suspended sediment transport conditions. Exponential regressions to these ã values reveal a median characteristic decay time of 8.8 years (interquartile range: 5.7–14.8 years). Values of ã tended to decline faster in basins with more intense landsliding, with a mean characteristic decay time of 6 years in the basins hardest hit by landsliding. Shortly after Morakot, changes in ã and b tended to be larger in basins with more intense landsliding, but this sensitivity to landsliding decayed away within 4–7 years. At stations that were not impacted or only minimally impacted by landsliding, neither ã nor b exhibited systematic responses to Morakot. To quantify the effect of landsliding on sediment discharge, we compared the measured sediment discharges after Morakot to the hypothetical sediment discharges that would have occurred if Morakot had induced no landslides, calculated by applying each station's pre-Morakot rating curve to its post-Morakot water discharge history. This analysis suggests that Morakot-induced landsliding increased sediment discharge by as much as >10-fold in some basins in the 1–2 years after Morakot. Together, these results indicate that the influence of Morakot-induced landsliding on rating curves was large shortly after Morakot but diminished in less than a decade in most of the study rivers, and will be imperceptible in another few decades in all of the study rivers. To the extent that these results are applicable to other landscapes, this suggests that periods of elevated sediment transport efficiency after landsliding should persist for years to decades, even if the landslide deposits persist for centuries to millennia.

Gregory Ruetenik, Ken Ferrier, and Odin Marc

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1278', Aaron Bufe, 08 Aug 2023
  • RC2: 'Comment on egusphere-2023-1278', Harrison Martin, 08 Jan 2024
Gregory Ruetenik, Ken Ferrier, and Odin Marc
Gregory Ruetenik, Ken Ferrier, and Odin Marc


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Short summary
Fluvial sediment fluxes increased dramatically in Taiwan during Typhoon Morakot in 2009, which produced some of the heaviest landsliding on record. We analyzed fluvial discharge and suspended sediment concentration data at 87 gauging stations across Taiwan to quantify fluvial sediment responses since Morakot. In basins heavily impacted by landsliding, rating curve coefficients sharply increased during Morakot and then declined exponentially with a characteristic decay time of <10 years.