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The Fiumi Uniti and Savio rivers are two small sandy-bed river systems which sediment yield contributes feeds part of the Emilia-Romagna beaches (Italy). Since the twentieth century the northern Adriatic coast has been affected by well-known beach retreat phenomena. As the sediment supply of these local rivers is not well known, an analysis of bed-load transport rates for the last 10 kilometres of the river has been done. Multiple analysis, supported by direct field measurements of the 2005-06 and 2017 years (still ongoing) have been realized. Complementary hydraulic investigation and modelling have been performed. Repeated bed-material samplings have been carried out during different flood conditions, obtaining a significant number of observations. Aiming at defining the behaviour of sediment-transport in these two rivers, this paper focuses on the analysis of threshold sediment transport condition. Several formulae available in literature were used to test the field measurements. All these criteria seem inappropriate to predict the threshold conditions for bed particle entrainment in terms of the value of bed shear stress. The inaccurate estimation given by these formulae is due to the fact that they do not consider natural grain-shape factors or are mainly suitable for gravel-bed rivers.

Multiple intertidal bar (MITB) beach systems comprise a succession of subdued, shore‐parallel sandbars, developed under low‐energy conditions in meso‐ to macrotidal settings. Their relatively stable morphologies over long timescales are commonly attributed to a dynamic equilibrium, driven primarily by seasonal morphodynamics. The seasonal behaviour is, however, poorly understood. The relationship between temporal and spatial hydrodynamic forcing and morphological changes was investigated through monthly differential GPS surveys in Dundrum Bay (Northern Ireland) from April 2019 to March 2020, associated with variations in nearshore wave conditions, simulated using the SWAN wave model. During low‐energy summer conditions, southeast waves helped promote MITB stability and a slight increase in bar crest elevation, with the seaward‐most bar buffering and preserving the inner bar system through wave dissipation. In the winter, a progressive increase in wave energy, and a switch in wave direction (southwest to south), initiated the highest rates of cross‐shore bar migration and a lowering of bar crests. While the seaward‐most bar remained largely submerged, the shoreward‐most bar played a key role in protecting the upper‐beach and/or foredune. Winter conditions also forced a newly observed mesoscale longshore drift involving offshore sediment transport in the southwest and onshore transport in the northeast. Cross‐shore variability in MITB behaviour at seasonal timescales was, however, primarily driven by local hydrodynamic conditions, including variations in wave energy and direction. Conversely, alongshore variability was largely influenced by the complex nearshore bathymetry, headlands and an active ebb delta, all interacting with changing hydrodynamic forcing. These results challenge the seasonal equilibrium model for MITB systems and highlight longer‐term patterns of sediment movement. [ABSTRACT FROM AUTHOR]

Developing and implementing a quick response post-storm survey protocol has the potential to improve impact assessments of coastal storms. Pre- and post-event surveys are essential to properly quantify the storm impacts on the coast. In this study, a combination of traditional RTK GPS and Unmanned Aerial Vehicle drone platform was utilized as part of a coordinated storm response workflow. The comprehensive approach employed in this pilot case study was conducted on the Emilia-Romagna coast (Italy), in the immediate aftermath of an extreme storm event that impacted the shoreline on the 5th-6th February 2015 called the Saint Agatha Storm. The activities were supported by timing information on the approaching storm provided by the regional early warning system. We collected aerial photos from a commercial off-the-shelf drone immediately after the Saint Agatha Storm and generated both orthomosaic and digital elevation models utilizing structure-from-motion photogrammetry techniques. The drone-based survey approach allowed us to quickly survey an area of 0.25 km2 within a 10-minute flight resulting in a ground sampling distance of 2.5 cm/pixel. Flooding and erosion impacts are analyzed and presented for the target study area. Limitations and possible applications for coastal management of the quick response post-storm surveying protocol are highlighted. [ABSTRACT FROM AUTHOR]

Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a variety of morphodynamic processes and controlling mechanisms. They provide valuable inhabitable land for small island nations, critical habitat for numerous species, and are threatened by climate change. Aiming to investigate the controlling mechanisms dictating the evolution of One Tree Island (OTI), a rubble island in the Southern Great Barrier Reef, we combined different remotely-sensed data across varying timescales with wave data extracted from satellite altimetry and cyclone activity. Our findings show that (1) OTI had expanded by 7% between 1978 and 2019, (2) significant gross planform decadal adjustments were governed by the amount, intensity, proximity, and relative position of cyclones as well as El Niño Southern Oscillation (ENSO) phases, and (3) the mechanisms of island growth involve rubble spits delivering and redistributing rubble to the island through alongshore sediment transport and wave overtopping. Frequent short-term monitoring of the island and further research coupling variations in the different factors driving island change (i.e., sediment availability, reef-wave interactions, and extreme events) are needed to shed light on the future trajectory of OTI and other rubble islands under a climate change scenario. [ABSTRACT FROM AUTHOR]

