How to measure coastal currents of Mullaitivu?

1. Where is Mullaitivu?

Mullaitivu is a district in the Northern Province of Sri Lanka. It is one of those places that has variation in landscapes and is very appealing. The area contains a beautiful beach running along the coast, bordering the azure waters of the Indian Ocean.

The local population is one of the most vibrant mixes of different ethnic groups, with a rich tapestry of culture. Since fishing forms a great part of the local economy, it has given the people here close affinity with the sea. Inland areas are just full of lush greenery; the palm trees and other tropical vegetation that grow in the area make the climate warm and humid.

Geographically, Mullaitivu directly borders the Indian Ocean. This oceanic condition is influenced mainly by the large-scale circulation of the Indian Ocean itself. Marine life in this coastal water includes colorful fish up to various types of crustacean species. The shorelines become important nesting sites for sea turtles as well. Nearby lagoons and estuaries add to the complexity of the coastal ecosystem. These water bodies interact with the ocean, creating a unique environment where fresh and saltwater mix.

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

In this context, coastal currents near Mullaitivu will be influenced by a host of factors. The major ocean currents in the Indian Ocean are a significant factor. The North Indian Ocean Gyre, consisting of the South Equatorial Current and the Somali Current, exerts an indirect influence on local coastal currents. The seasonal monsoon winds also exert a profound influence.

Strong winds during the southwest monsoon blow from the southwest to northeast direction. These winds push the surface waters and create a current which travels in a northeasterly direction. During northeast monsoon, the wind is opposite, so the currents move in the opposite direction too.

Currents are sometimes altered by the topography of the seafloor underwater around a coastline. Submarine reefs, sandbars, canyons may accelerate, decelerate, or change the direction of a current. Tidal forces are another factor. The semi-diurnal tides in the area result in a regular ebb and flow of the coastal waters, which may further interact with the other current-forming factors to provide a complex and dynamic current regime.

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

Surface Drifter Buoy Method

Surface drifter buoys are a traditional and relatively simple way to observe the coastal water flow. These buoys are released on the surface of the water and thus drift with the currents. In using satellite-based tracking systems or any other positioning methods, their locations can be determined at later times. This will provide the direction and speed of the surface currents. However, it does have its shortcomings, mainly in that it only concerns the surface layer and might not provide an accurate picture of the structure of the current of the whole water column.

Moored Ship Method

In this technique, a ship is moored at a given location along a coastline. The instruments onboard make measurements of the flow of water at various depths around the ship. This allows for continuous monitoring at a fixed point. But there are drawbacks, the ship's own movement due to waves and wind can introduce errors in the measurements. Also, it only provides data for the area immediately around the moored ship and may not represent the overall coastal current situation.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP technique is a far more refined and efficient way of measuring coastal currents. ADCPs send out acoustic pulses into the water and analyze the Doppler-shifted echoes that return. In that way, they can simultaneously measure the velocity of water at several depths. This gives a detailed profile of the current from the surface down to some depth, depending on the instrument's capabilities. This therefore presents a more complete picture of the current behavior of the water column, thereby proving to be an added tool in studying the currents at Mullaitivu coasts.

4. How do ADCPs using the Doppler principle work?

ADCPs work according to the Doppler principle. They send high-frequency acoustic signals into the water. The frequency of the reflected signal is altered by the Doppler effect when these signals hit moving particles in the water, like suspended sediments or small organisms.

If the particles are moving towards the ADCP profiler, then the frequency of the reflected signal is higher than the emitted frequency, while if they are moving away, it is lower. These frequency shifts, measured at different angles and depths, enable the ADCP to calculate the velocity of the water at different points in the water column. This enables it to give a detailed current profile indicating the speed and direction of the water movement at different depths. From the ADCPs, the obtained data can be analyzed in terms of spatial and temporal variability of the coastal currents.

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

Since it should be a high-quality measurement of the coastal currents near Mullaitivu, equipment has to have reliable materials that will enable operation in seawater with very corrosive constituents, along with harsh marine conditions. Moreover, it should have compact size, light weight, and low power consumption.

A small and light device is easier to deploy and retrieve, from either a boat or a coastal installation. Low power consumption allows monitoring over longer periods without frequent exchanges of batteries. Cost is also an important factor, at least when one is considering making measurements on a large scale.

The casing of the ADCP meter is preferably made of a titanium alloy. Titanium alloy has very good corrosion-resistant properties, and it is especially suitable for application in the sea environment. It also possesses a very good strength-to-weight ratio, which means it will provide the necessary structural integrity and at the same time will keep the device relatively light.

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

Based on Usage

Ship-borne ADCP: When the measurements are to be taken from a moving vessel, a ship-borne ADCP flow meter is quite suitable. In this case, it gives continuous data as the ship travels along the coast and hence is helpful in understanding the spatial variability of the currents.

Bottom-mounted ADCP: When long-term or fixed-point measurements near the seabed are to be conducted, a bottom-mounted ADCP should be chosen. This will be able to give detailed current profiles at one point, useful in studying the local current behavior.

Buoy-mounted ADCP: When flexibility in deployment and measurement of surface and upper-water-column currents is required, a buoy-mounted ADCP current profiler is the way to go.

Based on Frequency

The choice of frequency depends on the water depth. For shallow waters up to 70 m, a 600 kHz ADCP is usually sufficient. For water depths lying between 70 m and 110 m, a 300 kHz ADCP will be more fit. And for deeper waters, for example, as might occur in some areas off Mullaitivu, a 75 kHz ADCP is fitted for measuring up to approximately 1000 m depth.

There are well - known ADCP current meter 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.