Mariano Bresciani, Nicola Ghirardi, Giulia Luciani, Claudia Giardino
Institute for Electromagnetic Sensing of the Environment, National Research Council of Italy
(CNR-IREA), Milan (Italy)
Research theme aims
This research them focuses on the use of satellite remote sensing to map coastal erosion vulnerability in two Italian sites: Pianosa Island (Tuscany) and Piscinas (Sardinia). For both areas we focused on the land/water transitional ecosystem, with the aim of identifying potential coastal erosion phenomena and to demonstrate the role of benthic habitats in preserving the value of coastal environments. By merging the satellite products, the coastal erosion vulnerability maps have been generated based on substrate type in shallow waters and sand volume variation on land: rocky bottoms and stable meadows of phanerogams seemed preserving the coast, while the substrate characterized by a loss of phanerogams and a decrease in sand volumes might be considered more vulnerable.
Materials and Methods
Pianosa Island is the fifth, by extension, of the seven islands of the Tuscan Archipelago National Park with a total area of 10.2 km2 and a coastal perimeter of approximately 20 km. The island is almost completely flat, with some small undulations. The highest elevation is 29 m above sea level (a.s.l.), while the average is about 18 m a.s.l. While the coastal dune system of Piscinas is an area of about 1.5 km2 located in the South of the Oristano Gulf in the Sardinia Island, near Arbus. It has one of the highest dune systems in Europe (for this reason it is part of the UNESCO World Heritage) and the coastline extends for about 7 km, with a maximum height about 100 m.
Remote sensing data
The remote sensing data used to develop this research theme were basically acquired by the MultiSpectral Imager (MSI) on-board of Sentinel-2 (S2). For the case of Pianosa, the selected images were acquired on 18/07/2016, 04/05/2017, 11/05/2017, 03/06/2017, 13/02/2018, 19/04/2018, 18/07/2018. While for Piscinas on 29/10/2016, 15/11/2016, 28/12/2016, 18/03/2017, 04/05/2017, 28/12/2018. For this second site, SAR images (COSMO SkyMed) were added to the analyses (synchronous or as much as possible close to the optical images acquisition; 27/12/2016, 17/03/2017 and 04/05/2017). All these images were chosen because cloud-free, without sun-glint and other radiometric noise. Before processing the optical images, a comparison between Level 1 (L1) and Level 2 (L2) products was performed (Copernicus Open Access: https://scihub.copernicus.eu/dhus). The L1 products, corrected atmospherically using the 6SV code (Second Simulation of the Satellite Signal in the Solar Spectrum code-vector version)  were chosen due to their good performances in retrieving water leaving reflectance in inland and coastal waters [2,3]. To run 6SV, MSI-S2 imageries were downloaded as L1, containing Top-Of-Atmosphere (TOA) reflectances. Then, all bands were resampled at 10 m through SNAP S2-toolbox, with nearest neighbour method, and TOA reflectances ρtoa (λ) were converted into Ltoa (λ) before being ingested by the interface for atmospheric correction through 6SV, according to:
As an output, among other products, 6SV provides three coefficients xa(λ), xb(λ), and xc(λ) applicable to obtain water reflectance ρw from Radiance at Top of Atmosphere Ltoa as follows:
For each site, target elevation was accounted for, and a Lambertian ground surface was assumed. The 6SV-derived atmospherically corrected ρw was then converted into Remote Sensing Reflectances (Rrs) above water dividing it by π. The Rrs products were then processed using the Sen2Coral add-on-tool (Pianosa) and the non-linear optimization algorithm called BOMBER (Bio-Optical Model-Based tool for Estimating water quality and bottom properties from Remote sensing images)  (Piscinas) in order to obtain maps of three different substrates (sand, rocks and phanerogams) and bathymetry. The bio-optical model was applied in shallow water mode and, for each image, was applied to a mask that includes water up to 1000 m from the coastline (maximum depth between 10 and 12 m). Materials used for Pianosa site also include ancillary data such as: a bathymetric map and a georeferenced coastline of 2014 (ISPRA’s website); a nautical chart published by Istituto Idrografico della Marina; a RapidEye image of summer 2017 and Google Earth imagery. While the bathymetric outputs obtained with the BOMBER were compared with a bathymetric map available at “https://webapp.navionics.com” website.
Results and Discussion
For the Pianosa site, the analysis of the temporal evolution of the coastline obtained from NIR bands, shows that the position of rocky segments doesn’t change in time; also, shows that no significant modifications to the shoreline occurred for the two sandy area analysed in the period 2016-2018 (Fig.1). For the Piscinas site it was possible to make a comparison between the coastline produced by optical images and that produced by SAR+Rrs (“write function memory” method [5,6]). This comparison shows differences in the range from a few meters up to 20 m between the two coastlines. In fact, the interpretation of optical images was complex, especially in the so called “mixed” pixels, located at the water/sand interface, while SAR+Rrs images allow to better distinguish between these two surfaces. For this reason, the comparison of coastlines in the period 2016-2017 shows that along the sandy coast the coastline had a greater variability (up to 20m) than near the rocky shores (up to 5m) (Fig.1).
