Dredging companies increasingly have to ensure that they do not increase sedimentary concentrations too drastically with their dredging works. They have to do this in particular when they are working in the vicinity of valuable and vulnerable ecological areas. After having already developed a tool to monitor these sediment values using satellites, VITO is now working on a similar technique based on drones.
During dredging works, the seabed is excavated (for example to deepen a waterway) or sand is sucked up and then sprayed onto land (in order to build a dike). This can sometimes loosen a large quantity of sediment and cause it to move. This floating sediment makes the (sea)water turbid and can harm vulnerable and ecologically valuable ecosystems, such as coral reefs and mangrove areas – which need sunlight. Excessive sediment values can also cause problems for aquaculture, however. The sediment may contain contaminants that can be absorbed by, for example, oysters and mussels. Moreover, high concentrations of sediment are detrimental to the growth of shellfish.
Real-time monitoring
In many countries, therefore, standards apply to these sediment values, in terms of both concentration and duration. Dredgers therefore do everything they can to restrict the spread of sediments, but, in order to do so, they have to map them first. They also like to have sediment value data for the purpose of efficient business operations. They still generate this data themselves by means of (amongst other things) point measurements and buoys.
“If the standards are exceeded, a dredging company must, in the worst case scenario, suspend works for a certain period of time so that the sediment can sink back to the bottom”, says Els Knaeps of VITO. “Real-time monitoring of the values can help to prevent the standards from being exceeded in this way.”
VITO has previously already developed a software package enabling dredgers to extract the sediment values from satellite images. VITO now wants to extend its services to dredging companies – and to other companies or organisations that operate at sea – with images produced by drones. “They have the significant advantage of also being able to take photos when it is cloudy”, says Knaeps.
Detailed spatial overviews
In order to refine sediment monitoring via drones even further, a VITO team travelled to the Dutch island of Texel in the Wadden Sea at the end of last year. There, dredging company Jan De Nul Group is reinforcing the coastline at Prins Hendrikpolder. Over a distance of three kilometres, it is constructing a dune that will take on the flood-defence function of the old dike. The Wadden Sea is also one of UNESCO’s World Heritage Marine Sites, therefore it is strictly protected.
“The problem with the current monitoring techniques is that they only show a limited number of measuring points at any given moment,” says Liesbeth De Keukelaere of VITO. “However, a sediment plume (comparable to a plume of smoke, but underwater) can stretch several kilometres and continuously mutates, as a result of currents and tides.”
The drones use their multispectral cameras to capture images that allow accurate quantitative analysis of the sediment concentrations. The VITO-software recognises the concentrations automatically and in real time, and then immediately matches them to the correct location. “In ideal conditions, a drone that flies at a height of 120 metres can cover a water surface area the size of a full football pitch in a single image capture”, says De Keukelaere.
According to Emile Lemey, environmental engineer at Jan De Nul Group, the monitoring technique (which, to be clear, is still in the test phase) saves time, manpower and fuel. “We are currently still doing everything via manual measurements, having to sail from buoy to buoy. In one movement, the drones give us a detailed spatial overview of the sediment plume. This signifies a big step forward in our day-to-day operations.”
