3:30 PM - 5:00 PM
[HGM02-P09] Evolution of River Morphology and Landslide Distribution in Shenmu Village Watershed, Central Taiwan
Keywords:River Morphology, Landslide Distribution, Mountain River
The characteristics of mountainous rivers include steep slope, distribution of boulders, and high seasonal variation of discharge. The river morphology is adjusted by the floods and sediment inputs from the upstream and the slopes. The study area is on the Shenmu village watershed and its three tributaries (Aiyuzi, Hosa, and Chushui creeks) and their downstream Heshe creek. The landslide data from 2004 to 2018 interpreted by the authority were collected, and were used to analyze the evolution of the landslide distribution (newborn or restoration) by GIS spatial analysis tool. Based on the collected aerial photo and topography data and the orthophotos taken by an unmanned aerial vehicle, the changes in river morphology from 2004 to 2022, including channel width, channel number divided by bars (braided index), valley confinement, and riverbed elevation, in the study area were analyzed in GIS software.
Although there were three storm events having maximum 24-hour rain over 650 mm between 2004 and 2009, the watershed landslide area ratio was around 2.2±1.4% in the study area. However, the storm brought by Typhoon Morakot in August 2009 had 24-hour extreme rainfall up to 1138 mm, and it induced many large-scale landslides, resulting that the watershed landslide area ratio was jumped up from 1.8% to 9.3%. A large amount of sediment transported into the Heshe creek resulted in channel bed severe aggradation with average depth of 9.7 m, the average channel width was expanded from 190 m to 237 m, and the channel braided index rose up from 3.9 to 7, while the channel width in the upstream tributaries did not change markedly due to the confinement of the valley. Between 2010 and 2022, the maximum 24-hour rainfall did not exceed 650 mm. The percentage of landslide areas in the watershed dropped to 5.3% in 2018. The incision and narrowing happened in Heshe creek from 2010 to 2022. The riverbed elevation decreased by an average of 4.9 meters, the channel width reduced to 130 m, and the braided index lowered to 2.55 in 2022.
Although there were three storm events having maximum 24-hour rain over 650 mm between 2004 and 2009, the watershed landslide area ratio was around 2.2±1.4% in the study area. However, the storm brought by Typhoon Morakot in August 2009 had 24-hour extreme rainfall up to 1138 mm, and it induced many large-scale landslides, resulting that the watershed landslide area ratio was jumped up from 1.8% to 9.3%. A large amount of sediment transported into the Heshe creek resulted in channel bed severe aggradation with average depth of 9.7 m, the average channel width was expanded from 190 m to 237 m, and the channel braided index rose up from 3.9 to 7, while the channel width in the upstream tributaries did not change markedly due to the confinement of the valley. Between 2010 and 2022, the maximum 24-hour rainfall did not exceed 650 mm. The percentage of landslide areas in the watershed dropped to 5.3% in 2018. The incision and narrowing happened in Heshe creek from 2010 to 2022. The riverbed elevation decreased by an average of 4.9 meters, the channel width reduced to 130 m, and the braided index lowered to 2.55 in 2022.