How to measure the coastal currents of Bhavnagar?

1. Where is Bhavnagar?

Bhavnagar is a city and a district in the state of Gujarat, India, and it is situated on the western coast of the country. It is a city that beautifully combines history, culture, and maritime activities.

It is a city famous for its palaces, among them the Nilambagh Palace, standing as a testimony to its rich historical heritage. The cultural ambiance is vibrant, with festivals and traditions being celebrated with great enthusiasm. The local people are engaged in different occupations, the majority of them being connected with fishing and other maritime trades.

The Bhavnagar coast is a unique ecosystem, with the Gulf of Khambhat bordering it. This gulf is a partially enclosed water body that has both tidal and riverine inputs. The coastline has sandy beaches, patches of mangrove forests in some areas, and an operational port. It serves as a barrier that saves the coast from erosion, besides being a home to various types of flora and fauna. The port here has emerged as an essential base for trade and transport-related activities, handling consignments for both local and foreign destinations.

2. What about the prevailing coastal currents off Bhavnagar?

The coastal currents off Bhavnagar are influenced by more than one factor. Tidal currents are one of the predominant forces. Because of the semi-enclosed nature of the Gulf of Khambhat, the tides can cause considerable water movement. The gravitational pull of both the moon and the sun results in a very intricate pattern of ebb and flow. The tidal range in the gulf can be quite large, which in turn results in strong tidal currents that change direction regularly.

The monsoon winds also have a major impact. The southwest monsoon from June to September has strong winds which can influence the surface currents. Such winds can drive the water masses and modify the normal current patterns. Topography: The shape of the gulf and the surrounding coastline also play their role. The narrowing and widening of the gulf, with the presence of headlands and bays, can redirect and accelerate the currents.

Besides, the fresh water river inflow and its mixing with the saline sea would influence the density-driven current. The internal wave formations may be developed due to the mixing of different water masses with different temperatures and salinity, which in turn would affect the overall structure of the current.

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

Surface Drift Buoy Method: The method involves the deployment of surface-floating buoys. They are allowed to drift in the currents and their position is tracked by satellite-based systems or other positioning techniques. By observing the track and speed of the buoy, we can obtain information about the direction and speed of the surface currents. However, this method is limited to the surface layer and may not provide a complete understanding of the currents at different depths.

Anchor Boat Method: In this, a boat is made to stay in the coastal waters by anchoring. Necessary instruments like current meters can be used to measure water flow around it at diverse depths. While it has greater detail in data outcome, especially in the region of the anchored boat, it is relatively labor and time-consuming. Furthermore, this particular method covers a limited area of the place near an anchored boat.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP flow meter method is rather advanced and more efficient. It involves the use of sound waves to measure the velocity of water at several depths simultaneously. It emits pulses of sound and analyzes the Doppler-shifted reflections from particles in the water to give a rather detailed profile of the current from the surface to a certain depth. This gives a fuller and more accurate representation of the coastal currents than the other means.

4. How do ADCPs that utilize the principle of the Doppler effect work?

ADCPs work by using the Doppler principle. They send out pulses of sound into the water. These sound waves scatter from particles in the water, including sediment grains, plankton, or other small life forms. Due to the movement of these particles, the frequency of the reflected sound waves changes. Particles moving towards the ADCP profiler will have a higher frequency of reflected waves than the emitted waves, and those moving away will have a lower frequency.

These frequency shifts are measured by the ADCP meter at various angles and depths. The instrument calculates the velocity of the water in various directions using advanced signal-processing algorithms. Several transducers on the ADCP send and receive the sound signals. This allows the ADCP current meter to create a detailed profile of the water velocity throughout the water column from the surface down to a specific depth, depending on the instrument's capabilities.

5. What is required to carry out high-quality measurement of Bhavnagar coastal currents?

The equipment required in conducting high-quality measurements of coastal currents around Bhavnagar needs to have appropriate materials. The marine environment includes saltwater corrosion, which may cause changes in temperature and even the effect of physical impacts by floating debris; hence, it calls for durability.

The size of the equipment should be small for easy deployment and installation. Whether placed on boats, buoys, or at the seabed, a small size allows for easy handling and installation. It also makes it easier to transport and maintain.

The weight should also be light, especially for floating or moored devices, to ensure that the buoyancy and stability are appropriate. The power consumption should be low to allow for long continuous operation with infrequent battery replacement and a small power supply.

It must be cost-effective. Measuring along coastlines calls for affordable equipment. To suit ADCPs for practical measurements, a casing of a titanium alloy can be ideal. The reasons behind it are that a casing of titanium alloy would resist the corrosive attack from salinity in sea water exceptionally well. It would be also pretty strong and fairly lightweight at the same time. Such properties are ideal to set higher benchmarks in current-measurement tools.

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

When choosing equipment for current measurement near Bhavnagar, the first consideration is the usage purpose.

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: These are used for fixed-point measurements from the seabed. The result is a long record of the stable, near-bottom water flow that has many applications where there is a need for detailed interaction between the topography and the coastal bottom currents.

Buoy-mounted ADCP: Used when the aim is to measure the currents in a specific area over an extended 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 water depths within 70m, a 600kHz ADCP will suffice for general applications based on frequency. In general, for depths up to 110m, a 300kHz ADCP is more appropriate, while deeper waters up to 1000m are best served by 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.