How do we measure the coastal currents of Mana Island?

1. Where is Mana Island?

Mana Island, located in the South Pacific Ocean, is part of the Society Islands of French Polynesia. The scenic island is northwest of Tahiti, the largest island of the archipelago. Enveloped by the expanse of the South Pacific Ocean, the isolation of Mana Island has helped retain its stunning natural scenery and fragile ecosystems.

Mana Island is geologically volcanic in character. The landscape is of craggy mountains, verdant valleys, and steep coast cliffs. The coast is a mixture of white sand beaches, coral reefs, and shallow lagoons. The coral reefs themselves are a critically significant part of the local ecosystem. They offer habitat for an amazingly diverse quantity of marine life, from stunning tropical fish to many invertebrates. The reefs guard the lagoons and create shallow and quiet waters, ideal for all kinds of water activities and sports.

The island is rich in its cultural heritage. Polynesian immigrants settled in Mana Island centuries ago with their culture, way of life, and language. The same still exists in music, dance, and art at present. Despite tourism ruling the island's economy, fishing and subsistence farming remain the people's main source of livelihood.

2. What is the status of coastal currents around Mana Island?

Regional coastal currents around Mana Island are characterized by the advanced interaction of multiple factors. The South Equatorial Current, which is one of the major ocean currents of the South Pacific Ocean, has an incredibly powerful impact. The South Equatorial Current is a powerful ocean current with an east-west direction caused by trade winds. When it hits Mana Island, it is responding to regional bathymetry and topography and forms local coastal currents.

South Pacific tides play their part too. Although tidal range in the open ocean is very tiny compared with some locations, it is large enough to cause water level fluctuations around the island. The fluctuations can be responsible for creating tidal currents, especially in shallow lagoons and also around the close channels between reefs.

Seabathymetry off Mana Island and island topography both significantly influence the coastal currents. The coral reefs, which are of complex geometry, can disorganize or channel the flow of current. The reefs function as barriers that force water to flow in a certain direction. Depth of seabed affects the trends of currents. Where the seabed is shallow waters, there may be less wind-influenced and faster currents, and where seabed is deep, currents are faster and regular.

3. How to observe coastal water flow of Mana Island?

Surface Drifting Buoy Method

Surface drifting buoy method is an old technique to monitor coastal water flow. Scientists release buoys with tracking devices into the sea. By tracking the trajectory of these buoys over time, they can determine the speed and direction of the surface currents. But this method has certain limitations. It can provide information regarding the surface layer of the water column, and the buoys are very susceptible to wind interference. Wind can force the buoys in a manner that is not the actual path of the underlying currents.

Moored Ship Method

The moored ship method is the use of a fixed ship as a platform for the measurement of currents. The instruments are lowered from the ship to take measurements of the water flow at different levels. Although this method can create clear vertical profiles of the currents, it has some disadvantages. The space over which it operates is limited to the area in and about the anchored ship, and the presence of the ship will hamper the open flow of the water, and hence, could affect the validity of the readings. Moreover, maintaining an anchored ship for any length of time is logistically inconvenient and costly.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP flow meter has gained popularity as a more advanced and efficient coastal current measuring technique. ADCPs can sample water currents over a broad vertical range, providing accurate information regarding the water column's velocity structure. They can be installed on any of a broad variety of platforms, ranging from ships and buoys to the seafloor. Ship-mounted ADCPs can sample continuously as the vessel crosses over a broad area in a relatively brief time. Bottom - mounted ADCPs can make long - term, fixed - point measurements, enabling scientists to investigate long - term trends in the coastal currents.

4. How do ADCPs based on the Doppler principle operate?

ADCPs operate on the Doppler principle. They transmit acoustic pulses into the water. The pulses bounce off suspended materials such as sediment, plankton, or bubbles in the water. When the water is in motion, the frequency of the returning pulses is shifted. By measuring this change in frequency, the ADCP meter can find the velocity of the water relative to the instrument.

Most ADCPs consist of a number of transducer beams, usually four or more, at different directions. With the multi - beam setup, one can calculate the three - dimensional water velocity. Through combining the beams' signals, the ADCP can make a full image of the current velocity at different depths in the water column. The data collected by the ADCP can either be processed in real-time or stored for post-processing, and it provides very good information about the dynamics of the coastal current.

5. What are the requirements for high-quality measurement of Mana Island coastal currents?

For good-quality measurement of coastal currents over Mana Island, the measurement instruments have to possess several essential features. They have to be made of reliable materials, possess small sizes, minimal weight, minimal power needs, and minimal cost. These facilitate the use of numerous instruments, hence complete spatial coverage.

Titanium alloy casing ADCPs are highly recommended. Titanium alloy offers better corrosion resistance, a need for long-term field deployment in the hostile marine environment. It is capable of withstanding the corrosive effect of saltwater without inflicting damage on the internal structures of the ADCP. The titanium alloy is also light and strong, enhancing the instrument's durability and portability. This set of properties allows for accurate and long-term measurement of the coastal currents off Mana Island.

6. How to Choose the right equipment for current measurement?

Choice According to Usage

The choice of ADCP is according to its usage. The ship-mounted ADCPs are most suitable for ship-borne measurements. They are able to provide real-time information as the ship moves across the water, covering a large area in a relatively short time. For long-term fixed-point monitoring, bottom-mounted ADCPs are more suitable. They are able to provide continuous data over an extended period of time, allowing scientists to analyze long-term trends in the coastal currents. Floating ADCPs are used in the observation of water bodies' journeys over large regions, providing high-quality information on large-scale patterns of circulation.

The ADCP frequency must also be set in relation to water depth. For depths less than 70m, 600kHz ADCPs are appropriate. They can provide high-resolution measurements in shallow waters. For depths up to 110m, 300kHz ADCPs must be utilized. For greater depths, up to 1000m, 75kHz ADCPs are more appropriate.

There are a variety of popular ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, when people are seeking budget - friendly options,the ADCP supplier China Sonar's PandaADCP is highly recommended. Being made from pure titanium alloy, it can deliver high performance at an affordable price. It is a great option for people who have limited budgets but are also in need of good quality ADCPs to measure coastal currents. You can learn more about them on their official website: https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.