Digital mapping of coastal boulders – high‐resolution data acquisition to infer past and recent transport dynamics

Abstract

Coastal boulder fields provide clues to long‐term frequency‐magnitude patterns of coastal flooding events and have the potential to play an important role in coastal hazard assessment. Mapping boulders in the field is time and labour‐intensive, and work on intertidal reef platforms, as in the present study, is physically challenging. By addressing coastal scientists who are not specialists in remote sensing, this contribution reports on the possibilities and limitations of digital applications in boulder mapping in Eastern Samar, Philippines, where recent supertyphoons Haiyan and Hagupit induced high waves, coastal flooding and boulder transport. It is demonstrated how satellite imagery of sub‐metre resolution (from Pléiades and WorldView‐3 imagery) enables efficient analysis of transport vectors and distances of larger boulders, reflecting variation in latitudes of both typhoon tracks and approaching angles of typhoon‐generated waves. During the investigated events, boulders with a‐axes of up to 8 m were clearly identified to have been shifted for up to 32 m, mostly along the seaward margin of the boulder field. It is, however, hard to keep track of smaller boulders, and the length of a‐axes and b‐axes including their orientation is often impossible to map with sufficient accuracy. Orthophotographs and digital surface models created through the application of an unmanned aerial vehicle and the ‘Structure from Motion’ technique provide ultra‐high‐resolution data, and have the potential to not only improve the results of satellite image analysis, but also those from field mapping and may significantly reduce overall time in the field. Orthophotographs permit unequivocal mapping of a‐axes and b‐axes including their orientation, while precise values for c‐axes can be derived from the respective digital surface models. Volume of boulders is best inferred from boulder‐specific Structure from Motion‐based three‐dimensional models. Battery power, flight speed and altitude determine the limits of the area covered, while patches shielded by the boulders are difficult to resolve. For some tasks, field mapping remains mandatory and cannot be replaced by currently available remote sensing tools: for example, sampling for rock type, density and age dating, recording of lithological separation of boulders from the underlying geological unit and of geomorphic features on a millimetre to decimetre‐scale, or documentation of fine‐grained sediment transport in between the boulders in supratidal settings. In terms of future events, the digital products presented here will provide a valuable reference to track boulder transport on a centimetre to decimetre‐scale and to better understand the hydrodynamics of extreme‐wave events on a fringing reef coastline.

Impact of the October 2018 Storm Vaia on Coastal Boulders in the Northern Adriatic Sea

“…UAVderived orthophotographs and digital surface models (DSMs) can provide excellent data and information on coastal boulder patterns. Orthophotographs allow for the mapping of a axes and b axes, including their orientation, whereas precise values for c axes and boulder volume can be taken from the DSM [50]. In recent years, after an initial decade when the analyses were mostly dedicated to explaining boulder detachment and transport mechanisms and distinguishing between those of tsunamigenic and those of storm origin, boulder studies are now more oriented towards the To investigate the boulder motions on the Premantura Promontory (as the nearshore bathymetry of the model may not be accurate due to a lack of both resolution and accuracy of the bathymetry data), the wave parameters were extracted using a point off Cape Kamenjak, at a depth of about 29 m (Figure 10) and the wave height at the breaking point was calculated using the Sunamura and Horikawa equation [44].…”

Measuring Change Using Quantitative Differencing of Repeat Structure-From-Motion Photogrammetry: The Effect of Storms on Coastal Boulder Deposits

“…They have been documented around the Mediterranean [72][73][74][75], as well as in the Atlantic on the Aran Islands, Ireland [70,76], the Shetland and Orkney Islands, Scotland [69], Banneg Island, France [77], and Iceland [78]. CBD are also located in both western and northeast Australia [66,79], Iran [80], Oman [81], the Philippines [67,68,82], the Caribbean [71,[83][84][85][86], South Africa [87], and elsewhere. Until recently, however they received little attention: about 90% of published studies are from the last 18 years [88].…”

“…Data were collected in 2015 and 2017 using a DJI Phantom 3 (FC300X) drone controlled by DJI Groundstation Pro (Table 1). Phantom drones are relatively inexpensive, costing between $400 and $1500, and have been used for similar SfM studies [57,82,101]. Images were collected at nadir along precisely calibrated flightpaths parallel to the coast (Figure 4), at altitude 90 m in 2015 and 50 m in 2017 (Table 1).…”

“…Agisoft's ease of use, relatively low cost, and high-quality output [123][124][125] have led to broad uptake in scientific communities including forestry [101], geology [126] , geomorphology [41,56,82,127,128], and archeology [32,129,130]. The software has an intuitive workflow that guides users step-by-step through the process of generating a 3D model [105].…”

Tsunami deposits: Present knowledge and future challenges

“…Moreover, in the same location, Spiske et al (2020) also reinterpreted boulder accumulations as remnants of in situ platform denudation that produces shell hash, coral clasts and boulders. Boesl et al (2020) demonstrated how satellite imagery with a sub-metric resolution contributed to a better analysis of boulder dislocation (transport vectors and distances), even in boulder fields which cover larger areas. Highresolution UAV (unmanned aerial vehicle)-based ortho-photographs and digital surface models provided manifold data and information on coastal boulder patterns and can strongly support time-intensive and labour-intensive field surveys.…”