This paper explores the combination of unmanned aerial vehicle (UAV) photogrammetry and an e-GNSS survey to investigate the short-term geomorphic change and dynamic evolution of a beach system in 2015. The RMS values of the ground control points and checkpoints validate that the accuracy of UAV-SfM photogrammetry is suitable for the rapid and frequent acquisition of 3D survey data from a beach environment. We produced four digital surface models (DSMs) to determine the short-term beach topographic response to well-known western Pacific typhoons, the NE monsoon and anthropogenic construction using multitemporal surface analysis. We investigated the magnitude of the erosion caused by intense typhoons; however, the beach-dune system generally recovers through sediment transport processes in winter. Beach monitoring also reveals that the overall sand volume of this beach was reducing in short-term period. Given that the sand budget of Shanshui beach would have been continuing to reduce, this may be a negative warning to this precious tourism resource. The dynamic evolution of this beach may result in the formation of a tombolo. This study demonstrates the feasibility and survey efficiency of UAV-SfM photogrammetry for coastal monitoring, and the technique will contribute to identifying the risk to beach systems and will benefit sustainable coastal management.
ABSTRACT:Penghu islands, in the southern Taiwan Strait, is a remnant of a middle-late Miocene basaltic shield volcano. We present a procedure to use UAV (Unmanned Aerial Vehicles) to perform photogrammetry survey and monitoring analysis in beach evolution scenarios. The aim of this study is to understand spatial-temporal change along the sandy beach in Penghu islands, especially as for the effects of typhoon and coastal structures. According to the study result, this example of application is provided to show the results and the potential of this methodology in real beach changes. In addition, we found the typhoon and coastal structures play important roles to shape the beach morphology and its evolution. The result of beach monitoring reveals that the reduction and change of sand volume in Shanshui beach resulted from the placement of detached breakwater complexes. This coastal structure likely resulted in the development of tombolo and therefor make the beach unstable and subject to conduct rip current and more erosion. .
Abstract. The widespread tsunami risks in the South China Sea have diverse origins from trench megathrust to intraplate earthquake or landslide and remain poorly understood due to the scarce historical and geological records. The cliff-top paleotsunami gravels and basalt boulders on Penghu Islands in the Taiwan Strait present facies constraints on sediment transport, wave estimates from incipient motion formulas, and stratigraphic links to the probable sources. The boulders are supported by a pumice-bearing mud matrix that reflects a suspension-rich turbulent flow process and the typical rolling–saltation transport that results from bore-like waves. Calibrating for ancient sea level height and 100 year surge indicates that the storm waves that are likely to form in the shallow interisland bathymetry only enable boulder sliding–rolling and are incapable of the 2.5 m high cliff-top deposition. The estimated minimum height of tsunami waves is also insufficient and needs to add to 3.0 m high for a minimum cliff-top overflow of 0.5 m depth for terminal rolling before deposition. Coeval gravels in two other outcrops also record the time and extent of tsunami deposition, and are characterized by beach-derived bioclasts and stranded pumices, sharp base, matrix support, poor sorting, and elevation reaching above the 100 year surge. The gravels mark the local minimum wave run-ups and reach 2.4–4.0 m above sea level. The 1575–1706 radiocarbon age of the studied boulder suggests a probable tie to the disastrous 1661 earthquake in the SW Taiwan Orogen and the megathrust source in the northern Manila Trench.
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