Abstract
Coastal environments are usually characterized by a brittle balance, especially in terms of sediment transportation. The formation of dunes, as well as their sudden destruction as a result of violent storms, affects this balance in a significant way. Moreover, the growth of vegetation on the top of the dunes strongly influences the consequent growth of the dunes themselves. This work presents the results obtained through a long-term monitoring of a complex dune system by the use of Unmanned Aerial Vehicles (UAVs). Six different surveys were carried out between November 2015 and December 2017 in the littoral of Rosolina Mare (Italy). Aerial photogrammetric data were acquired during flight repetitions by using a DJI Phantom 3 Professional with the camera in a nadiral arrangement. The processing of the captured images consisted of the reconstruction of a three-dimensional model using the Structure-from-Motion (SfM). Each model was framed in the European Terrestrial Reference System (ETRS) using GNSS geodetic receivers in Network Real Time Kinematic (NRTK). Specific data management was necessary due to the vegetation by filtering the dense cloud. This task was performed by both performing a slope detection and a removal of the residual outliers. The final products of this approach were thus represented by Digital Elevation Models (DEMs) of the sandy coastal section. In addition, DEMs of Difference (DoD) were also computed for the purpose of monitoring over time and detecting variations. The accuracy assessment of the DEMs was carried out by an elevation comparison through especially GNSS-surveyed points. Relevant cross sections were also extracted and compared. The use of the Structure-from-Motion approach by UAVs finally proved to be both reliable and time-saving thanks to quicker in situ operations for the data acquisition and an accurate reconstruction of high-resolution elevation models. The low cost of the system and its flexibility represent additional strengths, making this technique highly competitive with traditional ones.
Highlights
Beaches are usually characterized by vegetated dune systems performing a variety of functions valuable to the ecosystem, such as protection against the ingression of the sea water during storms and a sand reservoir that supplies sand to eroded beaches during storms and buffers windblown sand and salt spray
Once successfully performed the validation of each computed Digital Elevation Models (DEMs) generated, with discrepancies similar to those mentioned above, a series of further information was extracted from the DEMs and compared each other
The strength of the aerial imageries by Unmanned Aerial Vehicles (UAVs), with respect to any traditional discrete and time-consuming approach based on the use of total stations or GNSS geodetic receivers in Network Real Time Kinematic (NRTK) mode, lies in the possibility to make cross sections in any region of the model at any time, thanks to a practically continuous data
Summary
Beaches are usually characterized by vegetated dune systems performing a variety of functions valuable to the ecosystem, such as protection against the ingression of the sea water during storms and a sand reservoir that supplies sand to eroded beaches during storms and buffers windblown sand and salt spray. The incipient foredunes, or embryo dunes [3], are low dunes formed by aeolian sand deposition within pioneer plant communities on the backshore of beaches [2,4] Their formation is due to an increase in surface roughness due to the presence of some elements on the surface of the backshore responsible for a reduction of wind flow velocities, resulting in sediment deposition. From a geomorphological point of view, the embryo dunes present the most interesting shapes to be assessed because they are sensitive to even the smallest changes in any of the coastal environment factors. They can quickly be destroyed as they are created. The presence of embryo dunes represents a crucial parameter for the final selection of the case study location
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