Sand beaches on highly indented karstic coasts: Where the sands come from and what should be protected (SE Adriatic, Croatia)

Abstract This study deals with sand beaches of the SE Adriatic archipelago and Peljesac Peninsula. The beaches themselves, as well as their genetically related surroundings are taken into consideration, as both are critical for understanding beach features and processes. Two approaches have been followed in these two areas: the study of sediment composition including heavy mineral associations and the study of geology, morphology and sediment distribution. These features and their combination present the basis for interpretation of present-day processes, as well as for reconstructing how and where the beaches developed and why have they in some places disappeared. Sediment composition of the studied beaches clearly indicates Pleistocene deposits as the principal sediment source. However, much of this supplied sediment has a short residence time on the beaches due to marine reworking and the bulk of the beach sediment arrives from the sea floor. The positioning of the studied sand beaches at bayheads, their lengths and shapes, as well as their present status, have been influenced by several main factors. They include: (1) pre-Pleistocene tectonic deformations, (2) pre-Pleistocene weathering and erosion of carbonate sediments, mainly Cretaceous in age, (3) initial distribution of “soft” Pleistocene cover sediments, as well as (4) the history of their reduction and related decline in sediment source capacity or at some locations their complete disappearance. Other main factors are (5) the post-LGM sea-level rise and (6) two climatic events: an important pluvial period from 8.4 to 6 ka BP (maximum from 7 to 6 ka BP) and the transition to Mediterranean climate between 6 and 5.5 ka BP. Beaches with relatively high Pleistocene hinterland supply and correlative, relatively large widths seem to be quasi-stable or possibly even prograde. Beaches of small width and minor to negligible hinterland supply, or those completely deprived of it, suggest ongoing beach recession. Suggested priorities for the protection of the beaches are directly related to the preservation of hinterland supply. In the case of beaches with negligible or no hinterland sediment supply replenishment projects can be considered.