## 4 Discussion
### Change in the Laishe River Midstream  
Large amounts of landslide material in the mountains are the main source of debris flows [56]. In particular, riverbank landslides can directly cause riverbed sedimentation and form landslide dams; subsequently, heavy rains may cause dam breaks, leading to downstream flooding. Monitoring channel platforms is essential for analyzing channel stability as well as improving river management. Three DEM periods were used in this study to clarify the dominant changes in the Laishe River due to Typhoon Morakot and multiple other events from 2009 to 2015. The height difference of the DEMs indicates that the channel change was the highest between April 2009 and 2010 (Figure 4a), during which, a deposition thickness of approximately 34 m can be observed (Figure 5). During this period, only Typhoon Morakot caused severe disasters in southern Taiwan. Therefore, the channel change during this period can be considered as a single event. From 2010 to January 2015, serious erosion of the river channel can be detected (Figure 4b). During this period, the typhoons affecting southern Taiwan were Meranti, Fanapi, Megi, Nanmadol, Talim, Saola, Tembin, Soulik, Trami, Kong-Rey, Usagi, Matmo, Fung-Wong, etc., considering the changes in the river channel during this period as the result of multiple typhoon events. In short, it is found that the changes in the Laishe River channel were mainly caused by Typhoon Morakot in 2009, which induced significant deposition in the middle of the Laishe River. However, the subsequent typhoons caused small incisions and sporadic landslides, and no obvious channel deposition. After Typhoon Morakot, the only erosional part in the middle reaches (Figure 4a marked by a black arrow), was a natural earth dam that blocked the main channel and formed a landslide dam. It can be seen that the area of the landslide dam is shrinking (Figure 4b). Although the landslide dam is located in the upstream area, far from the main downstream residential area, the danger posed by landslide dams cannot be eliminated. Once the outlet of a landslide dam is blocked and accumulated by runoff, the dam may break, which would cause specific harm to the safety of the lives and property of the downstream residents, i.e., the case in Xiaolin village [41; 57; 58]. After six years, the geometric change appears dynamic stable (Figure 4c). However, the height of the channel is still higher than the condition of per-Morakot, and the landslide-induce volume still stored in the channel, which means the uncomented sediment could be the erosional material of the following typhoon events; Overall, the channel gradient in upstream, midstream, and downstream before typhoon Morakot are 14.4%, 6.1%, and 2.4%, respectively. After typhoon Morakot, the channel gradients in upstream, midstream, and downstream are 14.4%, 5.7%, and 2.69%, respectively (Figure 5a). Substantial landslide material remains stored in the channel (Figure 5b). The cross-section of the channel also proved the deposition of 10 m after typhoon Morakot and the incise more than 10 m in 2015 (Figure 5c). Nevertheless, the considerable erosion on Earth's surface is not a single case. The measurable change was observed in the adjacent Laonong River catchment[59]. The Laonong River catchment was also suffering the typhoon Morakot in 2009, but the drainage area of the Laonong River is larger (1373 km\({}^{2}\)). Using ADS40 images to establish a DEM to analyze the geometric changes in the Laonong River channel during Typhoon Morakot, the results show that the channel area increased from 13.78 M m\({}^{2}\) to 19.77 M m\({}^{2}\) after the event. The erosion volume of the main river channel is 31.19 M m\({}^{3}\), the accumulation volume is 147 M m\({}^{3}\), and the maximum deposited height in upstream reaches is about 30-40 m. The riverbank of low height change would also demonstrate the error correction in this study is acceptable.