How can we measure the coastal currents of Mutur?

1. Where is Mutur?

Mutur is a small town in the Trincomalee District of Sri Lanka. It lies on the northeastern coast of the island, facing the Indian Ocean. The surrounding landscape combines the beauty of the sea with the charm of the local vegetation.

The coastal area of Mutur boasts sandy beaches with crystal clear waters, which are home to a variety of marine life. The area is rich in coconut palms and other tropical plants that add to its natural beauty. The local culture is deeply linked with the sea since fishing is a major part of the livelihood of the people in this community.

Geographically, Mutur lies just by the Trincomalee Bay and thus the consequences of water in-and-out shift within the bay. By its connection with the open ocean, the waters within the coastal environment at Mutur are influenced by large-scale oceanic circulation patterns. Added to this is the occurrence of estuaries and small inlets along this segment of the coast and adds to the complications of coastal water flow.

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

The coastal currents near Mutur are influenced by a number of factors. Monsoon winds have a profound impact. During the southwest monsoon, which extends from May to September, the winds blow from the southwest to the northeast, driving the surface currents in a northeasterly direction. In contrast, during the northeast monsoon, extending from December to February, the winds push the water in a southwesterly direction.

The topography of the bottom topography offshore also alters the current patterns. Submarine features like sandbars, reefs, and channels will make the currents accelerate, decelerate, or change direction. Another important factor involves tidal currents. The area has semi-diurnal tides, which means two high tides and two low tides occur each day. This ebb and flow of the tides interact with the other currents to provide a complex, dynamic current regime.

The coastal currents also get influenced by the interchange of water between the bay and the open ocean and through the estuaries and inlets. These various movements of water can further cause eddies and local current patterns unlike that of the general ocean currents.

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

Surface Drifter Buoy Method

Surface drifter buoys are a practical way to observe the surface water flow. These buoys are put on the surface of the water and are moved by the currents. By applying satellite-based tracking systems or other positioning methods, it is possible to monitor the movement of the buoys over time. In this way, information is obtained on the direction and speed of the surface currents. However, this provides only surface layer data and does not provide comprehensive information about the current structure of the entire water column.

Moored Ship Method

This technique involves anchoring the ship in a position to take measurements at a spot very close to the shoreline. The instruments on board make the measurement of the water flow at various depths. This enables the continuative measurement at a fixed measurement point. But it also has some disadvantages. Movements of the ship itself, caused by waves and wind, may provide measurement errors. It also gives information for only the area around the moored ship and does not represent the general coastal current situation.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP method is a more advanced and comprehensive way of measuring coastal currents. ADCPs send an acoustic pulse into the water and analyze the Doppler shifted echoes that return. It does this to simultaneously provide measurements of water velocity through the water column at selected depths. This results in a profile of the current from the surface to somewhere near the bottom, at intervals dependent upon the depth rating of the instrument itself. It provides an overall better insight into what the present status of the water column is, besides being a pretty good alternative to investigate what is going on with coastal currents off Mutur.

4. The principle behind Doppler-independent ADCPs?

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 shifts according 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, while if they are moving away, it is lower. By measuring these frequency shifts at different angles and depths, the ADCP calculates the velocity of the water at various points in the water column. This enables it to generate a detailed current profile showing the speed and direction of the water movement at different depths. The data obtained 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 Mutur coastal currents?

The equipment needs to be with reliable materials to conduct high-quality measurement of the coastal currents around Mutur. The instrument should be in a model that could provide resistance from the corrosive nature of seawater and harshness in the marine environment. It will be beneficial to have one in compact size, with light weight, and using low power.

A small and lightweight device is easier to deploy and recover, both from a boat or from a coastal installation. Low power consumption allows longer-term monitoring without frequent replacement of batteries. Cost-effectiveness is also an important factor, especially for large-scale measurement projects.

The casing of the ADCP is preferably made from a titanium alloy. Titanium alloys are very resistant to corrosion, and therefore quite satisfactory for marine applications. Apart from that, it contains enough strength-to-weight proportion that can provide the unit with the required structural soundness without making it unnaturally heavy.

6. How to choose the right equipment for measuring current?

Based on Usage

Ship-borne ADCP: Where the measurements are to be made from a moving vessel, a ship - borne ADCP meter will suffice. It would offer continuous data as the ship travels along the coast, thereby giving an enhanced view of the currents spatial variability.

Bottom-mounted ADCP: It would serve very well for long- and fixed-point measurements near the seabed. The bottom-mounted ADCP flow meter provides detailed current profiles at a specific location, which can be very 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. A 600kHz ADCP will usually be appropriate for waters with a relatively shallow depth up to 70m. In waters with a depth range between 70 and 110m, an ADCP with a 300kHz is more fitted. In deeper waters, like would be expected around Mutur in some places, the 75kHz ADCP would do for up to around 1000m.

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.