Measurement of Coastal Currents of Kolkata

1. Location and Importance of Kolkata

Kolkata is one of the major Indian cities located within the state of West Bengal. It is situated quite close to the coastline of the Bay of Bengal and serves as a vital economic, cultural, and transportation center.

The city has a diversified economy in which major sectors include trade, manufacturing, and services. The port of Kolkata on the Hooghly River, which finally merges with the Bay of Bengal, is an important entity in the sea trade of India. Inland waterways and coasts are used for different economic activities such as fishing and transportation.

2. Situation of Coastal Currents around Kolkata

Tidal Currents

Tidal currents play an important role. The Bay of Bengal manifests a semi-diurnal tide-that is, a pattern of two high tides and two low tides-above roughly one day. Thus, the Hooghly River is connected with the bay, and for this reason, it receives the tidal impact. In fact, because of this range of tide, there is a rise and fall in the level of water, creating tidal currents. These currents can vary with speed and direction depending on the phase of the tide and the form of the river and coastal channels.

River Discharge and Currents

The coastal currents are also influenced by the discharge from the Ganges and other rivers into the Hooghly River. The freshwater volume discharged into the bay through the river system influences the salinity and density of the water, thus affecting the current patterns. The interaction between the river water and the seawater creates a dynamic environment with complex current systems.

Monsoon Influence

The monsoon winds play a vital role. During the southwest monsoon, that is, June to September, heavy rainfall increases the river discharge, while strong winds during this period can drive surface currents and have a great influence on the coastal and riverine current patterns. The northeast monsoon will affect the currents from November to February, though the wind directions and intensities are different.

Underwater Topography and Channel Effects

Currents are influenced by the underwater topography and channels in the Hooghly River estuary and the adjacent coastal area. The depth and width of the channels can channel or slow down the currents. Shoals and sandbars can cause the currents to change direction and split.

3. Methods to Observe the Coastal Water Flow near Kolkata

Surface Drifting Buoy Method

Principle: A floating buoy is dropped into the water and, well, one observes the current's surface. Its movement in time can be recorded via GPS or any other such tracking systems.

Advantages: It's a relatively simple and inexpensive means of obtaining an initial understanding of the surface current patterns. It can yield valuable data about the direction and speed of the surface current during different tidal and weather conditions.

Limitations: This method provides information about the surface current alone and might be influenced by wind and waves. For instance, during strong winds or high - energy wave conditions, the buoy movement itself may not represent the real direction and speed of the current.

Anchor-Boat Method

Principle: The current is measured using an anchored boat. 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: It will reduce the area of measurement as the boat cannot change its position, hence can only measure a specific fixed area, which is probably unable to capture all variability in currents in the whole large coastal area. Waves and tides could impact the boat and potentially produce errors in the measurement of current speed.

Acoustic Doppler Current Profiler (ADCP)

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 operate based on the Doppler effect. They emit high-frequency acoustic pulses into the water. Due to interaction with moving water particles, the frequency of the reflected pulses changes. If the water particles are moving towards the ADCP meter, then the frequency of the reflected signal is higher than the emitted signal; if they are moving away, it is lower.

The ADCP flow meter is equipped with several transducers that send and receive signals at different angles. It calculates the three-dimensional velocity by analyzing the Doppler shifts from different directions and depths, enabling it to measure the horizontal and vertical components of the velocity. This provides great detail in understanding the structure and behavior of the current.

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

Equipment Materials

For high-quality measurement of the coastal currents near Kolkata, the equipment should be fabricated with reliable materials. Due to the marine environment-a saltwater, potential storm exposure, especially during the monsoon, and tidal and ocean current influences-there should be a hard casing. The size of the equipment should be as small as possible for ease of deployment and retrieval, considering the complexity of the river and coastal landscape with channels, sandbars, and docks.

Design Issues

A light design is preferred since it makes the instrument easier to handle both during installation and maintenance. Low power consumption is another important factor for long-and autonomous-operated systems. Cost-effectiveness is similarly relevant for large-scale measurement deployments, given the need for accurate current measurements within shipping, fishing, and other related activities.

Casing Material for ADCPs

The casing of ADCPs is preferably made from materials with excellent corrosion resistance, such as titanium alloy or high-grade stainless steel. These materials can resist the corrosive effects of seawater. They are also strong and durable, capable of withstanding the pressure of the water at different depths. Their lightweight nature-especially the titanium alloy-makes them more convenient for various deployment methods.

6. Selection of Appropriate Equipment for Current Measurement near Kolkata

Purpose-Based Selection

While selecting the equipment to measure current near Kolkata, the purpose will be the first thing to consider. If the measurement is for a vessel's navigation or short - term research related to surface conditions (such as for ships and boats in the Hooghly River and near - shore areas), a ship-mounted ADCP or a simple surface - drifting buoy system can be a suitable option. The ship-mounted ADCP current profiler will provide current information in real time during the voyage, while the buoy can give an approximation of the surface current.

Seabed-Related Measurements

For measurements near the seabed, such as in studies of sediment transport-which is important for maintaining the depth of the river and estuary channels and understanding coastal erosion-or benthic ecology, a bottom-mounted ADCP is preferable. It can capture the details of the near-bottom current, which is so important for understanding the movement of sediment and its impact on the seabed and the river-estuary infrastructure.

Long-Term and Wide-Area Monitoring: A buoy-type ADCP current meter can be used if it is required to monitor the coastal current for a longer duration over a wider area. In this way, the continuous data for a longer period of time and relatively large area will be useful to understand the overall patterns and changes in the coastal currents, which are also helpful for port operations, fishing management, and the management of the coastal zone.

Frequency Consideration for ADCPs Regarding the frequency of ADCPs, for water depths up to 70m (which is relevant for much of the coastal and riverine area near Kolkata), a 600kHz ADCP will suffice. For deeper waters that could be found in some offshore areas or in the deeper parts of the river-estuary channels, a 300kHz ADCP or lower (depending on the depth) can be more appropriate. The choice of frequency also depends on the specific requirements of the measurement to be made, such as the amount of detail in the current profile and the 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.