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Multispectral imagery use in coastal works

Official designation of the project: Use of multispectral images to support maritime works: beach nourishment in Ilha de Tavira/Praia do Barril

Start date: January 2, 2018
Completion date: August 1, 2018

Detailed description: The Ria Formosa is characterized by the presence of a coastal dune cord, which protects a lagoon area, and its artificial reinforcement is currently part of a set of actions to combat coastal erosion, using sand from dredging. This type of maritime works requires frequent updating of bathymetric information, obtained through hydrographic surveys using SONAR systems (SOund NAvigation and Ranging), an expensive and demanding methodology in logistic terms.

Given the extremely dynamic nature of this ecosystem, it is necessary to have means to estimate depths in large areas, in a short period of time. With the advent of aerial photography and space terrestrial observation missions, and after the first uses of aerial images of marine areas of clear and shallow water, it was recognized that depth could be estimated in some way through remote sensing.

Presenting itself as an indirect method, it allows obtaining bathymetric models of relatively extensive areas, and although these models do not meet the requirements of traditional hydrographic surveys, the data thus obtained can be used to assess areas where a high level of detail is required, to be obtained by high resolution sensors, such as SONAR.

The scope of this project essentially refers to the reinforcement of the Ria Formosa dune cord in an extensive section of the Barrier Islands, in the area called “Praia do Barril”. The intervention in this area - start in January 2018 and end in August of the same year - at the request of Sociedade Polis Litoral Ria Formosa, was justified, given the fact that there is no real zones of sand in stabilization, and the elevations of the particularly low dunes and the face facing the estuary is cut off by channels perpendicular to the coastline.

If the reinforcement is not carried out, these channels may become small “bars of sand” that are difficult to control, due to unfavorable conditions of sea agitation. Simultaneously, and due to the maintenance dredging operations of the Armona bar, serving as a “borrowing zone” to obtain 300,000 m3 of sandy sediments to reinforce the dune cord, it was intended to provide greater lagoon hydrodynamics in the areas of influence of tides of this bar, by increasing the water renewal rate.

This improvement would allow the improvement of navigation conditions in the bar and channels for aquaculture exploration, the increase in the circulation of nutrients, a lower concentration of polluting products, and also a greater availability of oxygen in the water column.

As the determination of depths is fundamental for the characterization of the underwater topography in dredging operations or other maritime works, the execution of hydrographic surveys with acoustic sounding systems is mandatory, which implies a prior planning that must take into account several factors. In addition to choosing suitable maritime resources for the sounding zones, this type of operations needs external support, which may include the installation of tide gauges, GNSS (Global Navigation Satellite System) reference stations and land transports, among others.

Weather conditions also greatly affect the execution of these tasks. In this specific case, the use of multispectral images obtained by satellite and/or UAV for bathymetry estimation proved to be extremely useful in the management of a series of activities, both for observing the evolution of dredging works and for planning hydrographic surveys and choice of the most appropriate means for its execution. The ability to derive bathymetry through remote sensing techniques is a topic of growing interest in the scientific field, with LS-Engenharia Geográfica pioneering the development of this technology in the Iberian Peninsula, together with the European Space Agency. The adopted method consists of a methodology that uses images of several multispectral bands for the study of the water column, and allows to produce a bathymetric model.

Prior to the beginning of the dredging and dune reinforcement works, hydrographic surveys were carried out with a single beam system, in the intervention areas. The data obtained allowed us to assess the dredging and feeding volumes on the beach. A few weeks after the start of dredging operations, the location and evolution of these works was already visible from space, and the study of the images suggested that the strong dynamics of the lagoon system would hardly allow the maintenance of the opening of a channel in Barra da Armona, at least with the available dredging ships, and at the depths defined for the project.

The evaluation of this satellite data led to a change in the dredging work plans. In addition, it would also be necessary to carry out a hydrographic survey along the entire length of the beach (2500 meters), which would require the displacement of necessary resources in other places, and also at additional costs.

The observation of satellite images made it possible to evaluate the intertidal zone and estimate volumes of sand in the submerged areas, abandoning the option of carrying out the hydrographic survey and avoiding excessive costs.

However, one of the areas heavily affected by the March 2018 storm was Praia da Fuzeta (West extension of the Island of Tavira), forcing an intervention already scheduled for that year to be brought forward. The need to quickly carry out a hydrographic survey of the entire intervention area forced an advance study to understand what type of vessels could be used, what were the most favorable working hours (tide heights) and which were the priority areas, all of this given the changes in the water basin. Using a rotor UAV equipped with multispectral cameras and a GNSS positioning system, a set of images of an area of ​​interest was acquired, which made it possible, in a short time, to study the area and manage resources and tasks necessary to schedule the work.

This technology, based on tested and demonstrated remote sensing techniques, has allowed the estimation of bathymetry with a good degree of confidence, and is currently considered an indispensable resource for the management of hydrographic works and navigation support within the scope of ongoing maritime projects.

Challenges: The use of this technology has limitations below 10 meters depth in areas of high water turbidity.

Advantages: Large geographic coverage, short revisit period, zero-carbon emissions, no mobilization of equipment or personnel

Intervention areas: Remote Sensing

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