D2.2 Numerical modelling and field campaigns to support litter removal at the demo sites

Within the MAELSTROM project two complementary marine litter removal technologies are developed and tested. An automated seabed cleaning platform has been developed with a cable-robot able to remove marine litter from the seafloor at demo locations in the coastal area near Venice (Italy), and a Bubble Barrier system will be implemented to remove marine litter from the water column in the Ave estuary (Portugal). In order to select locations where the technologies can best be applied and to assess the impact of the litter removal on marine life, we have set-up and applied numerical models. Furthermore, results of field observations that were carried out in the framework of MAELSTROM are analysed and are put into context using the numerical modelling results.  
For the coastal area near Venice, the transport and fate of marine macrolitter from three main source types was derived from particle tracking modelling, making use of hydrodynamic model results for the entire Adriatic Sea. The source input was estimated for marine litter from cities, from rivers and from aquaculture and fisheries. This approach was followed to estimate locations where marine litter accumulates on the seafloor. Such locations have a much higher litter density than average, and we refer to them as “hotspots”. Despite the various uncertainties in the modelling methods, the resulting maps of the item density on the sea floor are coherent with the available literature and represent a first assessment on the distribution of marine macrolitter accumulation in this region. Potential hotspot maps were obtained for both the entire northern Adriatic Sea (Gulf of Venice) and for the area around the Venice Lagoon. At the large scale the potential hotspots are connected to the sources. Specific for the smaller scale is that a large seabed hotspot was identified in front of the most southern inlet of the Venice lagoon, which is in agreement with earlier observations.  
For the Ave estuary, very limited observations were available at the start of the project. In order to estimate the maximum flow velocities and flow patterns relevant for the design and exact location within the estuary, measurements campaigns were carried out and a hydrodynamic model was set-up from scratch. Observations were carried out at selected locations within the estuary, having a length of about 2 km from a weir to the Atlantic Ocean. The aim of the observations and modelling was to understand the temporal and spatial variation of the flow velocity. The bathymetry was measured for the estuary and surrounding coastal waters. In absence of river discharge observations, a hydrological model was set-up for the Ave River basin, resulting in a river discharge ranging from about 9 to 300 m3/s in the period June 2020 up to June 2022. We can conclude that the flow in the estuary is mainly determined by the tides and the river discharge, and the effect of wind on the flow is limited. During low river discharge, the observations and the hydrodynamic model show that the estuary is highly stratified, meaning that a fresh-brackish water layer is on top of oceanic water (salt wedge estuary). Simulated flow velocity magnitudes do not exceed 0.5 m/s in the area selected to implement the Bubble Barrier system during low river discharge. At mean and high river discharge, we have only limited observations. In November 2022 a maximum flow velocity magnitude was observed, being 0.63 m/s, in combination with a mixed and entirely fresh water column. Apparently, a higher river discharge occurred this day. Unfortunately, we did not have the meteorological data yet to estimate the river discharge from the hydrological model. From this observation we can conclude that a higher river discharge can result in flushing of the estuary. For the extreme river discharge of 300 m3/s and using the hydrodynamic model, we found that a maximal flow velocity magnitude in the water column can occur up to 1.9 m/s. Furthermore, we have highlighted a few areas with generally low dynamics during low river discharge, where marine litter may deposit or accumulate.