Origin and stability of gravel beaches in Croatia: problems and solutions.
Croatian coast is a typical example of Mediterranean rocky coast, known as being steep and dominated by limestones and dolomites, with restricted stretches built in clastics (mainly flysch). Due to its carbonate lithology, karstification and bioerosion dominate over mechanical weathering and wave action. As the result, beaches are rather an exception than rule. Where formed in carbonates, beaches are dispersed and small, mostly in form of pocket beaches. They principally developed in two ways. Many coarse-grained beaches are alluvial fans formed by both, present day and relict surface flows, whose valleys were cut in carbonate during the lower sea-level stand. Their orientation is influenced by the surface valley network, largely subdued to the coastal geomorphology. However, despite being backed by waterless karst, beach dimensions are positively correlated with the drainage area of torrential flows. Less present are gravel beaches formed by the mechanical wave action in sectors of previously weakened (tectonized or karsified) carbonate rocks. Length of the beaches formed in clastics is higher; however, the general length of the coast in clastic rocks is only ~ 6%. The main anthropogenic pressure along the Croatian coast is related to rapid touristic development, especially during the last decade. This increasing trend is generally raising demands for beach capacities. To meet this demand, many beaches are being enlarged and artificial beaches are being built. Due to the general lack of sediment supplied by rivers, the beach fill material used for these purposes is mainly quarried carbonate aggregate. However, many of these beaches are subjected to erosion and require repeated replenishment. Furthermore, hard structures built to protect the beach in some cases enhanced the erosion. As the consequence of the coastal development, 20% of the coast has been already changed so far. Just recently, this change of land use revealed the warning consequence along the coast: many of natural beaches have been decreased in width, mostly due to the cutting off and redirection of the torrential flows, or by sealing the bedrock due to the rockfall mitigation. It is, thus, expected that further 20% of the coast being planned for construction (besides natural pressures such as sea-level rise and intensified storminess) might pose a serious threat to beaches in the future.
In order to manage and protect beaches in the future a systematic beach monitoring is urgently needed to detect most affected coastal sections. As the first step in this direction, the integration of efficient monitoring method into the existing and well-established scheme of bathing area monitoring for water quality is proposed. One of the monitoring methods proven to be applicable in various coastal environments along the Croatian coast is based on Structure from Motion (SfM) photogrammetry. Once identified, erosional hot spots will be the target of further monitoring and management. Bearing in mind the general lack of sediment for the adequate beach nourishment, providing the appropriate source of sediment for beach replenishment might be a major challenge in the future.