# Mapping Recent Shoreline Changes Spanning the Lateral Collapse of Anak Krakatau Volcano, Indonesia
## 4 Results and Discussion  
The methodology detailed in Section 3, provides information on the areal extent of the island since 15 May 2018, as well as the area of the crater lake that formed following the December 2018 collapse (which only became visible after 13 January 2019) as part of the volcanic rebuilding phase (Figure 5) and is considered land in our results having a similar NDWI range. Within our area of interest, NDWI thresholds values range from 0.08 when clouds, waves or turbid water (including both sediment-laden water containing tephra from the eruption and/or water clouded by hydrothermal precipitates) perturb the Sentinel-2 acquisitions to 0.38 when calm sea conditions prevail. With an overall standard deviation of 0.16 km\({}^{2}\), the results reveal that the area of the island has varied between 2.78 km\({}^{2}\)(1 August 2018) and 3.27 km\({}^{2}\)(12 July 2019) with a sharp increase in area following the lateral collapse due to rapid post-collapse regrowth (not captured in Figure 5 due to the gap in data). The wider scatter in the measurements in February 2019 and April 2019 mainly relates to the continuous changes over the newly regrown western side of the island. Starting from May 2018, the shoreline changes show no increase in the island area, despite the elevated eruptive activity in Summer 2018 noted by [PERSON] et al. (2019) [32] but a slight decrease from 2.93 km\({}^{2}\) in May 2018 to 2.83 km\({}^{2}\) in November 2018. Within this period, there is a small increase in  
Figure 5: Change in the size of the AK land surface (red circle) and the crater lake (blue circles) before and after the December 2018 lateral collapse (dashed black line). The large time gap from early November 2018 to the end of January 2019 is due to the combined effects of rainy-season cloud cover and the obscuring effects of the eruption and associated atmospheric plumes in early 2019.  
area from August to October 2018, due to the shoreline advancement towards the south given by the newly erupted material. Large and rapid changes in area, characterized by an initial decrease due to the lateral collapse (not observable on Figure 5) and then an increase due to the post-collapse intense explosive activity, took place in December 2018 and January 2019. Based on the reduction in island volumes of 49% stated by [PERSON] et al. (2019)[PERSON] et al. (2019), the post-collapse island had an area of ~1.5 km\({}^{2}\), which then increased to over 3 km\({}^{2}\) in the following two months after the collapse. The finer detail of these changes is not observable in our dataset and only seen as a large net increase because of the data gap around the time of the lateral collapse. The first Sentinel-2 image available after the collapse shows that the size of the island rapidly increased of \(>\)15%, from 2.83 km\({}^{2}\) on 16 November 2018 to now, where the island is consistently larger at 3.19 km\({}^{2}\). Such an abrupt change in 68 days can only be explained by the extreme intensity of the post-collapse Surtseyan activity (Sutteyan, 2019), which emplaced pyroclastic deposits much faster than they could be eroded by waves and currents at the coast. In contrast, with the decline of volcanic activity and perhaps also increased wave activity in the southern hemisphere winter (with long swell waves originating from winter storm systems in the southern Indian Ocean and Southern Ocean entering Sunda Strait from the south-west), there has been a slight but consistent decrease in an area of 0.08 km\({}^{2}\) between July 2019 and November 2019. In contrast to the variation in the coastline of the island, the crater lake area since its formation in January 2019 shows a gradual initial decrease in area (January to May 2019) followed by an almost regularly growth from 0.14 to 0.15 km\({}^{2}\) (Figure 5) since May 2019.