How to Measure Coastal Currents of Karachi?

1. Introduction to Karachi

Karachi is a mega port city in Pakistan that borders the Arabian Sea. The city ranks as the country's premier metropolis, economic hub, and one of the country's pivotal cultural epicenters.

The Skyscrapers, historic buildings, and local markets together build the cityscape of Karachi. There is tremendous cultural diversity among its inhabitants due to several ethnic groups and traditions in the area. The natives of the area pursue many professions; however, the major ones being trade, industry, and services. Its port is the busiest among those of South Asia due to the immense cargo and marine activity that takes place.

The Arabian Sea next to it is an important constituent of the city. Marine life abounds in the waters along the coast: fish, prawns, and all types of shellfish. The sea modifies the climate of this place through the sea breeze that brings some respite from the sweltering heat. The coastline consists of sandy beaches, rocky tracts, and vast mangrove forests. These mangroves protect the coasts and provide a habitat for many species.

2. What is the condition of the coastal currents off Karachi?

During these coastal currents, numerous variables are considered in action in the area of Karachi, including the tidal current, which acts most vigorously; the tide arises mainly due to gravitational pull because of the moon and sun. These themselves have given rise to strong currents at a particular site. The coastline curvature and submarine topography further regulate the shape and flow velocity of such currents around Karachi. Such bottom topography might deflect and change course from sandy bars, channels, or other ridges.

The monsoon winds have a major impact. The southwest monsoon, between June and September, brings strong winds that can drive the surface waters and change the direction and speed of the coastal currents. The inflow of freshwater from nearby rivers and the mixing with the seawater can also affect the density-driven currents.

Besides, the large-scale ocean circulation patterns of the Arabian Sea can interact with the local coastal currents. Inflow of different water masses coming from other regions may bring changes in temperature, salinity, and overall characteristics of coastal currents.

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

Surface Drift Buoy Method: This method involves deploying buoys on the water surface. These buoys are allowed to drift with currents. By using satellite-based tracking systems or other positioning methods to monitor the movement of the buoys over time, we can get information about the direction and speed of the surface currents. But this method mainly provides data from the surface layer and might not give a comprehensive understanding of currents at different depths.

Anchor Boat Method: Under this method, a boat is anchored in the water of the coastal area. It then records the flow of water around it at depth using instruments. This, though more detailed for an area around the boat, can be rather labor-intensive and time-consuming. Also, it has limitations in terms of the area it can cover.

Acoustic Doppler Current Profiling method: ADCP current meter is comparatively more advanced and efficient in measurement with regard to the measurement of coastal currents. It uses sound waves that measure the velocity of water simultaneously at different depths. With the emission of sound pulses and analysis of the Doppler-shifted reflections from the particles in the water, it can create a detailed profile of the current from the surface down to a certain depth. This provides a more accurate and comprehensive picture of the coastal currents than the other methods.

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

The ADCPs work on the principle of the Doppler effect. They send pulses of sound into the water. These sound waves interact with particles in the water, such as sediment particles, plankton, or other small organisms; the frequency of the reflected sound waves changes due to the Doppler effect. Particles moving towards the ADCP current profiler would have a higher frequency than the emitted waves, while those moving away would have a lower frequency.

Working mainly with reflected sound waves at different frequencies, angles, and depth, it calculates water speed in various directions. There are several transducers mounted on the ADCP profiler; these transmit and receive a series of sound signals; using the frequency shifts of these received echoes, it can develop a highly detailed profile of the water velocity across the entire water column, from the surface right down to the bottom at a certain depth dependent upon the instruments' capabilities.

5. What is necessary for high-quality measurement of the Karachi coastal currents?

In order to have high-quality measurement of the coastal currents near Karachi, the equipment should be made of appropriate material. Considering the very aggressive marine environment, including salt-water corrosion, temperature fluctuations, and even physical impacts from floating debris, it needs to be robust.

The size of the equipment should be small for easy deployment and installation. It will be easy to handle and install whether placed at boats, buoys, or seabed because of its small size. It also makes transportation and maintenance easier.

The weight of the equipment shall be light, in particular for floating or moored devices, to ensure adequate buoyancy and stability. It is also very important to have low power consumption that will enable continuous operation over a long period without the need for frequent battery replacements or large power supply.

Cost-effectiveness is an important consideration. To make the measurements on a large scale along the coast, the equipment should be affordable. In the case of ADCPs, a casing made from titanium alloy is a good choice. The corrosion resistance of the titanium alloy is excellent, which is necessary in the salty marine environment. It is also strong yet relatively lightweight, meeting the requirements for high-quality current-measurement equipment.

6. How to select the appropriate equipment to measure currents?

For selection of equipment to measure currents along Karachi, the purpose of usage has to be kept foremost.

Ship-borne ADCP: It is ideal for measurements from a moving vessel, and as the ship moves along the coast, the instrument will be able to record the coastal currents continuously for a comparatively large area covered by the vessel.

Bottom-mounted ADCP: optimal for fixed-point measurements at the seabed. It provides a steady and long-term record of the water flow at the bottom, which plays a very important role in studying the relation between the bottom topography and the coastal currents.

Buoy-mounted ADCP: An ADCP utilized when the purpose is to measure the currents in a specified area over a longer period while floating on the water surface. It can be deployed in areas of interest and provide continuous data without needing a vessel to be present at all times.

With respect to frequency, for depths of 70m or less, the 600kHz ADCP is appropriate; for depths up to 110m, use a 300kHz ADCP while a 75kHz ADCP will go as deep as 1000m or so.

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.