How do we go about measuring the coastal currents of Vapi?

1. Where is Vapi?

Vapi is a town in the state of Gujarat in India. It borders on the Arabian Sea. Vapi is mainly an industrious city and is one of the biggest economic hubs in the vicinity.

It has a heterogeneous population indulged in various industries of manufacturing, chemical industries, and trade. It is a culturally cosmopolitan place with varied traditions. Vapi also has a varied agricultural landscape around it, with a climate that is influenced by the vicinity of the sea.

The Arabian Sea nearby also plays an essential role in the local ecology. It houses the population of marine life like fishes, crabs, and other shellfish in its water. Transportation ships and boats use the sea, transporting goods to and from the ports in the nearby area. The shoreline adjacent to Vapi consists of sandy beaches and tidal zones. Tidal action is prominent; ebb and flow of tides inshore affect the coastal environment, including sediment movements.

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

Various factors exert their influence on the coastal currents off Vapi. The tidal influence is considerable. Gravitational attraction of both the moon and the sun results in tides, and the tidal currents accompanying it may be very strong. Even the general shape of the coastline and underwater topography exert a modification. The coastline may have headlands, bays, and shoals that deflect and alter the current's course of flow.

These are the monsoon winds, especially during the southwest monsoon season from June to September, which may push the surface waters and, therefore, the direction and speed of the coastal currents. Further, the inflow of freshwater from the rivers nearby and the mixture with the seawater can have its effect on the density-driven currents. As a result, there is an interaction among the different water masses, hence forming complex current patterns.

Large-scale ocean circulation in the Arabian Sea may interact with the local coastal currents. Inflow of water masses from other regions may change the temperature, salinity, and other characteristics of the coastal currents.

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

Surface Drift Buoy Method: This method consists of deploying surface-floating buoys. These buoys can be allowed to drift with the currents, while satellite-based systems or other positioning methods can track their movements. We get information on the path and speed of these buoys, which again lets us know about the direction and speed of the surface currents. This method, however, applies to only the surface layer since, by this method, the information on the currents at several depth levels would be quite restricted.

Anchor Boat Method: In this, a boat is taken into the coastal waters and anchored. Current meters or similar instruments are used to measure the flow of water at several depths around the boat. This can provide more detailed data around the vicinity of the boat but is time-consuming and quite laborious. Another drawback of this method is that it covers only areas in the vicinity of where the boat has been anchored.

Acoustic Doppler Current Profiler Method: By contrast, ADCP current meter is an advanced and efficient method using sound waves to simultaneously measure the velocity of water at many different depths. It produces a detailed profile of the current from the surface to some depth by transmitting pulses of sound and analyzing the Doppler-shifted reflections that return from particles in the water. This provides a fuller and more accurate picture of the coastal currents than the other methods.

4. How do ADCPs using the principle of the Doppler work?

The ADCPs work on the principle of the Doppler effect. They send out sound pulses into the water. The sound waves interact with the particles in the water such as sediment grains, plankton, or other small organisms. Due to the movement of these particles, the frequency of the reflected sound waves is altered. If the particles are moving towards the ADCP profiler, the frequency of the reflected waves is higher than the emitted waves, and if they are moving away, the frequency is lower.

The ADCP measures frequency shifts at different angles and depths using these to calculate the velocity in various directions by means of advanced signal-processing algorithms. A number of transducers installed on the ADCP meter transmit and receive the sound signals. This gives the ADCP current profiler the capability for a very high-resolution profile of the water velocity through the water column-from the surface down to a certain depth, a function of instrument capability.

5. What is necessary to provide high-quality measurement of the Vapi coastal currents?

High-quality measurement of the coastal currents around Vapi will be a job that requires equipment with very reliable materials, because of the very aggressive atmosphere-the saltwater corrosion, the changing temperatures, and maybe the physical impacts from floating pieces.

The size of the equipment should be small for easy deployment and installation. Smaller in size, hence easy to handle and install at sea, whether on boats, buoys, or at the seabed, it will be more convenient to transport and even maintain.

It needs to be light in particular for floating or moored devices, so that it allows buoyancy and stability. The resulting low power consumption enables its operation for a very long period, thus eliminating quick changes of its battery or using a supply with big dimensions.

Cost-effectiveness is an important factor. For large-scale measurements along the coast, the equipment should not be too expensive. For ADCPs, a casing made of titanium alloy will be an excellent choice. The titanium alloy has excellent corrosion resistance, which is very important in a salty marine environment. It is also strong and relatively light, meeting the requirements for high-quality current-measurement equipment.

6. What are the selection criteria for Current-measuring equipment?

The first point to be considered while selecting any equipment for current measurement around Vapi is the Purpose of usage.

Ship-borne ADCP: To measure continuously from a moving vessel, the ship may bear a course along the coast to yield one continuous record of the coastal currents comprising a series of scattered points in space over a comparatively vast area traversed by the vessel.

The bottom-mounted ADCP: It was perfect for fixed-point measurement at the seabed; it was stable and offered a record over the longer term with respect to near-bottom water flow, so it could study the interactions between the bottom topography and the coastal currents.

Buoy-mounted ADCP: When the purpose is to measure currents in a particular area over a longer period while 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.

For the frequency, taking up to 70m depth, an ADCP at 600kHz is quite good; while for up to a maximum of 110m depth, it is 300kHz; and for further depths of up to 1000m, it is more fitting to use a 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.