2:00 PM - 2:15 PM
[AHW22-02] Response and Recovery of Mountain Rivers to Extreme Rainfall: A Case Study of Heshe River, Taiwan
Keywords:Typhoon Morakot, Mountain Rivers, River Morphology, Landslide rates
The changes in river morphology are the consequence of the interplay between sediment supply and transport. Mountainous river morphology often shifts after rainfall events due to sediment input from slopes or upstream channels. Limited accessibility has restricted the frequency of surveys in mountainous watersheds, making it challenging to assess the impact of rainfall events on landslides and river morphology. This study collects satellite imagery, topographic data, and landslide distribution data. Additionally, we conduct UAV surveys to generate orthophotos and digital terrain models. Using GIS software, we analyze the evolution of main channel width, braided index, riverbed elevation, and riverbed slope in five sub-reaches of the Heshe River and its upstream watershed landslide rates from 2004 to 2024.
The extreme rainfall from Typhoon Morakot in 2009 (1818.5 mm in 72 hours) caused significant morphological changes in the study area. The sub-reaches width expanded between 1.61 to 3.29 times; the braided index rose by 1.1 to 4.92 times, the riverbed elevation increased by 5.33 m to 10.9 m, the riverbed slope steepened by 4.3% to 14.3%, and upstream watershed landslide rates increased by 5.06 times. Between 2010 and 2024, only four years experienced 24-hour rainfall events exceeding the two-year return period. During this period, riverbed elevation gradually degraded, the main channel narrowed, the braided index declined, and watershed landslide areas recovered. As of 2024, relative to the morphological changes caused by Typhoon Morakot, the main channel width of the sub-reaches has recovered to 69.5%–97.9%, the braided index to 23.2%–109%, and riverbed elevation has decreased by an average of approximately 42%. The watershed landslide rate recovered to approximately 75%.
These results indicate that despite the significant remaining sediment load, some reaches have partially adjusted to their pre-Morakot state. The findings of this study provide references for river management and sediment control in mountainous watersheds.
The extreme rainfall from Typhoon Morakot in 2009 (1818.5 mm in 72 hours) caused significant morphological changes in the study area. The sub-reaches width expanded between 1.61 to 3.29 times; the braided index rose by 1.1 to 4.92 times, the riverbed elevation increased by 5.33 m to 10.9 m, the riverbed slope steepened by 4.3% to 14.3%, and upstream watershed landslide rates increased by 5.06 times. Between 2010 and 2024, only four years experienced 24-hour rainfall events exceeding the two-year return period. During this period, riverbed elevation gradually degraded, the main channel narrowed, the braided index declined, and watershed landslide areas recovered. As of 2024, relative to the morphological changes caused by Typhoon Morakot, the main channel width of the sub-reaches has recovered to 69.5%–97.9%, the braided index to 23.2%–109%, and riverbed elevation has decreased by an average of approximately 42%. The watershed landslide rate recovered to approximately 75%.
These results indicate that despite the significant remaining sediment load, some reaches have partially adjusted to their pre-Morakot state. The findings of this study provide references for river management and sediment control in mountainous watersheds.