The use of Unmanned Aerial Vehicles (UAV) on wetlands is becoming a common survey technique that is extremely useful for understanding tidal flats and salt marshes. However, its implementation is not straightforward because of the complexity of the environment and fieldwork conditions. This paper presents the morphological evolution of the Po della Pila tidal flat in the municipality of Porto Tolle (Italy) and discusses the reliability of UAV-derived Digital Surface Models (DSMs) for such environments. Four UAV surveys were performed between October 2018 and February 2020 on an 8 ha young tidal flat that was generated, amongst others, as a consequence of the massive sediment injection into the Po Delta system due to the floods of the 1950s and 1960s. The DSM accuracy was tested by processing (i.e., photogrammetry) diverse sets of pictures taken at different altitudes during the same survey day. The DSMs and the orthophotos show that the tidal flat is characterised by several crevasse splays and that the sediment provision depends strictly on the river. During the study period, the sediment budget was positive (gaining 800 m3/year and an average rate of vertical changes of 1.3 cm/year). Comparisons of DSMs demonstrated that neither lower flight altitudes (i.e., 20–100 m) nor the combination of more photos from different flights during the same surveys necessarily reduce the error in such environments. However, centimetric errors (i.e., RMSEs) are achievable flying at 80–100 m, as the increase of GCP (Ground Control Point) density is the most effective solution for enhancing the resolution. Guidelines are suggested for implementing high-quality UAV surveys in wetlands.
In the present context of sea-level rise, the reconstruction of previously reclaimed intertidal areas represents an opportunity to build dynamic coastal defences to decrease flooding under storm conditions by the dissipation of wave and surge energy across the vegetated domain. In Europe, this approach started in the late 1990s along the coast of eastern and southern England, but it is becoming common to many European countries around the North Sea margin. The process of salt-marsh restoration normally develops around the opening or removal of flood protection structures and gradual flooding of the hinterland. If the intertidal zone starts to experience vertical accretion, vegetation will colonize the area and a saltmarsh will develop. This paper presents the morphological evolution and sediment distribution in the Perkpolder basin, SW Netherlands (NL), following the conversion of a reclaimed area into a tidal flat, after the opening of an inlet in the flood defence structures in June 2015. The main focus of this study is the description of the evolution of the tidal flat since the opening of the inlet and the identification of spatio-temporal conditions for the evolution of a salt marsh. To reach this objective, several topographic surveys were undertaken, together with sediment surface sampling. Sedimentation rates at fixed sampling stations were assessed during the transition between neap and spring tides over a period of 1 month and 2 weeks. The morphological analysis of the inlet evolution proved that 6-8 months after the opening the inlet reached an equilibrium state. The average accretion rate across the whole study area was about 6-7 cm per year −1. The average deposited sediment was about 100 g per m −2 per day. Considering the sedimentation rates in the most elevated regions, 80-110 cm above NAP (Normaal Amsterdams Peil), and assuming that the sedimentation rate will remain constant in time, the conditions for the onset of salt-marsh formation will not be reached before 8-10 years. Projections indicate that the area located at +50 cm above NAP will not become a mature marsh before 50 years.
<p>In the period 1950s-60s, the Po river Delta (Northern Italy) was hit by several floods. Agricultural fields were covered by water and many of them remained submerged since. As a consequence of the massive sediment injection into the system, this lead to the birth of new tidal flats around the tip of the Delta. The evolution of these environments over 50 years was studied, as they may be taken as an example for future reconstruction of intertidal areas. The sediment distribution and the morphological evolution of a young tidal flat of about 10 ha located in the Northern part of the Po della Pila branch were studied by undertaking fieldwork since October 2018, including detailed topographic surveys using a UAV, sedimentological analyses, and a study of sediment deposition rates. An extended crevasse splay covers the central part of the flat. The granulometry is predominately fine (Silty clay and Clayey silt), except for the central area, where the sand percentage increases (Loam and Silty sand). This surface distribution is uniform down to ~10 cm; the sand percentage increases instead within the sediment column from ~10 to 25 cm next to the mouths of the channels. The tidal flat experienced a positive sediment budget and it was characterized by higher rates of accretion after the Po river floods. These observations suggest that the tidal channels are fed by sediment from the Po River branch. Orthophotos from the 1950s show that the tidal flat is about 17 - 20 years old and its formation was influenced by human intervention and river floods. The work aims at finally comparing this case study with other tidal flats and salt marshes worldwide characterized by similar and different tidal regimes, to identify the optimal elevation for vegetation to establish and flourish, to support the future restoration of these environments.</p>
<p>Extreme events can cause damage to coastal defences resulting in overtopping, breaching, or their total destruction. The resulting flood can impact the population and assets, causing short- to long-term losses to the economy. Two examples of extreme coastal storms are the Katrina Hurricane (2005) in the USA, and the Xynthia storm (2010), in France. The Northern Adriatic Sea is often impacted by coastal storms, impacting residential and commercial areas. The region of Emilia-Romagna (Italy) is so often affected that it is common practice to create an artificial dune on the beach during winter to protect the properties.<br />On 22 November 2022, a coastal storm developed in the Northern Adriatic Sea, impacting the coasts of Veneto and Emilia-Romagna Italian regions. This storm, coming from ESE, was characterised by non-extreme waves, but coincided with spring tides, producing an extreme surge, reaching a total water level of 1.48 m above MSL, recorded for the first time by tide gauge at Porto Garibaldi (Comacchio), which corresponds to a return period greater than 100 years. The Saint Agatha storm that hit the same areas in 2015, reached a TWL of 1.2 m at the same location. The event of November 2022, caused damages on the coast, erosion of beaches, artificial and natural dunes, damage to coastal infrastructures, and flooding of residential buildings and local business activities.&#160;<br />While this storm represented a success for the MOSE, which succeeded in protecting the lagoon of Venice against one of the most hazardous events of the last decades, its effects on the Ferrara coasts were, in some cases, devastating. The most affected area was the Lido di Volano (Comacchio), where the extreme event caused a dike breach in the inner part of the Po di Volano mouth, leading to the recurring flooding of the town, due to the high spring tides that followed the storm. Indeed, the breach remained open for several days after the event. The event required the heavy involvement of first responders.<br />The site was surveyed on 23 and 25 November 2022. The research team measured flood extension and flood markers with the use of DGPS and aerial images, while UAV aerial surveys were implemented on the emerged beach to assess the morphologic impacts.<br />The flood associated with the extreme event was simulated using a hydrodynamic model (LISFLOOD-FP) to verify the causes and evolution of the flood event. The model was set up using topo-bathymetric data from 2019, water levels from the tide gauge of Porto Garibaldi. &#160;A thorough calibration was implemented using the fieldwork data. The model was used to simulate scenarios considering different dike-breach configurations and/or forcing it with other recent events (e.g. Saint Agatha, 2015).&#160;<br />The model was able to properly simulate the flooding event of November 2022. The tested scenarios highlighted the role of the magnitude of the event and the effects that a breaching dike can have on the associated areas. This is a contribution to the ECFAS project (EU H2020 GA 101004211).</p>
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