Evolution of restoration projects of coastal wetlandsfrom tidal flat to salt marsh

Resumen

The way we conceive coastal defences has profoundly changed during the last decades and, as a result, our approach to the coastal environment has been modified as well. New management realignment strategies are being explored, and natural-based solutions are preferred to traditional coastal structures, such as dykes, seawalls, and groins. Several restoration projects were applied in coastal wetlands nowadays focusing on landscape recovery and a new equilibrium between natural and anthropic structures. This PhD thesis combines geomorphological and sedimentological analysis (i.e. drone surveys, sediment traps, sediment coring, historical orthophotos) with a multi-disciplinary approach to study the evolution of young tidal flats located worldwide, which origin was caused by human intervention, specifically due to the breach of a dyke or a channel excavation that has connected the sediment supplier with an intertidal area that was not a tidal flat. Three study sites were chosen based on their environmental characteristics and the tidal range: i) the tidal flat of the Barbamarco lagoon, located in the Po River Delta (microtidal) in Italy, ii) the artificial tidal flat of Perkpolder (macrotidal) located in the Scheldt Estuary in The Netherland, and iii) the management realignment of Converse (ultratidal) located in the Bay of Fundy in Canada. The analyses were improved by fieldwork carried out in the accidental marsh of Halfway River, and the marsh of Eldriken, which are other tidal flats located in the Bay of Fundy. The study aims to define the sedimentological evolution of different human-induced tidal flats, to discuss the processes that have led to their present conditions, and to understand whether and how the transition from flat to marsh will occurr, defining which factors should be considered for future restorations. The first part of the PhD focused on fieldwork activities carried out on the tidal flat of the Barbamarco lagoon in the Po Delta (Italy). Several UAV (Unmanned Aerial Vehicle) surveys were performed between October 2018 and October 2020 and a collection of DSMs (Digital Surface Model) were processed. The granulometric distribution was achieved through core sampling and the rates of deposition were obtained using an innovative structure for Petri dish sediment traps that were built with a 3D printer. A historical review was accomplished using orthophotos acquired from the IGM (Istituto Geografico Militare Italiano) and the Veneto Region archives, from the 1950s until nowadays; the investigation was then extended to the lagoons of the whole tip of the Delta. Results gave important interpretations about the evolution of the tidal flat, indicating that the river is the sediment supplier, and during the last 20 years, it provided sediment for the Barbamarco and other lagoons as well, like the Burcio lagoon, where a rapid marsh developed between 2011 and 2013, after a strong sediment injection. Between fieldwork and data processing, the tidal flat of Perkpolder (The Netherland) was investigated using a dataset of Lidar provided by Rijkswaterstaat (Dutch Ministry of Infrastructure and the Environment) and rates of deposition that were obtained by the author during the master thesis (2017-2018). The results highlighted that, despite the rapid sediment accretion of the system, no vegetation has been established (until the last studies of 2018) and the tidal flat needs no less than a decade to meet the right conditions for a salt marsh to develop. General knowledge regarding salt marshes and data processing were improved during a short period at the University of Cadice (Spain) (second year of the PhD). During the last year of the PhD, a period of three months at Saint Mary’s University in Halifax (Nova Scotia, Canada) allowed investigating the evolution of the restoration site of Converse (Bay of Fundy). A field campaign was carried out between 23 and 26 July 2021 and several measurements were achieved (i.e. hydrodynamics, hydrology, sediment deposition, suspended sediment). The author participated actively in the measurements that regarded ADCP, ADV’s, RBR turbidity sensor, M9 bathymetric surveys, sediment filter traps, suspended sediment collector, ISCO bottles; particle size analysis was then accomplished using the sediment collected by the traps. However, in this thesis, the results concern sediment deposition, particle size analysis, and water level variations. In addition, further analyses of sediment deposition were carried out in two different sites from the southern basin of the Bay of Fundy between 12 – 13 August 2021: Halfway River and Eldriken. The collected data were compared with the site of Converse and improved the final discussion. The combination of the results from all study sites, which is discussed in the final part of the thesis, enhanced how artificial morphologies led to morphodynamic instability that alter the rates of deposition and, consequently, future insights. The strong erosion of the inlets and the strong accretion inside the tidal flats suggest that a cannibalization process might be ongoing, causing an increase in the rates of vertical variations for the short-term evolution. The analysis showed how vegetation growth is strongly linked to elevation with respect to the MHT (Mean High Tide) and the sediment supplier. From a morphological point of view, if the tidal flat is not located between the vertical range of any low marsh species, the surface needs to accrete and increase in elevation, but this can only happen if the sediment input is positive and constant in time. Salt marsh restoration can be a quick process (i.e. few years) but it can take longer periods (i.e. decades). Future management realignments of coastal wetlands should consider how the design affects the future evolution of the morphology, the rates of sediment deposition, and the elevation of artificial tidal flats with respect to the MHT as a preliminary evaluation to predict the waiting time for salt marsh development.