How do we measure the coastal currents at Mirbat?

1. Where is Mirbat?

Mirbat is an intriguing coastal city that is situated in the southern part of Oman, overlooking the Arabian Sea to the backdrops of some mountain ranges and arid land stretching further inland. This places the town in a very peculiar geographical position; hence, it is considered to be a place where nature's beauty meets historic importance.

The surroundings constitute a harmonious combination of land and sea. The seashore is sandy, rocky at certain places, and filled with small coves that provide an extra charm to its beauty. The adjoining waters of the Arabian Sea are teeming with marine life, where a wide variety of fish species, sea turtles, and other interesting animals reside. These waters have been a source of livelihood for the local fishermen, depending on the bounty of the sea for their catch.

Mirbat is a place with very strong human culture. It used to be a trading port in the past. Remains of its glorious past can be seen in old forts and historical buildings that speckle the town. Locals are attached to the sea, and the traditional way of fishing is passed down to generations. People here lead a very relaxed life. Most of their daily activities usually centre around the beach, where they spend most of the time mending nets, pushing boats, or simply relaxing in the sea breeze.

The gulf off Mirbat is a relatively tranquil and sheltered area; hence, it is really suitable for different types of water sports. It also provides a haven for boats during bad weather, and its clear waters attract both divers and snorkelers who come to see the underwater world.

2. What is the situation of the coastal currents near Mirbat?

The different factors that account for the shaping of the coastal currents near Mirbat include:. The major influencers include tidal forces. Tides in this area are controlled by the gravitational pull of both the moon and the sun. As a result of semi-diurnal tides, the regular pattern of water movements causes the coastal currents to change direction and strength during the day. This range or difference depends on the lunar cycle and other factors of astronomy, which then again go on to have their impact on the flow pattern near the shore.

Wind patterns also play a very important part. The prevailing winds of an area might change after every season. These are strong enough to act as drivers for the water on the surface and produce currents. Sometimes, in one season or another, the winds may be quite strong and really push the water in one direction, then that would affect the total circulation of the currents. The interaction of wind-driven and tidal currents produces a complex dynamic flow regime.

The major factors include the nature of the seabed topography around Mirbat. Features such as underwater ridges, shoals, and channels can cause diversions and changes in currents. For example, when water reaches a shallow shoal, it is sometimes forced to change direction to split or flow around an obstruction. These features on the seabed can also result in acceleration and deceleration of the currents as water navigates its way around the varying depths.

3. In which way is it possible to observe coastal water flow in Mirbat?

Surface Drifting Buoy Method

This method is realized by placing the buoys on the surface of the water, allowing it to drift freely with currents. The buoy movements are tracked, usually with satellite-based tracking systems or any other positioning technologies, while the data on the direction and speed of the surface currents are obtained. However, this method tends to consider only the surface and barely gives a full understanding of the currents within the water column.

Anchor Mooring Ship Method

A ship is allowed to anchor at a designated position in the coastal waters. It then drops its instruments, such as current meters, into the water at varying depths to measure water flow. The probe may provide more detailed insight into current conditions at one point in the water column, but this approach is rather stationary and thus cannot reflect the complicated structure of coastal current variability for greater areas.

Acoustic Doppler Current Profiler (ADCP) Method

An improved approach to measuring coastal currents was afforded with the ADCP current meter method. It continuously measures the velocity of water at different depths all at the same time by using acoustic pulses. By emitting sound waves and analyzing the Doppler shift of the reflected waves, it can accurately determine the speed and direction of the water movement. ADCPs can be installed on various platforms such as boats, buoys, and even on the seabed itself, which makes the instrument capable of operating at the widest range of locations for most detailed profiling of current across the water column.

4. How does an ADCP with a Doppler principle operate?

ADCPs are based on the principle behind the Doppler effect: they transmit acoustic signals into the water at known frequencies. When those signals reflect from the moving particles in the water, the frequency of the returned signals shifts via the Doppler effect. This shift in frequency is measured by the ADCP flow meter and mathematically interpreted to calculate the velocity of the water relative to the instrument.

Since ADCPs are able to emit signals at various angles and different frequencies, it can measure the components of velocity in multiple directions such as horizontal and vertical at more depths in the water column. This, in turn, provides a well-profiled picture of water flow with all specific details of the structure and dynamics of coastal currents.

5. What is necessary to have good measurements of Mirbat coastal currents?

Measurements of the coastal currents at Mirbat require a certain characteristic in the measuring equipment. The materials used to manufacture the equipment should be of a very reliable nature. Due to the saline marine conditions that cause corrosion, coupled with high humidity and constant action of waves, the equipment is supposed to last long without deterioration.

Other advantageous properties of these receivers include small size, lightweight, and low power consumption. The latter makes a device compact, lightweight, easy to be installed and transported whether on a small fishing boat or even just a floating buoy. Low power consumption ensures the operation of the equipment during an extensive period without changing the batteries very often or drawing from a big source of power.

The cost is another important factor. Equipment should be affordable, since large-scale measurements are normally required to understand the often-complex coastal current patterns. In this way, multiple units can be deployed in different locations along the coast.

For the ADCPs, it is highly recommended that the casing is of titanium alloy. Its corrosion resistance is very good, a matter of utmost importance in saltwater conditions such as in Mirbat. It has a very high strength-to-weight ratio; therefore, while the material can offer much robust protection to the internal parts of the ADCP profiler, it does not add too much to the general weight of the device.

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

Based on the Usage Purpose

• Shipborne ADCP: This is suitable for measurement when the ship is to be in motion along the coast. It can record continuously as the ship travels and provides a broad view of the current variations over different sections of the coastline.
• Bottom-mounted ADCP: Suitable for fixed-point observation in some location on the seabed, it can provide high-resolution and long-term current profiles right above its installation positions to precisely understand the local flow pattern.
• Buoy-mounted ADCP: When the objective is to monitor the surface and upper layers of the water column over a large area and for an extended period without being tied to a ship or a fixed bottom location, the buoy-mounted ADCP is a good choice.

According to the Water Depth

• For water depths within 70m, a 600kHz ADCP is usually adequate; it can provide accurate measurements with good resolution in the generally shallow coastal waters off Mirbat.
• For water depths up to 110m, a 300kHz ADCP will be more suitable. It can penetrate deeper into the water column and still obtain reliable velocity data.
• In case of operations in deeper waters extending up to 1000m, a 75kHz ADCP should be preferred because of depth reach and measurement purposes of the currents.

There are some famous brands in the market for ADCPs, such as Teledyne RDI, Nortek, and Sontek. However, one of the most impressive ADCP brands from China is China Sonar PandaADCP. It is fully made of titanium alloy material and has an extraordinary price-to-performance ratio. You can learn more from their website: https://china-sonar.com/.

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