Three Years of UAS-Based High Resolution Topographic Surveys of a Coastal Boulder Deposit to Monitor Change and Assess Tsunami vs Storm Wave Deposition in Java, Indonesia
Abstract
Boulder deposits can be important records of tsunami activity; however, distinguishing whether they were formed by tsunami or storm wave activity can be difficult, particularly when they are located in or near the littoral zone. We used uncrewed aerial systems (UAS) and structure-from-motion photogrammetry to generate 4 sets of 1 cm digital elevation models (DEMs) and 0.5 cm orthomosaics of a boulder deposit at Watu Karung (8.241S 110.983E) on the south coast of Java, Indonesia over 3 years as tools to measure change to the deposit and assess its origin. The deposit is ~700 m2, ~2 m asl, and comprises limestone and hardpan boulders ~0.5 to 2 m in length. Except at high tide, the deposit is separated from the sea by a steep (~25-35o) hardpan slope and 0.5-1.0 m tall hardpan berm, which inhibits landward transport of boulders. The deposit was surveyed four times: twice in 2016 (2016a, 2016b), in 2017, and in 2019. Surveys 2016a and 2016b were conducted immediately before and after a significant storm swell event (~4 m) contemporaneous with a spring tide. A larger swell event (~5 m; probably a 10 - 20 year event) occurred in July 2018. UAS used were quadcopters with a 24 MP Sony A5100 camera (2016a and b), a 20 MP DJI camera (2017), or a 12 MP DJI camera (2019). All of the surveys except 2019 included ground control points (GCPs) surveyed with dGNSS. The 2016 surveys used the same GCPs to improve DEM co-registration. The 2019 survey was georeferenced using natural control points that were assigned geographic positions using the 2017 data. DEMs were differenced against each other and hillshades and orthomosaics were compared to reveal locations of boulder erosion and deposition. Between the 2016a and 2016b surveys, ~10% of boulders moved, including one 1.7 x 0.7 x 0.2 m in size that moved 5.6 m. Between 2016 and 2017, approximately 300 of 650 large boulders (1.6 m average length) moved between 10 cm and 30 m. By 2019, most boulders had moved, and more than 150 boulders moved at least ~ 2 m relative to 2016; many of these rotated about horizontal axes. However, only one large (> 1.5 m length) boulder is recognized as having been added to the deposit. Thus, although storm waves extensively modified the deposit, it likely originally was formed by a tsunami, progressive landward erosion of the shoreline and exposure of boulders, or perhaps larger swell events.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMNH43F1007B
- Keywords:
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- 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDS;
- 4333 Disaster risk analysis and assessment;
- NATURAL HAZARDS;
- 4341 Early warning systems;
- NATURAL HAZARDS;
- 4564 Tsunamis and storm surges;
- OCEANOGRAPHY: PHYSICAL