How to measure the coastal currents of Pasni?

1. Where is Pasni?

Pasni is a coastal town in the Balochistan province of Pakistan and is located on the Arabian Sea. It is very famous for its fishing and has a long history associated with the sea.

The economy of the town is mainly based on fishing and other allied industries. The cultural identity of the place is a well-preserved local Balochi tradition. The locals are dependent on the sea very much for their livelihood. The Arabian Sea off Pasni is rich in all kinds of marine life: fish, crabs, and other shellfish. It contains sandy beaches with a port that acts as the epicenter for fishing boats and maritime trade in small-scale transactions.

2. What is the situation regarding the coastal currents near Pasni?

Many elements act in driving the coastal currents in surroundings of Pasni, and the most important is contributed by the monsoon winds. The strong winds accompanying the southwest monsoon, which falls between June and September, have the potential of driving surface waters, thus resulting in modifications in the direction and speed of coastal currents leading to different current patterns.

Tidal forces are also important. The gravitational pull of the moon and sun causes the tides, and the tidal currents that result can be very strong. The shape of the coastline and the topography of the ocean floor can influence the way that tidal currents flow. Submarine features such as sandbars and channels can also divert the water movement.

Added to that, the ocean current circulation patterns of a larger scale in the Arabian Sea interact with the local coastal currents. Due to the inflow of different water masses coming from other regions, there is variation in temperature, salinity, and overall characteristics of the coastal currents.

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

Surface Drift Buoy Method: This is a method where buoys are deployed on the surface of the water. These buoys are allowed to drift with the currents. With the help of satellite-based tracking systems or any other position-determining system which follows their motion over time we may find out how the surface currents travel with speed and in which direction. This method can provide mostly the data about the surface layer and will not allow an appropriate description of the currents at the different depth levels.

Anchor Boat Method: In this method, a boat is taken to the coastal waters and anchored. Instruments are then used to measure the water flow at various depths around the boat. While this can provide more detailed data for the area around the boat, it is relatively labor- and time-consuming. Also, it has limitations concerning the area it can cover.

ADCP Method: The ADCP current meter method is more sophisticated and efficient in the measurement of coastal currents. It uses sound waves to measure simultaneously, at different depths, the velocity of water. Sound pulses are emitted by this device, and then, through the analysis of the returned reflections, Doppler-shifted by particles in the water, it builds up a current profile from the surface to a certain depth. This provides a more precise and complete view of the coastal currents than the other methods do.

4. How do ADCPs using the principle of the Doppler work?

The ADCPs work based on the principle of the Doppler effect. They send pulses of sound into the water. When these sound waves interact with particles in the water-sediment particles, plankton, or other small organisms-the frequency of the reflected sound waves shifts due to the Doppler effect. If the particles are moving towards the ADCP meter, then the frequency of the reflected waves is higher than the emitted waves; if they are moving away, the frequency is lower.

From these frequency shifts of reflected sound waves, with measurements taken at varying angles and depth, the ADCP current profiler calculates water velocity in three directions. Dependent on capability, an ADCP flow meter uses multiple transducers to transmit and receive the respective sound signals, through which it develops a detailed profile of water velocity throughout the water column, from the surface to as deep as the instrument capability allows.

5. What is required to obtain high-quality measurements of Pasni coastal currents?

The equipment that is going to be used in carrying out the measurement of Pasni coastal currents should utilize use of durable materials. Such equipment has to be well durable owing to the kind of marine environment that exposes itself to salt-water corrosion and temperature fluctuations, which it may even be impacted upon physically from floating debris.

The size of the equipment must be small in size for easier deployment and installation. Be it from boats, buoys, or at the seabed floor, the smaller the size, the easier handling and installation will be. It also makes transportation and maintenance a lot easier.

Floating or moored weight of equipment has to be light. A low mass will ensure proper buoyancy and stability. In that case, the power consumption, similarly, needs to be quite low in order to maintain continuous operation over long lengths of time without frequency replacement of the batteries or a huge amount of power supply.

Cost-effectiveness is key. For large measurement along the coast, it has to be economically viable. In the case of ADCPs, one of the best options for casing is by using titanium alloy. Being in this salty marine environment, as would be expected, titanium alloy has a very good resistance to corrosion. Once more, it is strong yet relatively light, therefore meeting the requirements needed to make high-quality current-measurement equipment.

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

While selecting the equipment for current measurement at Pasni, the first consideration would be the purpose of usage.

Ship-borne ADCP: This is suitable for measurements from a moving vessel. It can record the coastal currents continuously as the ship moves along the coast, providing a series of data points over a relatively wide area covered by the ship's route.

Bottom-mounted ADCP: It is ideal for fixed-point measurements at the seabed. It gives a continuous and long record of the flow of water near the bottom that is very essential to understand the interaction between the bottom topography and the coastal currents.

Buoy-mounted ADCP: When the intention is to measure currents over a prolonged period in a given area while floating on the surface of the water. This can be deployed at points of interest and give continuous data with no need for a vessel to be on site all the time.

By the use of frequency, for a depth of 70m or less, a 600kHz ADCP is just right. For depths to a maximum of 110m, the 300kHz is quite suitable. And for depths to 1000m, the 75kHz is ideal.

There are well-known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth mentioning. It has a casing made entirely of titanium alloy and offers an excellent cost-performance ratio. You can find more information about it at https://china-sonar.com/.

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