Intense precipitation or seismic events can generate clustered mass movement processes across a landscape. These rare events have significant impacts on the landscape, however, the rarity of such events leads to uncertainty in how they impact the entire geomorphic system over a range of timescales. Taiwan is steep, tectonically active, and prone to landslide and debris flows, especially when exposed to heavy rainfall events. Typhoon Morakot made landfall in Taiwan in August of 2009, causing widespread landslides in southern Taiwan. The south to north trend in valley relief in southern Taiwan leads to spatial variability in landslide susceptibility providing an opportunity to infer the long‐term impact of such landslide events on channel morphology. We use pre‐ and post‐typhoon imagery to quantify the propagating impact of this event on channel width as the debris is routed through the landscape. The results show the importance of cascading hazards from landslides on landscape evolution based on patterns of channel width (both pre‐ and post‐typhoon) and hillslope gradients in 20 basins along strike in southern Taiwan. Prior to Typhoon Morakot, the river channels in the central part of the study area were about 3–10 times wider than the channels in the south. Following the typhoon, aggradation and widening was also a maximum in these central to northern basins where hillslope gradients and channel steepness is high, accentuating the pre‐typhoon pattern. The results further show that the narrowest channels are located where channel steepness is the lowest, an observation inconsistent with a detachment‐limited model for river evolution. We infer this pattern is indicative of a strong role of sediment supply, and associated landslide events, on long‐term channel evolution. These findings have implications across a range of spatial and temporal scales including understanding the cascade of hazards in steep landscapes and geomorphic interpretation of channel morphology. Copyright © 2018 John Wiley & Sons, Ltd.
Landscape form represents the cumulative effects of de‐stabilizing events relative to recovery processes. Most geomorphic research has focused on the role of episodic rare events on landscape form with less attention paid to the role and persistence of chronic inputs. To better establish the interplay between chronic and episodic extreme events at regional scales, we used aerial photography and post‐flood sediment sampling to assess stream and hillslope response and recovery to a 100–300 yr. flood caused by Tropical Storm Irene in New England. Within a 14 000 km2study area, analysis of aerial photographs indicated that the storm initiated (n = 534) and reactivated (n = 460) a large number of landslides. These landslides dramatically increased overall estimates of regional erosion rates (from 0.0023 mm/yr. without Irene to 0.0072 mm/yr. with Irene). Similarly, Irene‐generated LWD inputs of 0.25–0.5 trees/km exceeded annual background rates in a single event, and these concentrated inputs (101–102of trees/landslide) are likely to result in large jams and snags that are particularly persistent and geomorphically effective. Finally, we found that landslide scars continue to provide elevated sediment inputs years after the event, as evidenced by sustained higher suspended sediment concentrations in streams with Irene‐generated landslides. Overall, our results indicate that infrequent, high‐magnitude events have a more important geomorphic role in tectonically stable, more moderate‐relief systems than has been previously recognized. Understanding the role of these events has particular relevance in regions such as New England, where the frequency and magnitude of extreme storms is expected to increase. Further, these effects may force reconsideration of conservation and restoration targets (for example in channel form and large wood loading and distribution) in fluvial systems. Copyright © 2016 John Wiley & Sons, Ltd.
more » « less- Award ID(s):
- 1636415
- NSF-PAR ID:
- 10237917
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Earth Surface Processes and Landforms
- Volume:
- 41
- Issue:
- 10
- ISSN:
- 0197-9337
- Format(s):
- Medium: X Size: p. 1437-1447
- Size(s):
- ["p. 1437-1447"]
- Sponsoring Org:
- National Science Foundation
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