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Hydrographic survey in Faro Channel

Official designation of the project: Hydrographic survey in Faro Navigation Channel

Start date: July 8, 2021
Completion date: July 20, 2021

Detailed description: The characterization of shallow and dangerous areas in navigation channels is the main purpose of hydrographic surveys, which are also very useful in various fields of science, particularly in geological oceanography, in which phenomena related to hydrodynamics and sedimentation are studied.

LS Engenharia Geográfica carried out in the course of 2021, at the request of Doca Pesca SA, a hydrographic survey of Order 1B in the navigation channel that connects the Commercial Pier of Faro and the Esteiro do Ramalhete, in an extension of approximately 2700 meters. The surveys aimed to confirm the minimum service depths in areas with maritime signaling, with the secondary navigation channel in Faro being characterized by shallow and sandy bottoms, and local transit carried out by recreational boats, within the scope of tourism.

Hydrographic surveys, using acoustic sounders (SONAR) and global satellite navigation (GNSS), are relatively time-consuming and require demanding logistics, as well as access to vessels and specialized personnel. In addition to naval equipment and resources, this type of operations also requires external support, and in this case this included the installation of a tide gauge and a GNSS reference station.

This methodology allows the determination of depths with high precision, but ends up being limited by the operational costs that it requires, along with the reduced capacity of action in shallow areas, where it is only possible to work in high tide regimes.

According to publication S-44 (International Hydrographic Organization Standards for Hydrographic Surveys), a hydrographic survey of order 1b is intended for areas where the type of expected transit is such that a general representation of the bottom is considered adequate. Evenly distributed bathymetric coverage, albeit not total, is required for the survey area. This means that some features will not be detected, so this survey order is only recommended when the distance from the keel to the bottom is not considered critical. An example of this are areas where there is a low probability of having an object on the bottom that constitutes a risk factor for navigation.

In the execution of this project, a single beam sounder was used and installed in a receation boat, which carried out 280 lines transverse to the navigation channel. Through the SONAR technology used, we were able to accurately determine the depths, leaving only an uncertainty margin of approximately 0.04 meters.

The positioning of the soundings was ensured by the GNSS positioning system operating in RTK mode, a technique that allows a positioning accuracy of better than 0.03 meters and also the reduction of the tidal height in real time, using the tide gauge readings. installed at the Commercial Pier only as a form of validation or redundancy of the information obtained.

Two hydrographers and a local traffic master were part of this hydrographic survey project. The data obtained made it possible to produce a Digital Elevation Model for the general characterization of the area, where the average depth is about 3 meters in relation to the Chart Datum, with negative depths having been identified, but also some deeper areas, with a total of 8 meters deep.

Challenges: The characteristics of the site implied the use of recreation boats that were not prepared for the installation of computers and various equipment necessary for carrying out the surveys; the need for piloting skills to master the vessel's trajectory was also evident, since the surveys are carried out perpendicularly to the direction of the water flow, and the speed and force felt were high after the low and high tide periods, thus hampering the entire process.

Advantages: Updating of important hydrographic data, which make it possible to accurately identify shallow areas, study sediment transport processes and, if necessary, plan intervention actions such as dredging.

Areas of intervention: Hydrography and Geodesy

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