How do we measure the Coastal Currents of Thrissur?

1. Where is Thrissur?

Thrissur is a city in the southern Indian state of Kerala. It is an inland city, but not far from the Arabian Sea. The city is well known for its rich cultural heritage and is also called the "Cultural Capital of Kerala."

Geographically, Thrissur is set amidst a land of varied landscape. It is surrounded by alluvial plains, which are highly fertile and support intense agriculture. Nearby backwaters and canals form part of the inland water transport system, linking it with the large water body. Though not a sea-facing city, its relationship with the Arabian Sea is paramount. The coastal tracts around Thrissur are predominantly sandy beaches and mangrove forests. These mangroves act as a protective barrier against storms and erosion, while at the same time providing a peculiar habitat to a wide range of marine and bird life.

Culturally, Thrissur is noted for its festivals, especially the festival of color called Thrissur Pooram. It attracts thousands of people and is an example of rich cultural tradition in Kerala. The local food is a mouth-watering combination of spices and freshness, with a great influence from the sea because of its proximity.

The waters off Thrissur that open onto the Arabian Sea are influenced by the tides and the general circulation of the Arabian Sea. The sea breeze does its part, too, as the inland climate here is predominantly warm and humid. It carries a refreshing touch along the shoreline.

2. What is the condition of the coastal currents around Thrissur?

Various factors affect the coastal currents around Thrissur. Monsoon winds contribute a great deal. In June-September, it is southwest monsoon. The coming of the latter causes an abrupt change in the wind field. These are the winds that drive surface waters, which may account for variations in the speed and direction of the currents. The northeast monsoon comes during the period from October to December and exerts different characteristics.

Another important factor is the tidal currents. The tidal pattern in the Arabian Sea is semi-diurnal, and during a day, there is the occurrence of two high tides and two low tides. The ebb and flow of the tides create currents that interact with the wind-driven currents. Seabed topography near the coastal areas around Thrissur affects current paths. Underwater ridges, channels, and sandbars may deflect and modify the currents.

Freshwater inflow from the several rivers and canals in the area may lead to changes in the density of the coastal waters. This may result in the development of estuarine circulation patterns, which will further modify the coastal currents. Even larger-scale oceanic circulation within the Arabian Sea, like the Somali Current and the Arabian Sea Gyre, can indirectly influence the local coastal currents.

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

• Surface Drifting Buoy Method: A surface drifting buoy is a simple and efficient way of observing the surface current. The buoy is released into the water, and over time, the movement of it is tracked with GPS or other tracking devices. By monitoring the displacement of the buoy, one can estimate the speed and direction of the surface current. However, this method is limited to the surface layer, and the buoy's motion may be influenced by winds and waves, which may not yield accurate measurements of the actual water current.
• Anchor - Boat Method: This is a method whereby a boat is anchored at a certain point near the coast. Current meters are then deployed from the boat to measure the current at different depths. This gives a more detailed view of the current profile at a particular point. However, it is restricted to the area around the anchored boat and may not capture the full spatial variability of the currents in the region.
• ADCP Method: This is a more sophisticated and efficient means of measuring coastal water flow. It can measure simultaneously the velocity of water at more than one depth. In an ADCP profiler, by emitting sound waves and analyzing the Doppler shift of the reflected waves, one is able to comprehend the current profile from the surface to the seabed.

4. How do ADCPs using the Doppler principle work?

The principle of operation of ADCPs is based on the Doppler effect. They emit acoustic pulses into water. These pulses interact with moving particles in the water-sediment, plankton, and all other small organisms. Particles moving towards an ADCP will have a frequency of the reflected wave higher than that emitted, while those moving away from it will have a lower frequency of the reflected wave.

By precisely measuring this frequency shift, the ADCP is able to calculate the velocity of the water at different depths. Most ADCPs have multiple transducer elements that send out beams in different directions. This makes them capable of measuring three-dimensional structure in the current, which gives a real representation of the pattern in the flow of water.

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

The equipment for high-quality measurement in the coastal currents in Thrissur should have materials that are reliable. Because seawater is corrosive, the device has to be able to bear long-time exposure. It should also be small in size and light in weight for ease of deployment and retrieval at places where access may be difficult or conditions unfavorable.

Long-term operation, especially if the equipment is to be used for continuous monitoring, requires low power consumption. Cost-effectiveness is another important factor that will enable widespread and frequent measurements.

The casing of the ADCP meter is preferably made of titanium alloy. Titanium alloy has excellent corrosion resistance, which is very important in the aggressive seawater environment. It is also strong and able to resist the pressures exerted by the water column and the forces of waves and currents. Its relatively low density helps to keep the weight of the equipment down, making it more manageable during installation and maintenance.

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

It depends on the specific application.

• For Ship - based Measurements: A ship-mounted ADCP flow meter would be suitable. In this way, measurements could be made while the ship is in motion, enabling the mapping of currents over a larger area.
• For Near - Seabed Currents: A bottom-mounted ADCP can be used. This is particularly very ideal for studying the currents near the seabed, which may be very important in understanding sediment transport and benthic ecosystems.
• For Surface and Upper - Water - Column Measurements: A buoy-mounted ADCP will be a good option. It provides continuous measurements of the surface and upper-layer currents.

Considering the frequency of the ADCP current profiler, different frequencies are fitted for different water depths. With a 600 kHz ADCP, water as deep as around 70m will do, whereas with a 300 kHz ADCP, it's appropriate to go up to about 110m, and in some instances, a 75 kHz ADCP can accommodate up to 1000m of water depth.

There are several well - known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand - China Sonar PandaADCP is also a great option. It is made of all - titanium alloy material and offers an excellent cost - performance ratio. You can visit their website at https://china-sonar.com/ for more details.

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