engleski

Evolution of assumed natural beaches using remote sensing techniques (Case study: Island of Hvar, Croatia)

Beaches, as accumulations of unconsolidated material, are one of the most dynamic coastal geomorphological features. Their morphology changes due to both natural and anthropogenic processes. On the island of Hvar there are about 250 pocket beaches. They have been mapped using remote sensing methods based on satellite imagery and orthophotos from the State Geodetic Administration (SGA). Most of them are affected by some form of anthropogenic processes (75.3%). In order to define anthropogenic influences several parameters were selected, e.g., building constructions, promenades, and roads in a wider research area visible in the remote sensing data. All these processes affect on natural dynamics of beaches, so we singled out 10 natural beaches - without anthropogenic influence and studied their evolution. The 200 years of beach evolution was studied combining 1834 Franciscan Cadastre, archival aerial images, and UAV with combination of satellite imagery. Three different time periods were defined: 19th, 20th and 21st centuries. The first period was studied using the Franciscan Cadastre, which was created primarily for delineation of agricultural parcels with the aim of collecting taxes. But, some beaches were also mapped as individual parcels. The cadastral maps were georeferenced, while SGA archival images and satellite images were already projected. Beaches were digitized, and their area and shoreline length calculated in ArcGIS Pro software. Beach erosion was detected almost on all the natural beaches that we surveyed. From 1834 to the present, beaches eroded on average by 40%. The most recent period was also investigated with UAV during fieldwork (October 2021 and October 2022) using UAV DJI Phantom 4 pro v2.0. Collected data was processed in Agisoft Metashape and ArcGIS Pro software. The same procedure that was carried out with the satellite images was also carried out with the orthophotos from UAV. The measurement errors were defined on UAV which are the most accurate, and is calculated to be 7.2%. Consequently, we assume that the satellite imagery could be used for far, hard- to-reach locations. However, during the fieldwork we found that beaches assumed to be natural have anthropogenic influences as well, which are not visible on satellite images. Dry walls built mainly for agricultural reasons in 18th and 19th century, which disable sediment transport to the beach were not distinguishable. Though, erosion was detected on both, assumed natural and anthropogenically modified beaches and there is no significant difference. Satellite imagery proved adequate for measurements, but the lower resolution, due to vegetation or shadows, makes them less resourceful. Remote sensing techniques offer a variety of possibilities for beach investigations and coastal management, but this reveals that the fieldwork, if possible, is still the most important. This research was done with the support of the Croatian Science Foundation (HRZZ-IP-2019-04-9445) SEALeveL project.

beach ; geomorphology ; evolution ; remote sensing ; Adriatic ; Hvar ; Croatia

nije evidentirano