How to Measure the Coastal Currents of Cox's Bazar

1. Location and Significance of Cox's Bazar

Cox's Bazar is a city in Bangladesh, situated on the southeastern coast of the country along the Bay of Bengal. It's known for something quite interesting: having the longest natural sandy beach in the world, extending more than 120 kilometers.

Local people are highly dependent on fishing and tourism. The Bay of Bengal, near Cox's Bazar, is rich in marine life like different species of fish, shrimp, and crabs. A large number of local people depend directly on the fishing industry. Beautiful beaches and coastal scenery develop the tourism industry.

2. The Situation of Coastal Currents near Cox's Bazar

Influence of Tidal Currents

Tidal currents play a very important role in the coastal area of Cox's Bazar. In the Bay of Bengal, the semi-diurnal tidal pattern occurs with roughly two high tides and two low tides each day. The variation between high and low tide levels can be quite significant. Due to the ebb and flow of the tides, strong and complex current patterns develop.

Monsoon Winds and Currents

The monsoon winds have a major impact on the coastal currents. During the southwest monsoon (June - September), strong winds blow from the southwest towards the coast. These winds can drive surface currents and cause significant changes in the direction and speed of the coastal currents. The opposite occurs during the northeast monsoon (November - February), when winds blow from the northeast.

Influence of Underwater Topography The underwater topography near Cox's Bazar also affects the currents along the coast. The occurrence of sandbars, channels, and submerged ridges can make the current change direction and speed. The presence of a sandbar could split the current or slow it, while a channel can funnel it and accelerate its speed.

3. Methods of Observation of the Coastal Water Flow near Cox's Bazar

Surface Drifting Buoy Method

Principle: A surface-drifting buoy is thrown into the water to observe the surface current. The buoy movement is then tracked over a period using GPS or any other tracking systems.

Advantages: It's a rather easy and inexpensive method for the first glance at surface current patterns. Can return valuable data of direction and speed of the surface current for various tide and weather conditions.

Limitations This method gives information only about the surface current and is affected by wind and waves. For instance, under strong winds or high - energy wave conditions, the buoy movement may not represent the actual current direction and speed.

Anchor - Boat Method

Principle: An anchored boat is used to measure the current. Instruments are lowered from the boat into the water to measure the current at different depths.

Advantages: It allows for more detailed depth - wise measurements of the current. By using instruments such as current meters, one can get a better understanding of the vertical structure of the current.

Limitations: The position of a boat is fixed, hence, the area of measurement is restricted, probably not reflecting the whole variability of currents in a large coastal area. Also, the boat can be affected by waves and tides and this might introduce errors in measurement.

Acoustic Doppler Current Profiler (ADCP) Method

Principle: ADCPs emit acoustic signals into the water and measure the Doppler shift of the reflected signals to calculate the velocity of water at different depths.

Advantages: This gives a more complete profile of the current from the surface to the seabed. The ADCPs are less affected by surface disturbances and can operate continuously for long periods, giving a more accurate picture of the current structure.

How ADCPs using the Doppler principle work

ADCPs work on the principle of the Doppler effect. They transmit high-frequency acoustic pulses into the water. Due to the interaction of these pulses with moving water particles, the frequency of the reflected pulses changes. If the water particles are moving towards the ADCP current meter, the frequency of the reflected signal is higher than the emitted signal, and if they are moving away, it is lower.

The ADCP profiler has multiple transducers that send and receive signals at different angles. Analyzing the Doppler shifts from different directions and at different depths, the device can compute the three-dimensional velocity of the water. That is, it can determine the horizontal and vertical components of the velocity to give a good understanding of the current structure and behavior.

4. Requirements for High - Quality Measurement of Coastal Currents near Cox's Bazar

Equipment Materials

Materials in these instruments should be reliable, enabling high-quality measurement of coastal currents around Cox's Bazar. Because of the sea environments with saltwater, possibly storms-terrible experience of monsoon-and tides or ocean current influence, casings should be durable. Size: Small size is effective and efficient in deploying and recovering such equipment due to the quite rough seas at times, the long beach, and multiple coastal structures.

Design Considerations

The design should be lightweight to keep the equipment more manageable in installation and maintenance. Low power consumption is required for longand autonomous operation. Costeffectiveness is likewise necessary to create large measurements possible, especially for the generally limited resources available in the area for such research.

Casing Material for ADCPs

The casing of ADCPs is preferably made from those materials that have excellent resistance to corrosion, such as titanium alloy or high-grade stainless steel. These can sustain the corrosive action caused by seawater. They are also strong and durable to stand the pressure of water at depth. Their lightweight nature, especially for titanium alloy, makes them more convenient for various deployment methods.

5. Selection of Appropriate Equipment for Current Measurement at Cox's Bazar

Purpose-Based Selection

While selecting the equipment for the measurement of current near Cox's Bazar, first consider the purpose of use. If the measurement is for a vessel's navigation or short-term research related to surface conditions, such as for small fishing boats, a ship-mounted ADCP or a simple surface-drifting buoy system will suffice. The ship-mounted ADCP meter will provide real-time current information during the voyage, while the buoy can give a general idea of the surface current.

Seabed-related Measurements: For measurements near the seabed, like in the case of sediment transport studies, which are relevant in view of the sandy beach and potential coastal erosion, or benthic ecology, a bottom-mounted ADCP flow meter is preferred. It will provide the details of the near-bottom current, which is important for understanding sediment movement and its interaction with the seabed and beach.

Long-Term and Wide-Area Monitoring: If long-term and wide-area monitoring of the coastal current is required, a buoy-type ADCP current profiler is ideal because one can get continuous data for a long period and it is possible to cover a relatively large area and hence understand the overall patterns and changes in the coastal currents.

Frequency Consideration for ADCPs

Regarding the frequency of ADCPs, for water depths up to 70m (which is relevant for much of the coastal area near Cox's Bazar), a 600kHz ADCP will suffice, a 300kHz ADCP or lower (depending on the depth) can be more appropriate, and for deeper waters, as may be encountered near Tomakomai, greater than 1000m for instance, a 75kHz ADCP would be most suitable. The choice of frequency also depends on the specific requirements of the measurement, such as the level of detail needed for the current profile and the tradeoff between accuracy and range.

There are well-known ADCP current meter 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.