How to measure the coastal currents of Galle?

1. Where is Galle?

Galle is a city on the southwestern coast of Sri Lanka. It is a place with historical and natural significance. This city is famous for its well-preserved Dutch-era fort, which stands as a testament to its colonial past. The ancient walls and bastions of the fort overlook the ocean, bringing out a striking juxtaposition of history against the mighty sea.

The Galle coastline comprises rocky shores and sandy beaches. Its location makes the city a perfect destination for tourists who want to experience sun-drenched shores and crystal blue waters. The adjacent ocean is the Indian Ocean, and the bay near Galle is a haven for fishing boats and small vessels. Topographically, the sea floor off of the entire area ranges from generally shallower near the coast and gets progressively deeper towards the west. This particular topography further controls coastal currents and, for that matter, life in the sea.

The local community in Galle represents a dynamic and energetic amalgamation of cultures. The impact of Dutch, Portuguese, and British colonial rule is reflected in the architecture and local customs. Fishing is one of the most important economic activities, and the routine of the fishermen forms part of the coastal scenery. A great number of tourists visit the city in order to see the historical places, enjoy the beaches, and taste the local food.

2. What are the conditions of coastal currents in the vicinity of Galle?

The coastal current vicinity to Galle is under different factors. Monsoons' winds have the major controlling aspect. The southwest monsoon has strong winds within this country from May to September; therefore, these kinds of winds can drive a certain percentage of surface currents. This makes it go along the coast in certain directions to influence the general current distributions of the circulation pattern.

This also influences the larger-scale ocean circulation patterns of the Indian Ocean. In addition, there is a contribution to the general movement from the South Equatorial Current and the influence of the Indian Ocean Gyre. Then there are tidal forces to consider. The regular tides cause the water to rise and fall; in this way, there is interaction between tidal currents and other currents driven by the ocean. The shape of the coastline and the bathymetry around Galle may accelerate or decelerate the currents. For instance, the underwater rock outcrops and headlands will definitely change the direction and speed of the currents as the water is deflected around them.

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

Surface Drift Buoy Method: Surface drift buoys are a practical means of observing the surface water flow. These buoys are released into the water and are carried along by the surface currents. Using GPS or other tracking systems, the position of the buoys over time will give information on the direction and speed of the surface currents. However, this method provides information only on the surface layer and may not give a complete picture of the currents at different depths.

Anchor Boat Method: In this, the water flow can be estimated using an anchor boat. The boat is anchored in a particular position, and the current-measuring instruments are dropped at various depths from that boat. By doing so, the speed and direction of the currents in various vertical levels can be recorded. However, this method is fairly stationary and may not cover an enormous area efficiently.

ADCP Method: The ADCP is a more advanced and efficient method of measurement of coastal currents. It makes use of the Doppler effect for measuring the velocity of water at multiple depths. By emitting acoustic signals into the water and analyzing the Doppler shift of the reflected signals from particles in the water, ADCP meter is able to provide a detailed profile of the current from the surface to a certain depth. This provides an in-depth insight into the vertical structure of the coastal currents near Galle.

4. How do the ADCPs employing the Doppler principle work?

ADCPs, employing the principle of the Doppler, basically send out acoustic pulses into the column of water. These pulses interact with suspended particles of water, like plankton or sand grains, or any other type of small debris. In case the water is flowing, the particles move with it. If acoustic pulses are reflected back to an ADCP from such particles, there is a frequency shift in the received signal, which was emitted. The velocity of the water flow directly affects the frequency shift, better known as the Doppler shift.

The ADCP flow meter has several transducers that are capable of sending and receiving signals in different directions. It can calculate the speed and direction of the water currents at various points in the water column by accurately measuring the Doppler shift of the received signals from different angles and depths. This makes it possible to create a three-dimensional representation of the current field, providing important data about both the horizontal and vertical components of the coastal currents.

5. What’s needed for high - quality measurement of Galle coastal currents?

For high-quality measurements in the Galle current, the equipment used must be marked by the following characteristic traits, reliability of material by not being corrosive or destructive from sea water and the violent condition of the marine atmosphere. Small in size, lighter in weight-the benefits associated are that these kinds of instruments are easier to manipulate, handle, and deploy in different manners of platforms such as on a boat, buoy, or others.

The ability of low power consumption is required for long-term measurements; it should be possible to run the equipment without frequent battery replacements or a large-scale power supply. Another important factor is cost-effectiveness, the equipment should not be so expensive that it prevents the use of enough pieces and multiple deployments for adequate monitoring.

In the case of ADCPs, a casing made from titanium alloy is quite appropriate. Titanium alloy is highly resistant to corrosion in seawater and thus guarantees long-term durability of the equipment. It has a high strength-to-weight ratio, meaning it will provide structural integrity while keeping the weight to a minimum. In addition, titanium alloy withstands high pressures, which would be suitable for different water depth scenarios in the Galle coastal area.

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

Several factors need to be considered while choosing the right equipment for current measurement in Galle's coastal waters.

Different types of ADCPs exist depending on the usage purpose. For measurements from a moving vessel, a ship-borne ADCP current profiler is appropriate. It will measure the currents while the vessel is moving along the coast in a continuous stream. If the aim is to monitor the currents at a fixed location near the seabed, a sit-bottom ADCP would be a good choice. It can be placed on the ocean floor and record the water flow passing over it. For applications where a floating platform is more convenient, a buoy-mounted ADCP is the way to go.

Regarding the frequency of ADCPs, it depends on the water depth. For water depths up to 70m, an ADCP with a frequency of 600kHz is recommended as it provides high - resolution measurements. For depths up to 110m, a 300kHz ADCP is more appropriate. And for deeper waters up to 1000m, a 75kHz ADCP is a better choice to ensure accurate and reliable current measurements.

There are well - known ADCP current meter  brands such as Teledyne RDI, Nortek, and Sontek. However, for those seeking a cost - effective alternative, the China Sonar PandaADCP is a great option. It is made of titanium alloy and offers an excellent cost - performance ratio. You can find more information about it on its website: https://china-sonar.com/.

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