Using Point Clouds Generated from Unmanned Aerial Vehicles Imagery Processed with Structure from Motion to Address Tsunami vs Storm Wave Boulder Deposition in Watu Karung, Indonesia

Tsunamis pose a major hazard to coastal communities along the south coast of much of Indonesia due its location on the Australian-Sunda arc. Furthermore, tsunamis and high-energy wave events are the principal drivers of geomorphic change in the area and it is difficult to distinguish the effects of each. A potentially useful indicator of past tsunami activity is coastal imbricated boulder deposits. To address whether an imbricated boulder deposit located on a beach in Watu Karung (Java, Indonesia) could have been formed by non-tsunami wave activity and to investigate coastal geomorphic change, we generated three pairs of digital surface models (DSMs) over an approximately one year period using photographs taken from a small unmanned aerial vehicle and structure-from-motion photogrammetry. The first two DSMs were made from photographs taken on 7/30-31/2016 and 8/2/2016, immediately before and after a significant 4.2 m swell struck the beach during a +2.5 m spring high tide. The third DSM pair was made from imagery collected 7/12/2017. Each pair of DSMs consists of a 1 cm pixel DSM of the boulder deposit and a 4 cm DSM of the larger beach area that surrounds the boulders. In addition, prior to the 2016 wave event 21 boulders up to 75 kg were marked and hand-placed shoreward of the boulder deposit; their movement was tracked with RTK GPS measurements. In the 2016 wave event, every hand-placed boulder moved, with an average displacement of 27.6 m. At the same time, approximately 20 of 650 naturally - occurring boulders, up to 2 m in length, moved more than 10 cm and up to 5.6 m. Between 2016 and 2017, approximately 300 of 650 naturally - occurring boulders with an average length of 1.6 m moved varying distances of at least 10 cm and up to 30 m. In addition, changes in beach sand volume occurred in ten 25 m2 localized zones on the beach with an average volume change of approximately 65 m2. Changes in both boulder position and sand volume occurred during the 2016 to 2017 time period when no tsunamis affected Watu Karung—thus indicating that all changes were the result of storm wave events.