Researchers from the CIM Geoma group publish the largest study to date on the subtidal of the Samil bay

Multibeam bathymetry of the Samil bay, together with acoustic images of the ancient pipeline and the results of the model under different wave conditions.

The prestigious Journal of Marine Science and Engineering has just published the largest study to date on the subtidal of the Samil bay, 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 Mercantile Society Waters of the Basins of Spain, as a preliminary evaluation of the impact of the new pipeline on the sandbank.

To do this, they used an innovative methodology that combined experimental observations on the bottom and physical characteristics of the sediment with a numerical study that simulates the action of waves and 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 pipeline was not affected by the waves and seasonal rhythms of sand movement in the bay and, on the other, that its construction did not alter the natural dynamics of the beaches ”, Daniel Rey explains.

Despite urban pressure, the balance remains 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 multitool approach to fully understand the diversity of possible influences on local beach behavior and, at the same time, it also offered the highest resolution bathymetry carried out so far in the Ria de Vigo, as well as a series of acoustic images of the bottom where the need to replace the old pipelines was clearly appreciated. In addition, something also relevant, was to get a unique sedimentologic and hydrodynamic perspective on the natural balance of sedimentary environments and the underwater environment of the Marine Science Station of Toralla, Ecimat.

This work revealed the dynamic balance that Samil beach reached under the enormous urbanistic pressure it suffers. “At this time there is no significant loss of sand and the balance is reasonable,” Rey emphasizes, adding that to delve into the changes 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 present study is based on high resolution bathymetry data, seabed physical characterization, a granulometric study of the superficial sediment, and a numerical simulation of the tide, wave climate, and sediment transport in low and high energy conditions using open source Delft3D software.  This approach improves the reliability of the results of the inter and subtidal areas of the beach, where studies are scarce due to the complexity of data acquisition, and allows clarifying its dynamics with the general context of the Ria.

“The multibeam was the basis of a very precise hydrographic survey that, together with the calculation of the sediment grade size, allowed modeling of how transport would take place under normal and extreme conditions (storms) based on significant mean values ​​from the historical series of waves collected by Ports of the State during the last 30 years”, the director of the study explains. In addition, these data were also used to calculate the depth of closure of the beach, 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 in which the structure of the old pipelines were affected by waves and currents, causing erosion and sedimentation processes when meeting the pipeline.

“The combination of these three independent types of results demonstrated that the area of ​​interest is a low energy area, which is significantly shielded from wave attack, where fine sand predominates,” Daniel Rey explains. However, the field data indicated an interaction (accretion-erosion) in the submerged obstacles between 0 and 12 m deep. “The model revealed that there is significant sediment movement above a 7.4 m isobath, and that the pipeline would not alter the general transport dynamics of the beach, but would interact in the shallowest section,” the researcher stresses, thus emphasizing what he considers the main conclusion of the study, and that the future structure would not alter the global sediment dynamics of the beach. In addition, Rey explains that in order to guarantee the safety of the new pipeline, “we also consider that it should not emerge above 8-12m deep, depending on the area.”

Inndaga, “an ideal platform for on-site inspection and monitoring”

The use of Inndaga, a multipurpose platform developed by the Geoma group in an 8.5 m long inflatable boat and designed to carry out oceanographic surveys in coastal areas, was crucial in the research. This concept allows the vessel to operate safely and with great flexibility of maneuver in areas where the largest research vessels cannot access (rocky areas, ports …) and at low operating cost.

Fully integrated into an information management system to provide efficiency and effectiveness in strategic decision making process, the vessel is very flexible and has a wide range of applications. As they explain from the Geoma group, in the 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, the hydrographic conditions and to improve their maintenance and operation of Compliance with EU environmental directives and 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 it was the case in this study, or designing strategies to combat and manage possible dumping ”, Rey stressed.

Furthermore, a fully developed post-prospecting scheme enables a 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 process.

Source: DUVI