How can we measure the coastal currents of Porbandar?

1. Where is Porbandar?

Porbandar is a coastal city and a municipality in the state of Gujarat, India. It is situated on the Arabian Sea coast and is famous for its historical and cultural background.

This city plays an important part in the history of India. It is the birthplace of Mahatma Gandhi. The city has a rich cultural heritage. The culture is full of art, music, and dancing. The local people survive with fishing, trading, and handicrafts.

Porbandar's coastal area is in length a stretch of beautiful sand beaches and crystal blue water. The Arabian Sea near Porbandar is extensive and dynamic. It shelters many different marine life species, such as fish of many kinds, turtles, and in some areas, even corals. It acts as a significant source of income among the local fishermen through their fishing activities. It also serves as an important transportation route, with ships and boats plying regularly for trade and other purposes.

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

The coastal currents off Porbandar are caused by many factors. This includes the monsoon winds. The southwest monsoon, from June to September, has strong winds with a great influence on the direction and speed of the currents. These winds push the surface waters in different directions with the result of different current patterns arising.

Tidal forces are the other important factor. Tides are due to the gravitational pull of the moon and sun, while tidal currents can be strong in association. Tidal ebb and flow may alter the water movements off the coast. The shape of the coastline and the topography of the seabed have an effect as well. The bottom topography, like ridges and valleys, can deviate the currents, while the curvature in the coastline can affect the flow of currents.

Moreover, larger-scale ocean circulation patterns of the Arabian Sea interact with local coastal currents. Inflow of different water masses from the open sea alters the temperature, salinity, and overall characteristics of the coastal currents.

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

Surface Drift Buoy Method: It involves deploying buoys on the water surface that are designed to drift with the currents. By tracking, using satellite - based tracking systems or other positioning methods, the movement of the buoys over time, we derive information on the direction and speed of the surface currents. However, this approach gives essentially data from the surface layer and cannot be used to determine currents at different depths.

Anchor Boat Method: This method involves anchoring a boat in the coastal waters. Instruments are used to measure the flow of water at different depths around the boat. While it may provide more detailed data in the area surrounding the boat, this is a relatively labor-intensive and time-consuming method. This also has limitations in terms of the area it can cover.

Acoustic Doppler Current Profiler (ADCP) Method: ADCP current meter is one of the more modern and efficient ways of measuring coastal currents. It uses sound waves to measure the velocity of water at multiple depths simultaneously. It can provide a detailed profile of the current from the surface down to a certain depth, giving a more accurate and comprehensive picture of the coastal currents compared to the other methods.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They send out sound pulses into the water. Once these sound waves encounter particles in the water-like sediment particles or small organisms-the frequency of the reflected sound waves shifts through the Doppler effect. When the particles move towards the ADCP meter, the reflected waves have a higher frequency than the emitted waves, while when they are moving away, the frequency is lower.

By measuring the frequency shift of the reflected sound waves at various angles and depth, the ADCP current meter will calculate the velocity of water in various directions. For this, the ADCPs use a set of transducers that transmit and receive these signals of sound, thus generating a profile of the current water velocity from the surface to a considerable depth from the bottom, based on the instrument's capability.

5. What is required for high-quality measurement of Porbandar coastal currents?

The measuring instrument for the coastal currents around Porbandar has to be made of durable materials. It should be resistant to the harsh marine environment that includes saltwater corrosion, temperature fluctuations, and impacts from floating debris.

The size of the equipment should be as small as possible to enhance ease of deployment and installation on either boats, buoys, or the seabed. A smaller size is also more feasible in transportation and maintenance.

The weight of the equipment should be light. For floating or moored devices, a light weight is essential to ensure proper buoyancy and stability. Low power consumption is also very important to enable continuous operation over long periods without the need for frequent battery replacements or a large power supply.

Cost-effectiveness is another important consideration. In making large-scale measurements along the coast, equipment should be reasonably affordable. For ADCPs, casing made of titanium alloy will be excellent. This material possesses excellent corrosion resistance, which is essential in the salty marine environment. It is also strong but relatively light, as needed by high-quality current-measurement equipment.

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

While selecting equipment for current measurement off Porbandar, the first factor to consider is the purpose of usage.

Ship-borne ADCP: This is suitable for measurements from a moving vessel. It can record the coastal currents continuously as the ship moves along the coast, providing a series of data points over a relatively wide area covered by the ship's route.

Bottom-mounted ADCP: Ideal for fixed-point measurements at the seabed. It provides a stable and long-term record of the water flow near the bottom, which is important for understanding the interaction between the bottom topography and the coastal currents.

Buoy-mounted ADCP: deployed when the purpose is a measurement of currents over certain areas for longer periods just floating on the surface; it can be deployed, and it will provide continuous records in the area of interest independently of a vessel to have been present all the time.

In terms of frequency, for water depths within 70m, the 600kHz ADCP is quite a good one. For the depth of up to 110m, the 300kHz is more appropriate. And for even deeper waters, up to 1000m, the frequency recommended is 75kHz ADCP.

There are well-known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth mentioning. It has a casing made entirely of titanium alloy and offers an excellent cost-performance ratio. You can find more information about it at https://china-sonar.com/.

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