INLAND MIGRATION OF COASTAL BOULDER DEPOSITS ON INISHMAAN, IRELAND, MEASURED USING USING STRUCTURE-FROM-MOTION PHOTOGRAMMETRY

Coastal boulder deposits (CBD) accumulate above the high-water mark along steep coastlines with open-ocean exposure. They include megagravel with masses in the many 10s of tonnes, and therefore preserve a record of high-energy events; but because they are activated only by very large waves they can be essentially dormant for long periods of time, and undergo substantial changes only episodically. Thus their dynamics are not well known. The 2013-2014 “storm factory” in the northeastern Atlantic region brought a series of extreme wave events to western Ireland, and in its aftermath we used structure-from-motion photogrammetry to map CBD on Inishmaan, one of the Aran Islands off Ireland’s west coast. We used image analysis to make quantitative comparisons between t the photogrammetric data and pre-existing orthophotography so that we could measure changes in the CBD.

Photogrammetric images were captured in June 2016 using a DJI Phantom II flying at 60 m altitude above the coastal platform. Vertical images were acquired along programmed flight paths with 80% in-track overlap between successive photographs and 60% sidelap between adjacent parallel tracks. The 3D structure-from-motion models were created in Agisoft PhotoScan Pro. We georectified and registered the photogrammetric model with 2012 orthoimagery (available in Bing Maps).

The CBD on Inishmaan are dominated by coast-parallel boulder ridges 2-5 m high and 10s of m wide, and although they are constructed predominantly of cobble to boulder-sized material, they also contain megagravel clasts up to 80 t mass. The ridges sit at elevations up to 25 m above high water, and up to 220 m inland. Most of the boulder ridges had undergone some reorganisation, and several segments showed substantial inland migration. Clasts were transported from the front of the boulder ridges over the ridge crest and onto the landward side. We measured up to 6m inland displacement of seaward faces of the boulder ridges. On the landward side of the ridges, walls were knocked down and over-run by boulders, and newly-deposited boulders carpeted adjacent fields. Most of the moved boulders were fairly small (<1 t) but several were in the range 10- 40 t.

These findings underscore the role of storm waves in building and modifying coastal boulder deposits.