Evolution of pocket beaches on the island of Hvar during the last 200 years (Croatia, Eastern Adriatic Coast)

Beaches as dynamic coastal forms occur in different shapes, areas and lengths. They form at the interface between sea and land, where natural and socioeconomic processes interact. Their morphology changes on different temporal scales, from hours to centuries. In this work, the evolution of 25 beaches on the island of Hvar in Croatia was studied, covering the period from 1834 to 2021. The evolution of beaches was studied through three different periods. The oldest, starting from the year 1834, was analysed using the Franciscan cadastre (0.5 m error) mapped during the Austro-Hungarian Empire. Their surface areas were calculated using GIS software. Some of the beaches were already mapped as individual cadastral parcels. The obtained calculations significantly correlate with data from the Cadastral supplement (expressed in Klafter quadrimetre unit). The surface areas of beaches for the second period were estimated from archival orthophotos, provided by the State Geodetic Administration, using the method of repeat photography. The archival aerial photographs date from 1958 to 1968 and are in black and white colour with a relatively satisfactory resolution. The repeat photography method relies on more than 150 old photographs (1910-1980) that were used for benchmarking, beach boundary determination, and the reconstruction of mid-20th century situation. For the most recent period, which is the most accurate, we used UAV DJI Phantom 4 Pro in combination with a Trimble GNSS receiver. The fieldwork was conducted from November 2020 to March 2022. As a final product, very accurate orthophotos, DEM and DSM were produced by photogrammetric techniques in ESRI Drone2Map and Agisoft software. The analysis showed that beaches had the largest area during the cadastral survey, were slightly smaller in the mid-20th century and were the smallest in recent times. Consequently, beach erosion was found in all 25 sites surveyed. On average, the beaches have eroded by about 32% since the 19th century. Mina and Sveti Ante beaches were the most affected with more than 70%. Overall, the shoreline has moved inland of about 4.3 metres on average. It was found that cadastral data, archival maps, and old photographs proved to be useful and reliable data sources for analysing beach surface changes in the past.

This research was made with the support of the Croatian Science Foundation (HRZZ-IP -2019-04-9445).