How can we measure the coastal currents of Mannar?

1. Where is Mannar?

Mannar is a town in the Mannar District of Sri Lanka. It is a place of immense geographical and cultural importance. Geographically, it lies on a strategic location near the Mannar Strait, separating the island of Sri Lanka from the Indian mainland.

The landscape around the town consists of sandy beaches, lagoons, and extensive coastal plains. The local ecosystem is a rich tapestry of diverse flora and fauna. The area is known for its unique mangrove forests lining the shores, playing a vital role in coastal protection and as a habitat for numerous species.

The people of Mannar are very culturally active, and the place is a crossroads of many trade and migratory routes. The local fishing communities have an abiding affiliation with the sea, drawing their livelihoods from the rich resources of the strait and surrounding waters. It is a city with the historical remains of ancient civilizations and trade ports, attesting to its importance throughout history.

The waters off Mannar are part of the complicated interplay of the Mannar Strait and the Palk Bay. The strait, though relatively shallow at a number of areas, has a very characteristic hydrographic feature brought about by tidal current actions, freshwater river discharges from its vicinities, and with partial exchange of waters with the open sea.

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

These currents along the coast of Mannar result from a combination of many interacting factors. Tidal currents are one of the dominant forces. The area has semi-diurnal tides, which means two high tides and two low tides occur in a day. These tidal currents may be pretty strong and hence play an important role in the movement of water in and out of the strait and the lagoons.

The monsoon winds also have a great effect. The southwest monsoon and the northeast monsoon bring different wind - driven forces. During the southwest monsoon, the winds tend to push the surface waters in a northeasterly direction. In contrast, the northeast monsoon can reverse this flow. The topography of the seabed near the coast, including the presence of shoals, channels, and submerged reefs, can cause the currents to change speed and direction.

Inflow of fresh water from rivers and the exchange of water with the open sea through the Mannar Strait result in complex current patterns. Salinity gradients due to the mixing of fresh and saltwater may further influence the density-driven currents, adding another layer of complexity to the coastal current regime.

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

Surface Drifter Buoy Method

Surface drifter buoys are an effective way of observing the surface water flow. The buoys, when deployed on the water surface, are taken by the currents. They can then be tracked by satellite-based systems or other positioning methods to know their movement over time and, correspondingly, the direction and speed of the surface currents. However, this method gives information about the surface layer only, and the current structure of the whole water column is not represented.

Moored Ship Method

The moored ship method measures the water flow at various depths. The ship is moored at a location some distance from the coast and records the water velocity with instruments onboard the ship. This method can measure continuously at a fixed point. However, it has limitations: the ship's self-motion due to waves and wind could introduce errors into the measurements; and it only provides data around the area where the ship is moored and might not be representative of the general coastal current situation.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP profiler method is more modern and holistic in the measurement of coastal currents. ADCPs emit acoustic pulses into the water and analyze the Doppler-shifted echoes that return. By this process, they can measure the velocity of water at multiple depths simultaneously. This provides a rather detailed profile of the current from the surface down to a certain depth, depending on the instrument's capabilities. It gives a fuller view of the present behavior of the water column, which is an ideal method to study the coastal currents near Mannar.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They transmit high-frequency acoustic signals into the water. When these signals interact with moving particles in the water, such as suspended sediments or small organisms, the frequency of the reflected signals changes due to the Doppler effect.

If the particles are moving towards the ADCP meter, then the frequency of the reflected signal is higher than the emitted one; if they are moving away, it is lower. The ADCP measures these frequency shifts at different angles and depths and can hence calculate the velocity of the water at various points in the water column. This allows it to create a detailed current profile that shows the speed and direction of the water movement at different depths. Data from ADCPs can be used to analyze the spatial and temporal variability of the coastal currents.

5. What's needed for high-quality measurement of Mannar coastal currents?

The equipment should be manufactured with appropriate, reliable material for high-quality measurement in the coastal currents near Mannar. Because seawater contains corrosive material and a marine environment is strong, the device has to be designed for both conditions. It is desirable to have an instrument that is compact in size, lightweight, and can work on low power consumptions.

A small and light device is easier to deploy and retrieve, whether it's from a boat or a coastal installation. Low power consumption allows for longer - term monitoring without the need for frequent battery replacements. Cost - effectiveness is also a crucial factor, especially for large - scale measurement projects.

The casing of the ADCP flow meter is preferably to be made of titanium alloy. Titanium alloy has excellent corrosion-resistant properties, making it well-suited for use in the marine environment. It also has a good strength-to-weight ratio, which means it can provide the necessary structural integrity while keeping the device relatively light.

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

By Usage

Ship-borne ADCP: An ADCP current profiler would be considered ship - borne if the measurement is required to be taken while the vessel is on the move. This is relevant for cruises where data on sea currents, particularly those flowing parallel, are available continuously as a vessel follows the coast while mapping flow spatial variability.

Bottom-mounted ADCP: For long - term and fixed - point measurements near the seabed, a bottom-mounted ADCP is a good choice. It can provide detailed current profiles at a specific location, which is useful for studying the local current behavior.

Buoy-mounted ADCP: Where flexibility in deployment and measurement of surface and upper- water-column currents is needed, a buoy - mounted ADCP current meter should be employed.

Based on Frequency

The choice of frequency depends on the water depth. For relatively shallow waters up to 70m, a 600kHz ADCP is usually appropriate. For water depths between 70m and 110m, a 300kHz ADCP is a better fit. And for deeper waters, such as those that might be found near Mannar in some areas, a 75kHz ADCP is suitable for measuring up to around 1000m depth.

There are well - known ADCP brands like Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option with excellent quality, the China Sonar PandaADCP is a good alternative. It is made of titanium alloy and offers a great price - performance ratio. You can find more information at https://china-sonar.com/.

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