How to measure the current is observed in Chattogram Coastal Currents?

1. Where is Chattogram?

Located in Southeastern Bangladesh, Chattogram is a major port city of Bangladesh. It is situated along the Bay of Bengal. The city is very busy economically and one of the leading centers in Bangladesh concerning maritime trade and commerce.

A landscape in Chattogram has combined features: partly urbanized with natural coastal features, a long coastline with sandy beaches, tidal flats, and estuaries. The city is well-equipped with infrastructures to manage the high volume of shipping traffic. The local population is very diverse, ranging from port-related jobs, fishing, and various industries.

The Bay of Bengal, which extends westward off the cost of Chattogram, is a much larger, more dynamic body of water. It is a tidal current-heavy body and influenced by the monsoon climate. A wide variety of marine life can be found in these seas. The mangrove forests along the coast, such as the Sundarbans, partly situated in the vicinity, help protect the shorelines from erosion and create a habitat for numerous species, including the famous Royal Bengal Tiger.

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

A few factors control the coastal currents near Chattogram. The most predominant factor is the influence of monsoon winds. During the southwest monsoon, the winds are very strong and drive the surface currents. These winds may alter either the direction or speed of the flow of water and, consequently, may affect shipping and fishing activities.

Tidal currents dominate. The Bay of Bengal has a great range of tides, and due to the continuous ebb and flow of the tides in this bay, there is the development of complicated current patterns. Discharge of freshwater from the rivers, such as the Karnaphuli River, can alter the salinity and density of coastal waters and thus affect the flow of the current.

The shape of the coastline and the underwater topography also play a crucial role. The presence of sandbars, shoals, and channels can cause the currents to diverge, converge, or even form eddies. The interaction of coastal currents with larger oceanic currents in the Bay of Bengal further adds to the complexity of the current situation.

3. How to Observe the Coastal Water Flow of Chattogram?

Surface Drifting Buoy Method: This involves the use of a surface-drifting buoy that can trace the current in the surface. The buoy is cast into the water and, after sometime, usually with the aid of GPS or other tracking devices, its movement is tracked. However, this method has its own drawbacks. It only gives information concerning the surface current and is also easily susceptible to wind and waves. For instance, a powerful onshore wind may cause a buoy to appear as if the current is flowing in a direction other than that in which it is really flowing.

Anchor - Boat Method: An anchored boat can be utilized in measuring the current. Instruments are lowered from an anchored boat into the water to measure the current at different depths. While this method allows more detailed depthwise measurements, it also has a series of drawbacks. Because the boat is in a fixed position, it can sample the current only in a relatively small area around the anchor point and may not capture the full variability of the currents in a large coastal area.

ADCP Method: The ADCP current meter method is more modern and thus efficient in measurement than that of the coastal currents. ADCPs emit an acoustic signal into the water and measure the Doppler shift of the reflected signals to calculate the velocity of water at different depths. This gives a fuller profile of the current from surface to seabed. Consequently, ADCPs are less affected by surface disturbances than other instruments and can operate uninterruptedly over long periods, providing a truer representation of the current structure.

4. How do ADCPs Using the Doppler Principle Work?

ADCPs work on the principle of the Doppler effect. They emit high-frequency acoustic pulses into the water. As these pulses meet the moving water particles, their reflected frequency changes. If the water particles are moving towards the ADCPs, then the frequency of the reflected signal is higher than the emitted one, while in case of moving away, it is lower.

Accordingly, multiple transducers fitted on the ADCP current profiler send and receive signals at various angles. The device calculates the three-dimensional velocity of the water by analyzing the different directions and depths of the Doppler shifts. In this case, it determines the horizontal and vertical components of the velocity, offering a detailed view of the structure and behavior of the current.

5. What does Chattogram coastal current require for high-quality measurement?

The equipment for high-quality measurement of the coastal currents near Chattogram needs to be made of reliable materials. In view of its harsh marine environment, where saltwater, humidity, and even storms prevail, its casing needs to be durable. The size of the equipment also needs to be small enough to deploy and retrieve easily, especially in coasts with highly complex geometries.

A lightweight design is preferable as it makes the equipment more manageable during installation and maintenance. Low power consumption is crucial for long - term and autonomous operation. Cost - effectiveness is also important to enable large - scale measurements.

The casing of ADCPs is made from titanium alloy preferably. The corrosion resistance of this alloy is pretty good, which plays an important role in fighting the corrosive effects of seawater. It is strong and durable enough to bear the pressure of water at different levels. Its lightweight nature makes it more convenient for several deployment methods.

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

While selecting equipment for current measurement near Chattogram, the purpose of its usage has to be considered first. If the measurement is for a vessel's navigation or for short-term research, then a ship-mounted ADCP flow meter will suffice. The ship-mounted ADCPs provide real-time current information during the voyage.

For the measurement of flow characteristics near the seabed, for example, the study of sediment transport or benthic ecology, a bottom-mounted ADCP flow meter is preferred. It can capture the details of the near-bottom current. If long-term and wide-area monitoring of the coastal current is required, a buoy-type ADCP meter is ideal.

The frequency of ADCPs- For depth ranges up to 70m, a 600kHz ADCP gives good results. For ranges up to 110 meters, a 300kHz ADCP is more appropriate. For deeper waters that may occur near Chattogram, such as over 1000m, a 75kHz ADCP is recommended.

There are well - known ADCP profiler brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth considering. It is made of all - titanium alloy material and offers an excellent cost - performance ratio. You can find more information on their website: https://china-sonar.com/.

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