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Time-lapse photography of an active coastal-bluff landslide, Mukilteo, Washington, August 2015 - May 2016

July 21, 2020

A time-lapse camera was used to document periodic reactivation of a complex landslide on a steep coastal bluff in Mukilteo, Washington. This landslide is one of four monitoring sites initiated by the U.S Geological Survey to investigate hill-slope hydrology and landslide hazards affecting the railway corridor along the eastern shore of Puget Sound between the cities of Seattle and Everett (Mirus et al., 2016; Smith et al. 2017). The camera was installed in the crown of the landslide above the main scarp facing roughly North, with a field of view that includes the head of the landslide body and a minor scarp below. The attached file ‘CameraLocation.PNG’ provides an overview figure of the landslide and the camera’s location relative to the different monitoring stations. It recorded imagery from August 19th, 2015 through May 25th, 2016. The time-lapse photos were taken three times daily (at 9 am, 12 pm, and 4 pm, PST) and stored onsite on a memory card; corresponding intervals between the photographs were 3 hours, 4 hours, and 17 hours (overnight). The time-lapse photos were compiled into a video and five periods of distinct ground movement were identified. Apparent slow and consistent slope-surface movements are recorded during these periods, but subsequent site visits suggest that these slow displacements indirectly triggered topples and debris-avalanche movements both up slope and down slope of the camera’s field of view. The approximate sizes of topples and debris avalanches were on the order of 105 -107 cubic centimeters. The video captures slope movements during the time periods of December 8-9, 2015; January 21-30, 2016; and March 9-14, 2016. In addition, the video shows two seemingly "instantaneous" events during the nights of March 23 and March 26. Each of these periods of slope movement also correspond to observed rainfall events and associated subsurface hydrologic responses documented elsewhere (Mirus et al. 2017; Smith et al. 2017). The time-lapse video can be found in the attached .mp4 file "mukilteo_timelapse_video.mp4" The individual time-lapse photos can be downloaded from the attached zip file.

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