How to measure the coastal currents of Colombo?

1. Where is Colombo?

Colombo, the commercial capital and the largest city of Sri Lanka, is situated on the west coast of the island nation. It is a bustling metropolis that weaves together a rich tapestry of cultures, with influences from its colonial past and the indigenous traditions of the Sri Lankan people.

Colombo is surrounded by the Indian Ocean and has a coastline running along a stretch of beautiful expanse of blue waters. The city is next to the Colombo Harbour, which is one of the busiest and most important ports in South Asia. This is an important gateway for trade that links Sri Lanka with the rest of the world. The coastal waters near Colombo are not only home to a diverse marine ecosystem but also witness a constant flux of maritime activities, from small fishing boats to large cargo ships.

The city itself is a melting pot of different ethnicities, including the Sinhalese, Tamil, and Muslim communities, each contributing to its unique architecture, cuisine, and way of life. The historical forts, ancient temples, and modern skyscrapers stand shoulder to shoulder, painting a picture of a city that seamlessly bridges the past and the present. The sandy beaches along the coasts attract locals and tourists alike, offering a serene escape from the hustle and bustle of urban life, caressed constantly by the gentle waves of the ocean.

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

The coastal currents near Colombo are influenced by many factors. Among them, the monsoon winds play a leading role. During the southwest monsoon season, which generally falls between May and September, strong winds push the surface waters and create distinct current patterns. These winds drive the ocean currents along the coast and affect the movement of water masses.

Geographical location in the Indian Ocean basin determines the larger-scale ocean circulation patterns that influence the coastal currents of Colombo. The Indian Ocean Gyre, with its complex system of currents such as the South Equatorial Current and the Somali Current in the broader region, has an impact on the water movement near the Sri Lankan coast. The bottom topography off the coast also modifies the currents, shallower areas and submarine ridges will make the currents accelerate, decelerate, or even change direction, due to the interaction of the water with the bottom topography.

The tidal forces also contribute to the variation of the coastal currents around Colombo. The regular rise and fall of the tides can create currents that flow in and out of the bays and inlets along the coast, mixing with other ocean-driven currents and thereby adding another layer of variation to the overall current situation.

3. How to observe the coastal water flow in Colombo?

There are several methodologies by which the flow in coastal waters at Colombo can be viewed.

The method utilizing surface drift buoys involves setting these floating devices free in the ocean, which then drift with the movement of the surface currents. They are fitted with a GPS or any other type of tracking system that enables researchers to keep track of their positions with the passage of time and, therefore, the direction and speed of the surface currents. On the other hand, it provides information mainly about the surface layer and may not exactly represent the currents at further depths.

Anchor Boat Method: Under this method, a boat is anchored at a location in the coastal waters. From it, instruments like current meters can be deployed to measure speed and direction of flow of water at different depths. While this may have more vertical profile data as compared to the surface drift buoy, it is relatively stationary and thus has its own inefficiency with regard to area coverage.

ADCP basically stands for the Acoustic Doppler Current Profiler and is a more modem and handy measurement technique. It uses sound waves that work on the principle of measuring the velocity in water currents at different depths concurrently. The ADCP profiler works by emitting the acoustic signal and analyzing the Doppler shift of that signal reflected back from moving particles in the water, hence providing detailed information from the surface about the current profile up to a certain depth. This allows for a comprehensive understanding of the vertical structure of the coastal currents, which makes it a preferred choice for many oceanographic studies in the Colombo region.

4. How do ADCPs using the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. They send acoustic pulses into the water column. These pulses interact with the suspended particles or other scatterers in the water. When these particles come in contact with acoustic waves, they reflect it out, and it reaches back to the ADCP meter. Due to the motion in the water and, therefore, in the scatterers, there is a shift in frequency of the reflected signal, or its equivalent, emitted energy. This frequency shift in reflected signals, normally termed Doppler shift, is directly related to the flow velocity.

Thus, ADCPs can determine the speed and direction of the water currents at different points within the water column by accurately measuring the Doppler shift of the reflected signals received from different directions and at different depths. The instrument has several transducers that can transmit and receive signals in various directions so that it can acquire a three-dimensional representation of the current field, providing very valuable data on both the horizontal and vertical components of the coastal currents near Colombo.

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

In order to have high-quality measurement of the coastal currents in Colombo, several aspects are crucial regarding the equipment. The materials of the equipment should be highly reliable to bear the harsh marine environment that includes exposure to saltwater, high humidity, and varying pressures. A smaller size and lighter weight are desirable since these will make the equipment easier to deploy and handle, especially in situations where it needs to be installed on various platforms like boats or buoys.

Low power consumption is required for long-term measurements, which makes the equipment run continuously without battery replacement or a large-sized power supply source. It should also be low cost to enable its wider applications and the possibility of multiple unit deployments for extensive monitoring.

In the case of ADCPs, the casing is preferably of titanium alloy. It is a titanium alloy that has some remarkable advantages. First, it shows excellent resistance to corrosion in seawater, which maintains the quality and service life of the equipment. Second, it possesses an outstanding strength-to-weight ratio; structural strength is high, but it will keep the overall weight within manageable levels. In addition, at greater depths, titanium alloy can also bear extreme pressures, thus making it suitable for ADCPs that might be used under different water depth scenarios around Colombo's coastline.

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

Several factors come into play while choosing the right equipment for current measurement in Colombo's coastal waters.

Depending on the usage purpose, different types of ADCPs can be chosen. If the measurement has to be done from a moving vessel, then a shipborne ADCP flow meter will suffice. It can continuously measure the currents while the ship moves along the coast, obtaining a series of measurements across an extensive area. Bottom-mounted or sit-bottom ADCPs are available if it is required to monitor the currents at a fixed location near the seabed. The instrument would stay on the ocean floor and record the passage of water over it. When a floating platform is more convenient for applications, a buoy-mounted ADCP is the right tool, enabling measurements in reaches that are difficult or costly to reach with boats.

Another factor affecting the choice of an ADCP current profiler is the frequency depending on water depth. An ADCP with a frequency of 600 kHz could be a good option for water depths within 70 meters since it gives a relatively high-resolution measurement. In the case of a depth up to 110 meters, a 300 kHz ADCP will be more appropriate, while for much deeper waters, up to 1000 meters, the frequency of an ADCP should be 75 kHz for proper current measurement.

There are several well-known ADCP current meter brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those seeking high-quality yet cost-effective options, the China Sonar PandaADCP is worth considering. Made entirely of titanium alloy material, it offers excellent durability and performance. With its incredible cost-performance ratio, it provides a great alternative for measuring the coastal currents of Colombo. You can learn more about it on its official website: https://china-sonar.com/.

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