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Morphological changes of natural and artificial gravel beaches assessed by SfM photogrammetry: A Dugi Rat case study (Croatia)

Sustainable beach management within coastal management should be based on the understanding of beach processes and morphology. Gravel beaches are recognized as effective dissipators of wave energy and thus they play important role in coastal protection. Therefore, various coastal studies have been focused on gravel beaches during the two last decades (e.g. BUSCOMBE & MASSELINK, 2006 and reference within). Natural gravel beaches are common at high latitudes where glacially transported sediment has been reworked after winnowing of finer particles from the source material. They are also common at other latitudes near eroding cliffs and/or along coasts backed by mountains. An example of the latter is coastline around Dugi Rat (Middle Dalmatia), situated in the longest zone with flysch deposits outcropping along the Croatian part of the eastern Adriatic coast. Steep and rugged, this coast strip is a typical fold and thrust structure, characterized by older carbonates thrust over younger flysch. Along this zone mostly gravel beaches have been developed, regardless the composition of original flysch and associated deposits (PIKELJ & JURAČIĆ, 2013 ; PIKELJ et al., 2014). In general, the Croatian coast is a high rocky karstic coast, where small, mostly gravel and scattered beaches are developed. Human pressure in terms of rapidly growing tourism together with natural pressure due to the climate change resulted in artificial beach construction and nourishment of natural beaches. Many of them are regularly replenished and/or protected by coastal structures due to the sediment losses during storm events. However, there is little understanding of how these beaches behave during a range of environmental conditions and whether the sediment losses are permanent. Regular monitoring of changes of beach morphology in various environmental conditions is thus crucial for improving the knowledge, which in turn can inform beach management. The novel Structure-from-Motion (SfM) photogrammetric method for 3D reconstruction utilizes digital photographs to produce digital elevation models (DEM) of centimetric precision (FONSTAD et al, 2003) for objects viewed from distances of 10’s m (JAMES & ROBSON, 2012). SfM results can be comparable LIDAR in terms of resolution, but the approach is cheaper, quicker and reduces field labour. As such, SfM photogrammetry is considered to be convenient for frequent data collection at several small gravel beaches at the same time. The method was applied at two study sites located in municipality Dugi Rat. Once an industrial settlement, Dugi Rat has turned to tourism industry since 2003 and its shoreline is heavily influenced by human activities. Several artificial gravel beaches have been constructed to enhance touristic offer and to provide coastal protection to newly built promenade. After being constructed, artificial beaches are nourished on a yearly basis due to the loss of the sediment. Nourishment is usually held during spring, after storm events occurring mostly during winter and before the start of the summer tourist season. Two gravel beaches, one natural (Glavica) and one artificial (Dugi Rat), were monitored using SfM technique during energetic wave conditions caused by three-day lasting Jugo wind in February 2015. Glavica is a 170 m long natural beach oriented south-southwest to nort-northeast on a predominantly NW-SE (Dinaric) coastline and directly exposed to Jugo waves. Dugi Rat is an 80 m long embayed artificial beach, divided in half by a perpendicular recreational concrete pier. This construction serves partly as a groin across the beach ; therefore two parts of the beach have been studied separately. The western part of the beach is oriented nearly west-east, while the eastern part curves and it is oriented approximately northwest-southeast. The beach is partially protected from Jugo waves by the Dugi Rat headland. In total, 300-400 photos were collected at each site, using a single SLR camera (Canon EOS 450 D). The camera was held at two different heights ; at the eye level and overhead by mounting it on a pole, following the recommendations given by JAMES & ROBSON (2012) and JAMES et al. (2013). Coordinates for the ground control points, which were used for georeferencing, were obtained by virtual reference station real-time kinematic (VRS RTK) positioning method, using Trimble R8 GNSS receiver and CROPOS VPPS service. Achieved accuracy was within 2 cm in horizontal and 4 cm in vertical position. Point clouds and DEMs were generated from digital photographs, using Agisoft PhotoScan 3D reconstruction package, while contour maps were created using Golden Software Surfer. DEMs and contour maps were used to calculate beach volume change and to assess changes in beach elevation. The sediment loss on Glavica beach was 21 m3 (~2%) after the Jugo event. The most prominent change in beach elevation occurred on the southwest part of the beach (>-25 cm), as well as in the middle of the beach, north of the concrete platform. It is hypothesized that the sediment was lost offshore. Increased heights associated with a discontinuous berm fronted with a sharp step occurred in the upper part of the beach. This beach step is a typical steep break in the beach slope, common at gravel beaches exposed to more energetic waves. Another area of accretion was observed just onshore of the toe of the beach, created by swash zone processes of more gentle post-storm waves. The sediment loss on the eastern part of the Dugi Rat beach was 4 m3 (~3%) after the Jugo event. South-eastern side of this beach was visibly lower (~-25 cm), while sediment was accumulated at the western (≥35 cm). This sediment relocation indicates the direction of the longshore sediment transport from east to west. The accumulation was pronounced in form of two parallel ridges formed at the onshore swash boundaries, created by waves of different energy. Unlike eastern portion of the beach, western part has gained sediment (3%). Here the erosion also took place on the eastern side of the beach, and accretion on the western side and only one ridge dominates the accretion zone. Since no sediment exchange between two parts has been observed (except negligible sediment discharge over the pier) it is assumed that sediment excess occurred mainly by cross-shore swash processes. However, there might be that some sediment was transported from eastern part to western part of the beach around the pier in the nearshore zone. Based on the results presented, SfM photogrammetry is considered as a suitable method for quick and frequent beach surveys. In the frame of beach monitoring for beach management purposes, a sequence of surveys should be done under the various wave conditions over various temporal scales. Furthermore, this study showed interesting result that the artificial beach was more resilient then natural beach. However, the beach orientation and configuration, sediment grain size, prevailing winds/waves, wind heights etc. need to be considered to explain the observed behaviour.

beach morphology; SfM photogrammetry; storm event

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