Unusual shore-normal and barred-like rhythmic features were found in Camposoto Beach (Bay of Cádiz, SW Spain) during a monitoring program using unmanned aerial systems (UAS). They appeared in the backshore and persisted for 6 months (October 2017–March 2018). Their characteristics and possible formation mechanism were investigated analyzing: (1) UAS-derived high-resolution digital elevation models (DEMs), (2) hydrodynamic conditions, and (3) sediment samples. The results revealed that the features did not migrate spatially, that their wavelength was well predicted by the edge wave theory, and that they shared characteristics with both small-scale low-energy finger bars (e.g., geometry/appearance and amplitude) and swash cusps (e.g., wavelength, seaward circulation pattern, and finer and better sorted material in the runnels with respect to the crests). Our findings pinpoint to highly organized swash able to reach the backshore during spring tides under low-energy and accretionary conditions as well as backwash enhanced by conditions of water-saturated sediment. This study demonstrates that rhythmic features can appear under different modalities and beach locations than the ones observed up to date, and that their unusual nature may be attributed to the low spatiotemporal resolution of the traditional coastal surveying methods in comparison with novel technologies such as UAS. [ABSTRACT FROM AUTHOR]

Recent advances in structure-from-motion (SfM) techniques have proliferated the use of unmanned aerial vehicles (UAVs) in the monitoring of coastal landform changes, particularly when applied in the reconstruction of 3D surface models from historical aerial photographs. Here, we explore a number of depth map filtering and point cloud cleaning methods using the commercial software Agisoft Metashape Pro to determine the optimal methodology to build reliable digital surface models (DSMs). Twelve different aerial photography-derived DSMs are validated and compared against light detection and ranging (LiDAR)- and UAV-derived DSMs of a vegetated coastal dune system that has undergone several decades of coastline retreat. The different studied methods showed an average vertical error (root mean square error, RMSE) of approximately 1 m, with the best method resulting in an error value of 0.93 m. In our case, the best method resulted from the removal of confidence values in the range of 0–3 from the dense point cloud (DPC), with no filter applied to the depth maps. Differences among the methods examined were associated with the reconstruction of the dune slipface. The application of the modern SfM methodology to the analysis of historical aerial (vertical) photography is a novel (and reliable) new approach that can be used to better quantify coastal dune volume changes. DSMs derived from suitable historical aerial photographs, therefore, represent dependable sources of 3D data that can be used to better analyse long-term geomorphic changes in coastal dune areas that have undergone retreat. [ABSTRACT FROM AUTHOR]

In the current scenarios of sea level rise and increasing coastal erosion processes, dune conservation is one of the potentially more efficient solutions for Disaster Risk Reduction policies. Coastal dunes represent a natural defence barrier from flooding and can protect strategic infrastructures and populated urban areas located behind them.The abstract reports the results of a yearly monitoring programme (March 2017 April 2018), performed using UAV surveys to describe the geomorphic evolution of a coastal dune system, recently subjected (2015) to a preservation scheme by local authorities, which included re-vegetation with pioneering plants (2016). The project mainly consisted in the construction of wood walk-boards and a fence to prevent trampling by beach-goers. A total of 5500 local dune plants were planted to accelerate sedimentation and growth of the incipient dune that developed at the shortly after the construction of the protection scheme. The entire dune area is 600x150 m (90000 m2) while the area of interest (incipient dune) is 4700 m2.The estimation of sand transport over time on the incipient dune was monitored comparing the DoDs (Dem of Difference) of the four UAV surveys. Surveys were performed flying at 80 m with a Phantom DJI 3 professional equipped with a digital camera of 12 megapixels. Automatic flight plans with 72% of sidelap and frontlap were realized after the deployment and measurement of 20 GCPs (Ground Control Points) with an RTK-GNSS. A ground resolution of 3.2-3.3 cm/pix was achieved for each survey. GCPs returned an error always below 10 cm and a validation of elevation values between RTK-GNSS field measured and drone derived ones was done, with a resulting RMSE of 7-9 cm.During the monitoring period, the incipient dune showed the ability to withstand storms and to grow under strong wind action in a few months. The buffering role played by the wooden fence and the planted vegetation resulted to be important for the generation and growth of this incipient dune that locally reached and exceeded 0.5 m of vertical growth. The vegetation contribution by the reintroduced plants in overestimating the volume growth of the dune was estimated to be 11%., The incipient dune currently represents a small deposit of loose sand potentially able to feed the facing beach after big storms or to enlarge the portion of stabilized dune at its rear. The latter process has already started since the crest of the incipient dune, after its most important vertical growth, showed also a landward migration that locally joined the stable foredune located behind.Since the evaluation of extreme storm impacts is often site-dependent, the observed geomorphic change can be functional to determine local coastal vulnerability to erosion and inundation hazards. The lifetime of this kind of coastal protection scheme will be also determined relatively to the occurrence of more energetic storms which may occur in the future. [ABSTRACT FROM AUTHOR]