How to measure Puttalam coastal currents?

1. Where is Puttalam?

Puttalam is a district in the west of Sri Lanka. It is one of those districts in the country that represents various landscapes and ecosystems. The area borders with the Indian Ocean, where its coastline is one of its major geographical features.

Puttalam coasts host picturesque sandy beaches stretching along for miles in distance. These beaches attract many tourists and are an important part of the local fishing industry. This district is further made up of wide areas of lagoons like the Puttalam Lagoon, one of the largest in the country. In the lagoon itself lies an enormous variety of flora and fauna in an elaborately developed ecosystem. It serves as a breeding ground for many fish species and is a haven for migratory birds.

The cultural heritage of Puttalam is immensely rich. Locals have a long-standing relationship with the sea and lagoon. Fishing and the production of salt are two such traditional occupations that are practiced generation after generation. Inland areas in the district consist of agricultural lands with small villages. The climate is typically tropical, with distinct wet and dry seasons that influence the local economy and way of life.

The waters off Puttalam are part of the larger circulation system of the Indian Ocean. Local conditions are given to be influenced by changes in the Southwest Monsoon and the Northeast Monsoon, where wind and sea conditions take on variations that are quite characteristic. Equally important, its proximity to the lagoon carries an added implication of continuity in water exchange between itself and the ocean, creating a characteristic coastal environment.

2. What is the condition of the coastal currents near Puttalam?

Various factors influence the coastal currents near Puttalam. The most significant influence is from the monsoon winds. During the Southwest Monsoon-from May to September-the strong winds blow from the southwest to the northeast, which in turn moves the surface currents in a northeasterly direction. In contrast, during the Northeast Monsoon-from December to February-the winds reverse to blow from the northeast to the southwest, and the currents tend to flow in a southwesterly direction.

Topography of the seafloor off the coast further influences the pattern of currents. Underwater ridges, troughs, and sandbars are responsible for accelerating, decelerating, and even changing the direction of currents. Another major contributor to these is tidal currents. Semi - diurnal tides with two high and two low tides in a day prevail in this area, resulting in a marked variation in magnitudes and directions of coastal currents.

The exchange of water between the ocean and the Puttalam Lagoon also affects the coastal currents. The lagoon acts as a buffer, and the inflow and outflow of water through narrow channels can create local current patterns that are different from the larger - scale ocean currents.

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

Surface Drifter Buoy Method

Surface drifter buoys are simple, effective ways to observe the surface water flow. The buoys are set in the water surface and carried away by the currents. The buoy movements can be tracked using satellite-based tracking systems or other positioning methods. By this, one gets an idea about the direction and speed of the surface currents. This technique will only deliver information about the surface waters and might not be useful to study the whole water column's contemporary configuration.

Moored Ship Method

The mooring ship technique keeps a ship anchored at one location from the shore. The instrument on board measures the flow in the water at various vertical levels. It facilitates continuous recording at one fixed point. But it has its drawbacks. The ship's motion itself due to waves and wind can lead to error in measurement. Also, the data it provides relates to the surrounding area of the moored ship and is not representative of the whole coastal current situation.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP method is the more sophisticated and comprehensive way to measure coastal currents. ADCPs emit an acoustic pulse into the water and measure the Doppler-shifted echoes returning from the water. In this way, they can concurrently measure the water velocity at several depths. This gives a very detailed profile of the current from the surface down to a certain depth, depending on the instrument's capabilities. It provides a more complete view of the current behavior of the water column and is a very good choice for studies on coastal currents near Puttalam.

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

ADCPs operate based on the Doppler principle. They emit 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 profiler, then the frequency of the reflected signal is higher than the emitted one; if they are moving away, it is lower. The ADCP calculates the velocity of the water at different points in the water column by measuring these frequency shifts at various angles and depths. It thus enables the creation of a current profile in great detail, showing the speed and direction of the water movement at various depths. The data from ADCPs can be analyzed to find out the spatial and temporal variability of the coastal currents.

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

In view of the high-quality measurement at Puttalam, equipment must be made of reliable materials. Due to seawater being corrosive in nature and the marine environment being harsh, it needs to be able to bear these conditions. A compact size, light weight, and low power consumption are also desirable.

A small and lightweight device is more easily deployed and retrieved both from a boat and from a coastal installation. Low power consumption makes it possible to perform long-term monitoring without frequent exchanges of batteries. Cost-effectiveness is another important point, especially in the case of projects with large numbers of measurements.

The casing of the ADCP is preferably made from Titanium alloy. The selection of titanium alloy is ideal as it has very exceptional resistance to corrosion, making its deployment in the marine environment more useful. It also has quite an efficient strength-to-weight ratio; hence, it allows enough structural integrity with rather minimum weight for the apparatus.

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

Based on Usage

Ship-borne ADCP: This type of ADCP meter would be ideal for making the measurements from a moving vessel. In this case, the ADCP can give continuous data while the ship is traveling along the coast to capture the spatial variability of the currents.

Bottom-mounted ADCP: Used in most long-term or fixed-point measurements near the seabed; therefore, a bottom-mounted ADCP flow meter would be applied. Current profiles provided to this aspect can be detailed on spot and very useful in analyzing local characteristics of the currents.

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 will depend on water depth: for relatively shallow waters, a 600 kHz ADCP can normally be used; waters between 70 and 110 meters deep are best fitted by a 300 kHz ADCP; deeper waters-like those that could be obtained in some spots around Puttalam-would allow the 75 kHz to measure a depth of up to approximately 1000 m.

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.