How can we measure the coastal currents of Junagadh?

1. Where is Junagadh?

Junagadh is a city in the state of Gujarat, India. While it's not a coastal city in a strict sense, it's in close proximity to the Arabian Sea. The city is rich in history and culture, with a heritage that dates back centuries.

It has a reputation for hosting several ancient forts and monuments that act as the immediate pointers to the past glory it beheld, for example Uparkot Fort. Inhabited by a predominantly farming population with several instances of trade and small - scale industries. Its vicinity with the coastal area makes this town an important place to live.

The local ecology essentially depends on the adjoining coastal tracts along the Arabian Sea. The coastal waters support a rich marine life with numerous species of fish, shellfish, and other organisms. The sea modifies the climate through regulating temperature and adding moisture from sea breezes. Sandy beaches, rocky outcrops, and mangrove forests in some places line the coastline, which is important in coastal protection and as a habitat for wildlife.

2. What is the situation in the coastal currents near Junagadh?

The currents in the coasts in the vicinity of Junagadh are influenced by a plethora of factors. The important ones are monsoon winds. During the southwest-monsoon months (June - September), the strong westward winds of the southwest monsoon push across the surface waters of the Arabian Sea. Such winds produce periodic changes in the directions and speeds of the currents and thus present a varied flow pattern.

Other major determinants are tidal forces. The gravitational pull of both the moon and the sun creates tides, and the associated tidal currents can vary in strength and direction. The shape of the coastline and the topography of the seabed in adjacent areas also affect the currents. Submarine features like sandbars, ridges, and canyons may deflect the flow of water, either enhancing or disrupting the normal current patterns.

In addition, the large-scale oceanic circulation in the Arabian Sea can interact with the local coastal currents. The inflow of different water masses from other regions can bring changes in temperature, salinity, and density, which in turn can modify the characteristics of the coastal currents near Junagadh.

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

Surface Drift Buoy Method: This method involves deploying buoys on the water surface that are allowed to drift with the currents. With the use of satellite-based tracking systems or other positioning methods, the movement of these buoys is monitored over time to acquire information about the direction and speed of the surface currents. This method mainly provides data for the surface layer and may not be representative for the currents at deeper levels.

Anchor Boat Method: The methods may involve an anchored boat in open coastal waters, from where the instruments are measuring the flow of water around the anchored boat at a number of depths. Though it might give more accurate data concerning the area of the anchor point, this method is time-consuming and labour-intensive, nonetheless. Another drawback of the method includes its limitation regarding the general area that the method would be able to cover.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP current meter method is an advanced, much more efficient way to measure coastal currents. It can detect the velocity of the water at different depths in one go by using the propagation of sound waves through the water. It allows the detailed profiling of the current right from the surface down to a certain depth and thus gives a more proper and comprehensive picture of coastal currents than the other techniques.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They send sound pulses out through the water. Once this sound comes into contact with particles in the water, such as sediment particles or small organisms, because of the Doppler effect, the frequency of the reflected sound waves shifts from the frequency of the emitted waves. If the targets are moving towards the instrument, the frequency of the reflected waves is higher; if they are moving away, it is lower.

By measuring the frequency shifts of the reflected sound waves at different angles and depths, the ADCP current profiler can work out the velocity of the water in different directions. To do this, the ADCP flow meter uses several transducers to transmit and receive the sound signals, which gives it a very detailed profile of the water velocity throughout the water column from the surface down to a certain depth, depending on the capabilities of the instrument.

5. What is required to measure the Junagadh coastal currents with high quality?

In order to measure the Junagadh coastal currents at a high quality, it is necessary that the materials used in the equipment must be reliable. It should sustain the harsh marine environment by being resistant to salt-water corrosion, temperature fluctuations, and other potential impacts from floating debris.

The size of the equipment should be small for easy deployment and installation on either boats, buoys, or at the seabed. A small size also makes it more convenient for transportation and maintenance.

Its weight has to be light; for floated or moored devices, their weight needs to allow the right buoyancy or stability. Besides, high input power is relevant to let its continuous action for periods with less frequently replaced or small-sized batteries.

Cost-effectiveness is a significant factor. The equipment should be affordable to conduct large-scale measurements along the coast. In the case of ADCPs, a casing made from titanium alloy would be an excellent choice. This is because titanium alloy has very good corrosion resistance, which is essential in the salty marine environment. It is also strong and relatively light, fitting the requirements for high-quality current-measurement equipment.

6. How to Select appropriate Equipment for Current Measurement?

While selecting equipment for current measurement at Junagadh, the very first consideration should be for the purpose of usage.

Ship - borne ADCP: It is adequate for measurement from a moving vessel. It will record the coastal currents continuously while the ship moves along the coast and therefore it will provide a continuum of data points over a wide area covered by the track of the ship.

Bottom-mounted ADCP: It is used 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: This type is used when the aim is to measure currents in an area over some time, floating on the water surface. It can be deployed in areas of interest and provide continuous data without the need for a vessel to be present all the time.

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

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.