Feature ArticlesADCP Measurements in Protected Areas On Vessels of Opportunity
By Alberto Ribotti
Institute for the Coastal Marine
Italian National Research Council
Institute for Marine Sciences
Italian National Research Council
La Spezia, Italy
The use of 3D circulation numerical models with high spatial resolution to forecast currents and hydrocarbon diffusion and transport in marine areas requires continuous verification through the acquisition of hydrodynamic data along the water column. This is economically feasible with widely used acoustic current meters—moored, ship-mounted or towed.
In the open sea these instruments are almost exclusively installed under the keel of the ship, but coastal areas often require the use of small vessels that are not suited to this configuration. Different solutions have been found using both fixed and light structures, mobile or external, or towing instruments.
At the end of August, all of these solutions were taken into account to plan hydrodynamic measurements in the Strait of Bonifacio and around La Maddalena Archipelago in northeast Sardinia, Italy, under the framework of a two-year project funded by the Italian Ministry of Environment. The project was named SOS-Bonifacio and was coordinated by the Institute for the Coastal Marine Environment of the Italian National Research Council (CNR).
The work was achieved in collaboration with the La Maddalena Italian coast guard and CNR's Institute for Marine Sciences (ISMAR).
Under this project, an innovative system for marine circulation forecasting was created for the management of environmental emergencies due to oil spills. The main aim was the prevention and/or limitation of damages to local marine resources and coasts due to oil-spill events.
Two different approaches have been used to characterize the hydrodynamics in the area: Where depth was more than 50 to 60 meters (as in the strait), the acoustic instrumentation, a Teledyne RD Instruments (Poway, California) 300-kilohertz Workhorse acoustic Doppler current profiler (ADCP) and a 75-kilohertz Ocean Surveyor ADCP was mounted on CNR's RV Urania; in shallow waters (as near the La Maddalena Archipelago), a solution-like vessel of opportunity was used with a 600-kilohertz Teledyne RDI Workhorse Sentinel mounted on a local coast guard patrol boat.
Bonifacio Strait and Archipelago
In the western Mediterranean, the Strait of Bonifacio is an 11-kilometer-wide strait between Sardinia and Corsica, France, that divides the Algero-Provencal Basin (west) from the Tyrrhenian Sea (east) with a maximum depth of 100 meters. La Maddalena Archipelago covers the eastern part of the strait and its waters have a maximum depth of about 57 meters.
High current fields cross the area, particularly during strong Mistral (northwestern) or Greco (northeastern) wind events. During the summer, when tourism is strong, the wind speed is amplified due to the Venturi effect, causing high waves and decreasing safety at sea. Several accidents have occurred over the years involving both large vessels and small yachts, with a consequent increase in danger for the marine environment.
Finally, the area has high environmental importance, with two marine national parks, and it is part of the International Sanctuary for the Protection of Marine Mammals in the Mediterranean.
The acquisition of current data is done in strict collaboration with the local coast guard in charge of the marine area. The patrol boat used was a fiberglass unit model Class 500 with a length of 10.4 meters, a width of 3.75 meters and a draft of 0.65 meters.
The Workhorse Sentinel, used in broadband mode, was held in a three-meter aluminum pile fixed horizontally by aluminum bars to the patrol boat's stanchions. Ropes were used to increase security and decrease vibrations. The ADCP is fixed at the lowest extremity of the pile through flanges that were downward oriented, with the beam at 3° to 45° to starboard of the vessel stem.
The ADCP measures currents in a depth range of two to 52 meters. During the experiment, the sea state ranged from calm to moderate and weak wind.
Inside the patrol boat, a computer acquired current data from the ADCP through the RDI SeaSave software, linked in real time with a global positioning system (GPS). Outside the boat, the Jeppesen Marine (Massa, Italy) C-Map GPS acquired boat position with a three-second frequency. The power was 220 volts provided by the boat or, if necessary, by a 12-volt, 25-ampere-hour battery.
The acquisition was along transects perpendicular to the coast, closing the narrowest passages in the archipelago with a boat speed of four to 4.5 knots and 15 nautical miles in about four hours. Data from each transect were processed separately using the freeware program CODAS3, interfaced with MathWorks' (Natick, Massachusetts) MATLAB for the graphics.
The experience acquired during this experiment verified the feasibility of ADCP current measurements in shallow water using Italian coast guard patrol boats, leading to three important indications.
First, it is possible to acquire 3D current data in a coastal area repeatedly and in different meteorological conditions to validate forecast numerical models on circulation and oil-spill diffusion and transport. This kind of information is also vital in obtaining more accurate marine forecasts for the dispersal of pollutants at sea.
Second, this method allows for significantly reduced costs and assembly and disassembly times of the external support apparatus. The entire operation of assembly, measurements in small areas and disassembly can be completed in one day—considerably reducing costs and making it feasible to take advantage of small windows of good weather.
Third, this configuration gives several advantages to the agency carrying out the measurements and to the local coast guard. While the first can eliminate the cost of renting a boat with crew all year long, the latter can perform patrolling at reduced costs while increasing the public visibility of its environmental management work. A minor disadvantage for the agency is sudden stops for emergencies and reduced operation time.
The authors would like to thank Andrea Satta, Barbara Sorgente, the crew of the coast guard patrol boat 527, commander Rodolfo Giovannini of the La Maddalena Harbor Office and Dr. Katrin Schroeder of ISMAR CNR. This work is part of the project SOS-Bonifacio, which is funded by the Directorate General for Nature Protection of the Italian Ministry for Environment, Land and Sea.
For a full list of references, please contact Alberto Ribotti at firstname.lastname@example.org.
Alberto Ribotti is a researcher in the Operational Oceanography Group of the Institute for the Coastal Marine Environment of the Italian National Research Council in Oristano, Italy. He has experience in physical oceanography and ocean monitoring focused on numerical ocean forecast model validation and climatological studies. He is involved in international operational oceanography projects.
Roberto Sorgente is a researcher in the Operational Oceanography Group of the Institute for the Coastal Marine Environment of the Italian National Research Council. He has extensive experience in physical oceanography and high-resolution ocean forecast numerical modeling.
Mireno Borghini is a technician at the Institute for Marine Sciences of the Italian National Research Council (CNR) in La Spezia, Italy. He has considerable experience in managing oceanographic instrumentation. He is involved in a CNR group for the management of national research vessels and in several Italian and European research projects in physical oceanography.