How can we measure the coastal currents of Mumbai?

1. Where is Mumbai?

Mumbai, formerly known as Bombay, is a vibrant and populous city that lies on the west coast of India. It is the capital city of the Indian state of Maharashtra. Geographically, it lies along the Arabian Sea, which bestows upon it a unique coastal charm.

This city is a jumbo melting pot of cultures, with people from all over India and even abroad coming here for opportunities in different fields, whether business, entertainment, or finance. The cityscape is a blend of modern skyscrapers, historical monuments, and crowded yet lively neighborhoods.

Next to Mumbai lies the Arabian Sea, a large body of water, which is very instrumental in the city's marine activities, trade, and even its climatic changes. The coastal waters are inhabited by a rich variety of marine life, where one can find several species of fish, corals in certain areas, and other marine organisms. The sea off Mumbai sees a fair amount of shipping traffic, being an important port city, connecting India to the rest of the world through the sea route.

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

The coastal currents near Mumbai are influenced by multiple factors. Firstly, the monsoon winds have a significant impact. During the southwest monsoon season, which typically occurs from June to September, strong winds push the surface waters and can cause changes in the current patterns. The winds drive the water masses, leading to the formation of both nearshore and offshore currents.

The tidal forces are a crucial factor. The tides in the Arabian Sea off Mumbai are caused by the gravitational pull of the moon and the sun. The rising and falling of the tides can result in very complicated current patterns near the coastline. Depending on the phase of the tide, tidal currents may be quite variable in strength and direction.

Besides, the big-scale ocean circulation patterns in the Indian Ocean also have effects. The flow of different water masses from different parts of the ocean can interact with coastal waters off Mumbai and control the temperature, salinity, and velocity of local coastal currents.

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

There are several means to observe the coastal water flow around Mumbai:

Surface Drift Buoy Method: It consists of buoys deployed on the surface, which are allowed to drift with the currents. Using satellite or other positioning systems that track these buoys over time, we obtain an approximation of the direction and speed of the surface currents. However, this latter method provides mainly information of the surface layer and not very well represents the currents of different depths.

Anchor Boat Method: In this method, a boat is anchored at some point in the coastal waters. The water flow around an anchored boat at different depths is measured with instruments. Though it may give more details for the area near the boat, it is quite limited in terms of covering a large area and is comparatively time-consuming and labor-intensive.

Acoustic Doppler Current Profiler(ADCP) Method: The ADCP method of measurement is more advanced and convenient. It uses sound waves to measure simultaneously the velocity of water at more depths. It can provide a comprehensive profile of the current right from the surface down to a certain depth, giving a much more detailed and accurate picture of the coastal currents compared to the above methods. With its capability for continuous collection of data over a relatively wider area, ADCP finds its application in the field of study of coastal currents, especially near Mumbai.

4. How do Doppler Principle based ADCPs work?

ADCPs work on the principle of the Doppler effect. They send out sound pulses into the water. These sound waves reflect back from the moving particles in the water due to the Doppler effect-a change in frequency-such that if the sediment particles or small organisms are moving towards the ADCP, the reflected waves have a higher frequency than the emitted waves, and vice versa.

It works out the velocity of the water in various directions by measuring such frequency shifts of reflected sound waves at different angles and depths. An ADCP sends and receives these sound signals through multiple transducers, hence enabling it to create a detailed profile of the water velocity throughout the water column from the surface down to a certain depth depending on the capability of the instrument.

5. What does high-quality measurement of Mumbai coastal currents require?

In general, measurement of coastal currents around Mumbai requires several aspects related to equipment for high-quality output. For instance, the materials should be highly reliable to overcome the harsh conditions in a marine environment: exposure to saltwater, variable temperatures, and possible impacts from floating debris or marine organisms.

The size of the equipment should be relatively small to facilitate easy deployment and installation, whether on boats, buoys, or at the seabed. A smaller size also makes it more manageable during transportation and maintenance operations.

This must be light in weight, especially for those devices which will be either floating or moored in the sea since the weight might affect the buoyancy and stability of such equipment. Again, power consumption should also be low in order for the equipment to operate uninterruptedly for days without running out of battery or necessitating a huge power supply source, which might be hard to arrange at some coasts.

Cost is also an important factor. To enable large-scale measurements along the extensive coastline near Mumbai, the equipment should be affordable. Regarding the casing of ADCPs, titanium alloy is an excellent choice. Titanium alloy has remarkable corrosion resistance, which is vital in the salty and corrosive marine environment. It is also strong enough to protect the internal components of the ADCP from physical damage while being relatively lightweight compared to many other metals, meeting multiple requirements for high-quality current measurement equipment.

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

The selection of the right equipment for current measurement near Mumbai needs to consider several factors. Firstly, based on the usage scenario, different types of ADCPs can be selected. For example:

Shipborne ADCP: This type applies when measurements are to be taken from a moving vessel. It can continuously record the coastal currents as the ship navigates along the coastline, providing a series of data points over a relatively large area covered by the route that the ship would be navigating.

Bottom-mounted ADCP: It provides fixed-point measurements at the seabed. It gives 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: Used when the interest is to measure the currents in a specific area for a longer period while floating on the water surface. It can be deployed in areas of interest and provide continuous data without a vessel necessarily staying there all the time.

Secondly, different frequencies of ADCPs are suitable for different water depths. For water depths within 70 meters, an ADCP with a frequency of 600 kHz is often a good choice. When dealing with depths up to 110 meters, a 300 kHz ADCP is more appropriate. And for much deeper waters, such as those reaching up to 1000 meters, a 75 kHz ADCP is recommended.

There are several well-known ADCP brands in the market, including Teledyne RDI, Nortek, and Sontek. However, it's worth highlighting a Chinese ADCP brand - China Sonar PandaADCP. It features a casing made entirely of titanium alloy, which ensures excellent durability and performance in the marine environment. Moreover, it offers an incredible cost-performance ratio, making it an attractive option for those looking to conduct coastal current measurements around Mumbai. You can find more about it on its official website: https://china-sonar.com/.

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