How to Measure the Coastal Currents of Chattogram

1. Location and Significance of Chattogram

Chattogram (formerly known as Chittagong) is a major port city in Bangladesh. It's located on the southeastern coast of the country, along the Bay of Bengal. It's a crucial economic hub, handling a large portion of the country's maritime trade.

The economy is quite diverse, with a heavy emphasis on shipping, fishing, and related industries. The Bay of Bengal, near Chattogram, is very rich in marine resources, comprising a wide variety of fish species, shrimp, and other seafood. The port of Chattogram is one of the busiest in South Asia, through which a good amount of imports and exports of raw materials, manufactured products, and agricultural commodities is made.

2. The Situation of Coastal Currents near Chattogram

Tidal Currents

The dominant influence in tidal currents exists. A semi-diurnal tide happens inside the Bay of Bengal with two highs and two lows in each day. This is caused by large ranges in tide, especially as tidal currents are subject to large variations in a short expanse of flow, both at ebb and at flow, thus being responsible for complex patterns in the generation of the same currents. Variations depending on the place and kind of the tide and topographic structure can alter speed and direction.

Monsoon Winds and Currents

The monsoon winds play the major role. Strong winds blowing from the southwest towards the coast during the southwest monsoon from June to September will push the surface currents. Thereby, the directions and speeds of the coastal currents change appreciably. During the northeast monsoon, between November and February, the opposite happens. During this season, winds are from the northeast and produce a different response in the current patterns.

Influence of Underwater Topography

The underwater topography near Chattogram also affects the coastal currents. The presence of channels, shoals, and underwater ridges can cause the currents to change direction and speed. For example, a deep - water channel can funnel the current, increasing its speed, while a shoal can cause the current to slow down and split.

3. Methods to Observe the Coastal Water Flow near Chattogram

Surface Drifting Buoy Method

Principle: A buoy that drifts on the surface is thrown into the water and the movement of the same buoy after some time is measured by using GPS or any other tracking systems.

Advantages: It is a comparatively uncomplicated and inexpensive method to obtain a preliminary impression of the surface current pattern. It can give quite valuable data on the direction and speed of the surface current in various tidal and weather conditions.

Limitations: This can only give information about the surface current and is readily affected by wind and waves. For instance, under strong wind or high - energy wave conditions, the buoy drift may not be a reflection of actual current direction and speed.

Anchor-Boat Method

Principle: Current measurements obtained from an anchored boat. Lowering instruments from a anchored boat into the water automatically measures the current at the different depths of interest

Advantages: It allows for a high degree of detail for measurement depth-wise of current. Indeed, with instruments such as current meters, a far better appreciation in the vertical structure of current could be accessed.

Limitations: The boat position is fixed, which limits the measurement area. It may not measure the full variability of currents in a large coastal area. Also, the boat can be affected by waves and tides, which might introduce errors in the 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 provides a more comprehensive profile of the current from the surface to the seabed. ADCPs are less affected by surface disturbances and can operate continuously for long periods, giving a more accurate picture of the current structure.

4. 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. The interaction of these pulses with moving water particles changes their frequency. If the water particles are moving towards the ADCP profiler, the frequency of the reflected signal is higher than the emitted one; if they are moving away, it is lower.

The ADCP meter is equipped with several transducers, each transmitting and receiving the signals at different angles. Through analyzing the Doppler shifts from different directions at different depths, this device is able to compute three-dimensional water velocity, thereby being able to deduce horizontal and vertical components of the same velocity for detailed current structure and behavior.

5. Requirements for High-Quality Measurement of Coastal Currents near Chattogram

Materials of Equipment

Materials and equipment should be of dependable materials since high-quality measurement of Chattogram's coastal currents is wanted. In the marine setting, with saltwater facing potential exposure to storms-mainly during the monsoon time period-and under the influence of tides and oceanic currents, a casing type should be durable. This said, the size of such an instrument should be small in particular in light of the roughness of the sea at all points in time and the available port and other structures found on the coast.

Design Considerations

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, especially considering the importance of accurate current measurements for shipping and other coastal activities.

Casing Material for ADCPs

ADCPs are preferably housed in a casing made from materials of excellent corrosion resistance, including but not limited to titanium alloy and high-grade stainless steel, which are capable of sustaining the corrosive action of seawater. Besides that, they are strong, durable, and can support the pressure of the water at different depths. Because of their lightweight nature-actually, titanium alloy-they will also be more convenient for several methods of deployment.

6. How to Choose the Right Equipment for Current Measurement near Chattogram

Purpose-Based Selection

While selecting the equipment to measure current near Chattogram, the first thing one should consider is the purpose of use. If the measurement is for a vessel's navigation or short-term research related to surface conditions, such as ships entering or leaving the port, a ship-mounted ADCP or a simple surface-drifting buoy system can be a suitable option. The ADCP flow meter mounted on the ship can provide real-time current information along the voyage, while the buoy can give an impression of the surface current.

Seabed-Related Measurements

For measurements near the seabed, such as for the study of sediment transport important to maintaining the depth of the port and understanding coastal erosion, or benthic ecology, a bottom-mounted ADCP current profiler is preferred. It will capture the details of the near-bottom current, crucial for understanding the movement of sediment and its impact on both the seabed and the port infrastructure.

Long-Term and Wide-Area Monitoring If long-term and wide-area monitoring of the coastal current is needed, the buoy-type ADCP current meter would be ideal. That will provide continuous data for a longer period and cover a fairly larger area, which shall enable one to understand the general pattern and changes that the coastal currents undergo, beneficial for port operations and coastal management.

Frequency Consideration for ADCPs

For water depths up to 70m, which is quite relevant for much of the coastal area near Chattogram, a 600kHz ADCP is quite a good choice. In deeper waters, which might be found in some offshore areas or in channels, a 300kHz ADCP or lower (depending on the depth) can be more appropriate. Frequency choice is also depending on the needs from a measurement: what is the amount of detail for the actual profile and how to trade-off between accuracy and range.

There are well-known brands of ADCPs such as Teledyne RDI, Nortek, and Sontek. However, for those looking for a cost-effective option with good quality, the China Sonar PandaADCP is highly recommended. It is made of all-titanium alloy material and offers an incredible price-performance ratio. You can find out more about it on its website: https://china-sonar.com/.

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