How can we measure the coastal currents of Jaffna?

1. Where is Jaffna?

Jaffna is a city located at the northern tip of Sri Lanka. It is a place of great historical and cultural significance. Surrounded by the azure waters of the Palk Strait and the Indian Ocean, it presents a unique coastal landscape.

The city is also famous for its ancient temples and forts standing as envoys of its rich heritage. The people of Jaffna have a distinct culture, with their own traditions, art forms, and a cuisine that tantalizes the taste buds. The area is also famous for its palmyra palms dotting the landscape and being an integral part of the local economy and way of life.

Speaking about its water bodies, it has the Palk Strait separating it from the Indian mainland to the west, and is a relatively shallow body of water. The coastal areas house a variety of marine life, from colorful fish to several types of crustaceans. While on the eastern side lies the Indian Ocean with an expanse of deep blue, powerful waves, and strong oceanic currents, over the years, this has modified the coastline. The beaches of Jaffna vary from sandy stretches to more rocky outcrops, with their different kinds of charm and ecological importance.

2. What is the situation of the coastal currents near Jaffna?

Various factors prevail along the coastal currents near Jaffna. The dominant among them is the monsoon winds. The southwest monsoon brings strong winds that may drive the surface currents in a particular direction, often towards the east. In contrast, the northeast monsoon can reverse or modify these current patterns.

The geography of the region also affects the currents. The narrow Palk Strait and the presence of numerous islands and shoals can channel and redirect the water flow. Tidal forces are another crucial element. The regular tides in the Palk Strait and the Indian Ocean can cause the coastal currents to vary in speed and direction. The inflow and outflow due to tides can then interact with the existing oceanic currents, creating a complex and ever-changing current regime.

The bottom topography, the presence of the Sri Lankan landmass, and proximity to the Indian subcontinent are further factors affecting the currents. This shape of the coastline off Jaffna may converge or diverge the currents, allowing for areas of upwelling and downwelling. This upwelling and downwelling affects the distribution of nutrients within the water and thus the marine ecosystem.

3. How to observe the coastal water flow of Jaffna?

Surface Drifter Buoy Method

The surface drifter buoys are simple devices for tracing the movement of the water surface. They are set and left to float with the currents. By using satellite-based tracking systems or other means of position determination, the path of the buoys can be monitored. However, this method provides information only on the surface layer of the water and may not be representative of the currents at a deeper level.

Moored Ship Method

In the moored ship technique, a ship remains moored at a given place in waters off the coast. The shipment is instrumented to measure the water current over the vessel. This process serves for continuous current measurement along the position. However, this method has its drawbacks. Shifting of the ship due to tides and winds may not always result in accurate measurements since only data about the locality very close to the site where it is moored can be available.

Acoustic Doppler Current Profiler (ADCP) Method

This method is more advanced and efficient in the measurement of coastal currents. It makes use of acoustic waves for measuring the water velocity at different depths. In this, by emitting sound pulses and analyzing the Doppler shift of the reflected waves, a detailed profile of the current can be provided from the surface down to a certain depth. The outcome would be less dependent on the conditions of the surface and may yield a better record of the flow of water within the coastal waters off Jaffna.

4. How do ADCPs using the Doppler principle work?

The basic working principle of ADCPs relies on the Doppler effect. They emit acoustic pulses into the water. Interactions of these pulses with the water particles, like sediment particles or small organisms, produce reflected waves with a frequency different from the original, depending on the speed of the particles. In other words, if the particles move toward the ADCP meter, then the frequency of the reflected wave is higher than the frequency that has been emitted, and vice versa.

The ADCP profiler estimates the velocity at those points by measuring the frequency shift at different angles and depths. In this way, the instrument can obtain a profile of the current, showing the movement of water at different depths of the coastal area. The more pulses emitted and the more accurate the measurement of the frequency, the more precise the current profile obtained.

5. What's needed for high - quality measurement of Jaffna coastal currents?

For high - quality measurement of the coastal currents near Jaffna, the equipment needs to have several important features. First, the material should be reliable. It should be able to withstand the harsh marine environment, including saltwater corrosion, high humidity, and possible physical impacts from floating debris.

Other important factors include the size and weight of the device. A small and light ADCP would be easier to deploy and recover, whether it's on a boat, buoy, or seabed. A low power consumption is likewise desirable. This will enable operation for longer periods without constant battery replacements or recharging of power sources.

Cost-effectiveness is also an important factor. For large-scale measurement, the equipment cost should be low enough to be distributed at several locations. Titanium alloy is a very suitable casing material. It is one of the most corrosion-resistant alloys, which is one of the major requirements due to the corrosive action caused by seawater. It also has a good strength-to-weight ratio, making it suitable for constructing a durable yet lightweight casing for the ADCP.

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

Based on Usage

• Ship-borne ADCP: Where the measurements must be taken from a vessel in motion, the ship-borne ADCP can be applied. It will, therefore, give continuous measurement as the ship sails in waters along the coast to comprehend the spatial variation of currents.
• Bottom-mounted ADCP: For long-term and fixed-point measurements, the bottom-mounted ADCP flow meter is quite adequate. It can be laid on the seabed and record the current profiles continuously at that location, giving much valuable data about the local current conditions.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are useful in monitoring the surface and upper-water-column currents. They can be easily deployed and can drift with the currents to some extent, providing a more flexible way of measuring the coastal currents.

Based on Frequency

Depending on the water depth, either ADCP current profiler is chosen. Depths up to 70 m, an ADCP rated at 600 kHz works typically well; between 70 and 110 m depths, one with 300 kHz is good. In larger water depths, like at some deeper portions of the Indian Ocean offshore of Jaffna, there, an ADCP of even a lower rating of 75kHz, can make current measurements closer to a depth of up to 1000 meters.

There are several well - known ADCP current meter brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for a cost - effective and high - quality option, the China Sonar PandaADCP is worth considering. It is made of titanium alloy and offers excellent performance at an affordable price. You can find more information about it on its website: https://china-sonar.com/.

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