The prestigious Journal of Marine Science and Engineering has just published the largest study carried out to date on the bottoms of the Samil cove, a work carried out by a team of researchers from the Geoma group of the Marine Research Center of the University of Vigo, CIM-UVigo, led by Ana Bernabeu and Daniel Rey, after being requested by ACUAES, the State Waters Corporation of Spain, as a preliminary assessment of the impact of the new outfall on the sand.
To do this, they used an innovative methodology that combined experimental observations on the seabed and physical characteristics of the sediment with a numerical study that simulates the action of the wave and the currents that interfere with the transport of sediments. “The objective was twofold: on the one hand, to determine the ideal depth and location so that the outfall is not affected by the wave and the seasonal rhythms of the movement of sand in the cove and, on the other, so that its construction does not alter the natural dynamics of the beaches,” explains Daniel Rey, coordinator of the GEOMA group and director of the CIM.
Despite urban pressure, balance is maintained in Samil
The research, which was part of Aimar del Valle’s doctoral thesis, demonstrated the advantage of exploring the effect of submerged infrastructures with a multi-data approach to fully understand the diversity of possible influences on the local behavior of beaches and, at the same time, also offered the highest resolution bathymetry carried out so far in the Vigo estuary, as well as a whole series of acoustic images of the seabed where the need to replace the old pipes was clearly seen. In addition, something also relevant, was to obtain a unique sedimentological and hydrodynamic perspective on the natural balance of sedimentary environments and the underwater environment of the Toralla Marine Sciences Station, Ecimat.
Furthermore, this work has highlighted the dynamic balance that Samil beach has reached under the enormous urban pressure it suffers. “At the moment there is no significant loss of sand and the balance is reasonable”, emphasizes Rey, adding that to delve deeper into the alterations that may occur in the future related to climate change, a new study would have to be carried out on which to base future adaptation strategies.
High resolution technology
The study is based on high-resolution bathymetry data, physical characterization of the seabed in the area, a granulometric study of the surface sediment and a numerical simulation of the maritime climate (waves and currents) and sediment transport in low and high energy conditions using the open source Delft3D software. This approach improves the reliability of the results of the inter- and subtidal zones of the beach where studies are scarce due to the complexity of data acquisition, and allows to clarify their dynamics with the general context of the estuary.
“The multibeam was the basis of a very precise hydrographic survey that, together with the calculation of the sediment grain size, allowed modeling how transport would occur under normal and extreme conditions (storms) based on significant average values of the historical wave series collected by State Ports during the last 30 years”, explains the director of the study. In addition, these data were also used to calculate the beach closure depth, which is the depth at which the bottom sand stops moving seasonally. In the case of the side-scan sonar data, they were used to directly observe the depths at which the structure of the old outfalls was affected by waves and currents, causing erosion and sedimentation processes when encountering the outfall.
“The combination of these three independent types of results demonstrated that the area of interest is a low-energy zone, significantly protected against wave attack, where fine sand predominates”, explains Daniel Rey. However, the field data indicated an interaction in the submerged obstacles between 0 and 12 m depth. “The model revealed that there is significant sediment movement above 7.4 m depth and that the outfall would not alter the general transport dynamics of the beach, but would interact in the most superficial section”, emphasizes the researcher, thus emphasizing what he considers the main conclusion of the study, which referred to the fact that the future structure would not alter the overall dynamics of the beach. Furthermore, Rey explains that in order to guarantee the safety of the new outfall, “we also consider that it should not surface above 8 or 12 m depth, depending on the area”.
Inndaga, “an ideal platform for on-site inspection and monitoring”
Crucial to the research was the use of Inndaga, a multipurpose platform developed by the Geoma group itself in an 8.5 m long inflatable boat and designed to carry out oceanographic surveys in coastal areas. This concept allows the vessel to operate safely and with great maneuverability in areas where larger research vessels cannot access (rocky areas, ports, etc.) and at low operating costs.
Fully integrated into an information management system to provide efficiency and effectiveness in strategic decision-making, the vessel is very flexible and has a wide range of applications. According to the Geoma group, in ports it becomes an ideal platform to develop specific technologies and methodologies in situ for the inspection and precise monitoring of the different elements of the docks, hydrographic conditions and to improve their maintenance and operation in accordance with EU environmental directives and the UN Sustainable Development Goals (SDGs). “This concept allows the acquisition of high-resolution data necessary to improve hydrodynamic models, important for predicting the possible effects of dredging, analyzing the behavior of a structure, as was the case in this study, or designing strategies to combat and manage possible spills,” Rey emphasized.
On the other hand, a fully developed post-survey scheme allows for rapid data processing and analysis of water and sediment samples. These results are integrated into a Geo-referenced Information System (GIS) that allows the visualization of all relevant information for the environmental management of coastal areas, providing efficiency and effectiveness in strategic decision-making.
O desenvolvemento deste traballo contou co apoio da Xunta de Galicia e a Unión Europea, a través do cofinanciamento do CIM no marco do Programa Operativo FEDER/FSE Galicia 2014-2020.
Source: DUVI
