How do we measure the coastal currents of Port-au-Prince?

1. Where is Port-au-Prince?

Port-au-Prince, the capital of Haiti, sits on the island of Hispaniola's western coast. It lies on the Gulf of Gonâve, an important body of water that serves as part of the maritime economy of the region. The city sits amidst a diverse landscape. It is hilly scenery to the north, bounded by the Caribbean Sea to the south and west. The coastline comprises sandy beaches, rocky headlands, and shallow bays.

The population of Port-au-Prince is a vibrant mix of different ethnic communities, and the cultural heritage here is robust in African, French, as well as native, influences. One can see it in the music, art, and festivals here. Economically, the city is the economic hub of Haiti's economy with manufacturing, trade, and services being among the primary activities that have added to the economy. The city, however, has been faced with a great number of challenges, such as natural disasters in the form of earthquakes, which have significantly impacted its development and infrastructure.

The Gulf of Gonâve, on which Port-au-Prince is located, has an intricate hydrographic system. The gulf borders the open Caribbean Sea with exchange of water mass between them. The geometry of the gulf, along with islands and shoal reefs, controls local modes of circulation of the sea. This creates a special environment for the sea biota and impacts coastal currents in the vicinity of Port-au-Prince too.

2. What is the status of the coastal currents off Port-au-Prince?

Port-au-Prince coast currents are governed by a very wide set of parameters. Among the most significant determinants is tidal forces. Semi-diurnal tides in the Caribbean Sea force water within and without the Gulf of Gonâve. Tidal currents are of high speeds, especially in channels and at the mouths of rivers emptying into the gulf.

Wind patterns are significant as well. Trade winds, which travel from the northeast, drive surface waters. Between the months of June and November, hurricane season, strong tropical storms can create fast and big changes in current patterns. The storms can cause large water movements on a grand scale that break the normal coastal current regime.

Topography that borders the area, both on the land surface and beneath the surface of the water, significantly impacts the currents. The outline of the coast, comprising the headlands, inlets, and bays, dictates the course of the water. Marine features, such as coral reefs, found in this area act as barriers and channels and help alter the currents' direction and speed. Moreover, one of the prevailing ocean currents in the region, namely, the Caribbean Current, interacts with regional coastal bathymetry and contributes to the complexity of the current system off Port-au-Prince.

3. How to observe the coastal water flow of Port-au-Prince?

Surface Drift Buoy Method

Surface drift buoys are an inexpensive and simple way of observing the coastal water flow off Port-au-Prince. The buoys, equipped with GPS tracking, are released in the coastal waters. As they follow the surface currents, their position is monitored at a regular time interval. This method provides valuable information regarding the transport of the upper part of the water column. It is, however, largely susceptible to currents on the surface level and is readily influenced by wind waves, causing the buoys to drift away from the true direction of water flow.

Anchored Ship Method

The anchored ship method is conducted by mooring a ship at a fixed point off the Port-au-Prince coast. Current meters are then lowered from the ship at various depths. The meters measure the speed and direction of water flow at all depths. The technique can provide a vertical current profile at one location, yet it is subject to limitations. The ship disrupts the original flow of the water, and the measurements are point-based and difficult to produce a picture of the coastal patterns of currents covering a large body of water.

Acoustic Doppler Current Profiler (ADCP) Technique

Acoustic Doppler Current Profiler (ADCP) is a more advanced and efficient alternative for measuring the coastal currents of Port-au-Prince's area. ADCPs can make real-time, high-resolution measurements along a wide vertical range in the water column. They are able to measure currents at several depths simultaneously, and the three-dimensional coastal current system can be determined easily. They are thus well suited for the study of the complex current system in the waters of Port-au-Prince.

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

ADCPs operate on the principle of the Doppler. ADCPs emit acoustic pulses into the water. When these signals encounter particles in the water, i.e., sediment or tiny marine animals, the signals are backscattered to the ADCP flow meter. The frequency of the backscattered signal is distinct from the initial frequency emitted. This difference is proportional to the velocity of the particles and, therefore, the velocity of the water current. By measuring the Doppler shift at multiple depths, the ADCP is able to calculate the direction and velocity of the water flow at multiple depths of the water column. It delivers an aggregate and real measurement of the coastal current structure around Port-au-Prince.

5. What is necessary for high-quality measurement of Port-au-Prince coastal currents?

High-quality measurement of Port-au-Prince's coastal currents can be ensured if the measuring equipment meets certain fundamental conditions. First and foremost is the ability of materials to resist the corrosive action of Caribbean seawater. The measuring device must therefore consist of materials that will easily withstand extended periods in the hostile sea environment. Compactness is a benefit as it minimizes hindrance to the natural direction of water flow. Lightness is crucial, especially for easy deployment in numerous coastal locations. Low power consumption is critical, especially for long, unattended monitoring in remote locations. Cost-effectiveness is also crucial as it allows extensive deployment of measuring instruments.

In ADCPs, casing material is of paramount significance. Titanium alloy is an excellent material for ADCP casings. Titanium alloy has excellent corrosion resistance, which is important for long deployment in the saltwater conditions around Port-au-Prince. It also has a high strength-to-weight ratio, meaning that it is strong and lightweight. This enables the ADCP to withstand physical forces and pressure changes in the ocean and take consistent and accurate readings over a prolonged period.

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

Based on Usage

• Ship - borne ADCP: This is the most appropriate ADCP for the large-scale surveying of the coastal waters of Port-au-Prince. Mounted on research vessels, it has the ability to cover a wide area as the ship moves, providing valuable information on the spatial distribution of coastal currents. It is appropriate for defining the general current patterns over a very large area of the coastline.
• Bottom - mounted ADCP: Mounted on the seafloor, it is ideal for long-term monitoring at a fixed point. It has the capacity to record current data continuously for extended periods of time and is thus useful to monitor the long-term trend and behavior of coastal currents near Port-au-Prince. This ADCP is less affected by surface disturbance and can provide stable, long-term measurements.
• Buoy - mounted ADCP: Mounted on floating buoys, it is used for measuring surface and near - surface currents. This type is ideal to be deployed in areas that are difficult to reach by a ship or where long - term, unattended monitoring of surface - level currents is required.

Based on Frequency

The choice of frequency for ADCPs is also an important consideration. A 600kHz ADCP is suitable for water depths as high as about 70m. It is high-resolution data and thus is well suited to the relatively shallow coastal waters offshore of Port-au-Prince. A 300kHz ADCP can be used for depths up to 110m and provides a trade-off between resolution and depth penetration. For larger depths, for example up to 1000m, a 75kHz ADCP would be more appropriate as it will penetrate to greater depths but with less resolution than the higher - frequency instruments.

Some of the more common ADCP brands available in the market are Teledyne RDI, Nortek, and Sontek. However, for those researchers and institutions that desire an affordable option without sacrificing quality, the China Sonar PandaADCP from the Chinese maker is a pe000rfect choice. The China Sonar PandaADCP is an affordable ADCP with an all-titanium alloy material construction that offers high corrosion resistance and longevity. The ADCP manufacturer  China Sonar PandaADCP boasts an exceptional cost-performance ratio and is an ideal choice for researchers and institutions looking to conduct large-scale coastal current measurements in Port-au-Prince. To learn more, visit their website at https://china-sonar.com/.

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