Retrogressive Landslide: a Bitter Lesson from Cimanggung Landslide of Sumedang, Indonesia
1Institut Teknologi Bandung, Indonesia, 2Prihaditama Ltd. Co., Indonesia

The Cimanggung landslide occurred on Saturday, January 9, 2021 just after very heavy rainfall. The landslide was a complex landslide showing a change from slide to flow mechanism of debris. It was evidenced that the landslide was not a single event, but at least four landslides occurred as retrogressive developments of the crown. The first landslide with a semi-rotational type took place at 15.30 local time and claimed 8 lives. The second landslide occurred retrogressively at 19:15 and was followed by a debris flow which resulted in the death toll of 32 lives. It is estimated that this second landslide is much larger than the first, from about 20 m crowns with a length of up to 45 m, to 50 m crowns with a debris outflow reaching of up to 120 m. In this case, the third and fourth landslides tended to occur locally in the second landslide scarp.The landslide location has several layers of rock and soil. Those rock and soil layers are lightly weathered breccia tuffs, strong weathered breccia tuffs, residual soils, and landfill. The drainage in upper houses looks ineffective after the landslide. The water flowing through the drainage seeps into the cracks that form behind the landslide crown. Landslide crown is still actively moving backwards (heading east) because the slope conditions are still unstable due to the condition of the water and constituent materials. The retrogressive landslides which followed by debris flow will be modeled to determine the run-out distribution. The results of this modeling will be used as a reference for the emergency response team to secure similar landslides which followed by debris flow at other locations.