Measurement of Digha Coastal Currents

1. Location and Importance of Digha

Digha is one of the most important seaside resort towns in the state of West Bengal, India. It lies on the Bay of Bengal and has become famous for its extensive sandy beaches and general pleasant weather.

The local economy is predominantly dependent on tourism and fishing. Marine resources in the Bay of Bengal near Digha are abundant in fish, prawns, and crabs. The fishing activity is a principal employment source for the local people, while the arrival of tourists gives a considerable boost to the local economy.

2. The Situation of Coastal Currents near Digha

Tidal Currents

Tidal currents have a great effect on the coastal area of Digha. The Bay of Bengal has a semi-diurnal tide, with two high and two low tides each day. The tidal range is quite significant at times, which gives rise to strong and complicated patterns of current. These tidal currents can vary in speed and direction depending on the phase of the tide and the local underwater topography.

Influence of Monsoon

In these coastal currents, the most important role is played by the monsoon winds. During the southwest monsoon, strong winds blow from the southwest toward the coast between June and September. 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 when, between November and February, winds blow from the northeast and have a different impact on the current patterns.

Underwater Topography

Underwater topography near Digha affects the coastal currents, as well. Sandbars, shoals, and channels can make the current change direction and speed. A sandbar may slow the current and divide it, while a channel can funnel the current and accelerate it.

3. Methods to Observe the Coastal Water Flow near Digha

Surface Drifting Buoy Method

Principle: A surface-drifting buoy is thrown into the water and the surface current is observed. The movement of the buoy is followed for some time using GPS or other tracking systems. Advantages - This is a relatively simple and cost - effective way to initially understand the surface current patterns. It can give useful data on the time of day and in various weather conditions regarding the direction and speed of the surface current.

Limitations: It gives information on the surface current only. Wind and waves may greatly affect the buoy's movement, thus giving incorrect readings of the direction and speed of the current. For instance, during a storm or heavy winds, the buoy may not be representative of the current in reality.

Anchor-Boat Method

Principle: It involves measurement using an anchored boat. Instruments are moved from the boat into the water to measure current at different depths.

Advantages: It enables more detailed depth-wise measurements of the current. With the help of instruments such as current meters, a better understanding of the vertical structure of the current can be achieved.

Limitations: The area of measurement is restricted by the boat in a fixed position. It cannot perhaps capture the full variability in a large coastal area of currents. Besides, the boat may be disturbed by waves and tides, which could introduce errors into 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 gives a better profile of the current from the surface to the seabed. The influence of surface disturbances is minimal in ADCPs, and they can run continuously for long periods, thus giving quite an accurate picture of the structure of the current.

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. As these pulses encounter the moving water particles, the frequency of the reflected pulses is altered. If the water particles are moving towards the Acoustic Doppler Current Profiler (ADCP), the frequency of the reflected signal is higher than the emitted one; if they are moving away, it is lower.

The ADCP flow meter is equipped with multiple transducers that send and receive signals at different angles. By analyzing the Doppler shifts from different directions and at different depths, the device can calculate the three - dimensional velocity of the water. This means it can determine the horizontal and vertical components of the velocity, providing a detailed understanding of the current's structure and behavior.

5. Measurement of Coastal Currents near Digha - High Quality Requirements

Equipment Materials

The equipment should be fabricated with materials that can ensure reliable performance for high-quality measurement of the coastal currents near Digha. The casing should be durable to cope with the marine environment, saltwater, exposure to storms (especially during the monsoon season), tides, and ocean currents. The size of the equipment should be small, making deployment and retrieval easy, especially considering the sometimes rough sea conditions and the presence of a beach and other coastal structures.

Design Considerations

A lightweight design is preferred as this makes the equipment more manageable during installation and maintenance. Low power consumption is required for long - term and autonomous operation. Cost - effectiveness is also relevant to allow large scale measurements, considering the local economic and resource constraints.

Casing Material for ADCPs

The casing of ADCPs is preferably made of materials with excellent corrosion resistance such as titanium alloy or high-grade stainless steel. These materials can bear the corrosive effects of seawater. They are also strong and durable to bear 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 Equipment for Current Measurement near Digha

Purpose-Based Selection

While selecting equipment for current measurement near Digha, one should consider the purpose of its usage. If the measurement is for a vessel's navigation or short-term research related to surface conditions, such as for local fishing boats or tourist boats, then a ship-mounted ADCP meter or a simple surface-drifting buoy system will suffice. The ADCP mounted on the ship provides real-time current information during the voyage, while the buoy can give a general idea of the surface current.

Seabed-Related Measurements

Since it would be measurements near the seabed-for instance, studying sediment transport, which is of vital concern due to the sandy beach and potential coastal erosion, or benthic ecology-a bottom-mounted ADCP current profiler would be desirable. It captures details in the near-bottom current that are important for understanding sediment movements and their interaction with the seabed and the beach.

If long-term and wide-area monitoring of the coastal current is required, a buoy-type ADCP current meter is ideal. This can provide continuous data over an extended period and cover a relatively large area, helping to understand the overall patterns and changes in the coastal currents.

Frequency Consideration for ADCPs

For the frequency of ADCPs, a 600kHz is a good choice for water depths up to 70m, which applies to much of the coastal area near Digha. For deeper waters that might be found in some offshore areas or in channels, a 300kHz ADCP or lower can be more appropriate. 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 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.