Figure 1: On the left, the temporal evolution of the coastline (18/07/2016, 03/06/2017, 19/04/2018) obtained from NIR bands in the two segments analysed for Pianosa. In the middle, the comparison between the coastline obtained by Rrs image (black line) and the coastline obtained by SAR+Rrs images (red line), along a fraction of the Piscinas coast on 04/05/2017. On the right, the temporal evolution of the coastline derived from SAR+Rrs images acquired on 27/12/2016 (blue line), 17/03/2017 (green line) and 04/05/2017 (red line).
Vulnerability maps of coastal zones
The reflectance maps of Pianosa at 550 nm produced by Sen2Coral were classified based on reflectance range expected for each substrate type, so that values lower than 0.08 were assigned to phanerogams, values higher than 0.2 to sand and those in between to rocky substrates. Most of rocks are detected in shallow water along the coastline, sandy substrates can reach bottom depths at around 5 m and phanerogams are detected in deeper regions. While the bottom cover maps of Piscinas were obtained through the BOMBER outputs. In particular, in each maps the main class is the sand one, followed by the remaining two classes (phanerogams and rocks) which were concentrated near the coast (up to 150-200m from the coastline). The phanerogams class was concentrated along the rocky portions of the study area, while along the sandy shores it is easier to find mixed pixels (sand/rock). The analysis of sand volumes variations in the two sandy areas examined for the island of Pianosa, shows overall only minor variations in the period considered, resulting in slight increments of sandy volumes, which confirms a stable situation that is not conditioned by erosion processes. Bathymetry, on the other hand, varies considerably along the entire Piscinas dune system. Consequently, Piscinas can be identified as a not stable at bathymetric level, probably due to the synergistic effect of wind and sea currents on an area characterized principally by sand substrate. Finally, based on the bottom type and sand volume variation, maps of vulnerability of coastal zone of the two study areas were created (Fig.2). For Piscinas, a short-term (windy event that occurred between the images of 29/10/2016 and 15/11/2016) and a long-term map (2016-2018) has been created. The results show that the least vulnerable map is the short-term, i.e. the one relating to the windy event. The wind has led to the deposition of a large amount of sand, especially away from the coastline, and has not led to a marked decrease in the phanerogams. On the other hand, the most vulnerable map is the long-term, which is characterized by a marked decrease in sand volume and a decrease in phanerogams, especially in the rocky areas to the northeast.
Figure 2: Vulnerability maps of coastal zones obtained from the classification of substrates and bathymetric analysis. On the left, the vulnerability map of Pianosa (2016-2018); in the middle, the short-term vulnerability map of Piscinas (windy event: 29/10/2016 – 15/11/2016); on the right, the long-term vulnerability map of Piscinas (2016-2018).
The results confirm that the coast of Pianosa has no problems of coastal erosion, while the vulnerability maps of Piscinas seem to be closely linked to episodic events (e.g. strong wind), with minor-to-none impacts on the Piscinas dune system.
Future perspectives (PRISMA data)
The preliminary assessment of PRISMA data capabilities for mapping some parameters/variable useful to evaluate the coastal erosion and produce vulnerability map was performed over a study area very near to Piscinas characterized by similar coastal conditions, located along the western Sardinia coast, slightly north of Piscinas. PRISMA image (L1) acquired on 24/08/2019 and atmospherically corrected with ATCOR code , was compared with S2 image (L2) acquired on 25/08/2019 (Fig.3). The retrieved Rrs data from both PRISMA and S2 where then ingested by BOMBER to produce the thematic maps on bottom properties and water quality parameters. The comparison between the products shows a high consistency between PRISMA and S2, while due to the lack of in situ data is challenging to evaluate which is the more accurate.
Figure 3: Different maps (on the left bathymetry, top right particle concentration, down right substrate coverage classification) obtained from PRISMA image of 24/08/2019 and from Sentinel-2 image of 25/08/2019 in the western Sardinia coastal zone.
Publications and Presentations
- N. Ghirardi, M.Bresciani, G. Luciani, G. Fornaro, V. Zamparelli, F. De Santi, G. De Carolis, C. Giardino, “Mapping of the risk of coastal erosion for two case studies: Pianosa island (Tuscany) and Piscinas (Sardinia)” In book Eighth International Symposium “Monitoring of Mediterranean Coastal Areas. Problems and Measurement Techniques”, Vol.126, pag. 713-722, Editore Firenze, 2020.
- M. Bresciani, N. Ghirardi, G. Fornaro, V. Zamparelli, F. De Santi, G. De Carolis, D. Tapete, M. Palandri, C. Giardino, “Mapping Of The Risk Of Coastal Erosion For The Case Study Of Piscinas (Sardinia)”, In Proc. IGARSS 2021
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