Mapping the aquatic vegetation of Kaiafas Lagoon, using a USV-based system
08/2021 to 09/2021
08/2021 to 09/2021
As part of the research conducted by the team of Oceanus-Lab (in collaboration with Intelligent Machines Single Member P.C.) on the demographics of the aquatic vegetation (Potamogeton pectinatus and Chara hispida corfuensis meadows) of the Kaiafas Lagoon, a pilot survey was performed using a USV-mounted side-scan sonar in the north-northeastern part of the basin.
Kaiafas Lagoon is located near the city of Zacharo, in Western Greece. It is a medium-sized limited semi-enclosed lagoon with an average depth of 3.2m, while the maximum depth observed is 8.1m. The presence of Potamogeton pectinatus and Chara hispida corfuensis, which are the dominant species of aquatic vegetation in the lagoon and the balance between which is extremely important for the health of this ecosystem, is characteristic. The lagoon system is probably endangered by a neighboring wastewater treatment, while it is an attraction for many tourists. During the catastrophic fires of 2007, about 75.77 km2 were lost from the Natura area of Kaiafas.
For the conduction of the survey, on 13 August 2021, an unmanned surface vehicle was used, specially constructed for the monitoring of environmental parameters of marine and aquatic ecosystems, as well as for the imaging of important submerged natural and cultural heritage structures. The USV has an operational capability in offshore (0-50m depth), inland (lakes, rivers) and transitional environments (lagoons, estuaries, etc.).
The system consists of the autonomous vessel, an echosounder and a side scan sonar. The USV was built in the shape of a catamaran, in order to achieve greater stability in lateral movements, while it has a length of 1.7 m, a width of 0.6 m and a weight of less than 15 kg. Thus, there is no need for it to be towed with a trailer or similar external structures for its transport. Its floats are inflatable and not fixed.
The movement of the vehicle is carried out by 4 motors - T200-type thrusters by BLUE ROBOTICS. One innovation, concerning the vessel, is the possibility of positioning and using the motors in different layouts and types of operation. Also, the system has two separate GPS receivers in an automatic failover set-up.
The USV enables remote operation and control, as well as programming and fully autonomous execution of the desired routes, while it has the ability to instantly switch between these two methods. In the case of manual control, the vehicle has a remote-control Herelink Long Range HD Video Transmission System with the ability to transmit a signal in a range of up to 20 km. Control can also be carried out with a remote control system and not necessarily with the use of a tablet or laptop. Furthermore, the minimum telemetry range exceeds 4 km.
The catamaran-shaped boat has an RTK precision GNSS positioning receiver and a motion sensor, while it was equipped with a 200kHz sonar and a 455kHz Lowrance Ti 5 TotalScan side scan sonar with a 2x30m scanning range.
In the Kaiafas lagoon, 3 areas were selected, which were fully imaged, with USV cruising routes designed for the sidescan sonar data to overlap.
In the aforementioned field test, the USV performed excellently and without complications, providing excellent quality data, of equal value to those obtained by traditional mapping methods, with complete safety and a much lower cost, both in man-hours and in economic terms.
For the acquired data, the Reefmaster 2.0 program was used, where the tracks were processed, while in a GIS environment (ArcMap 10.8), the created mosaics were unified into polygons with common reflective characteristics, which in turn led to the distinction of the lagoon bottom acoustic types. The process was completed by creating a map of those acoustic types.
From the interpretation of the acoustic types, an increase in vegetation by and a significant decrease in Chara hispida corfuensis came to the surface, while in many spots, in place of the latter, characteristic scars were observed on the bottom of the lagoon.
Images by Oceanus-Lab.