How can we measure coastal currents in Qeshm?

1. Where is Qeshm?

Qeshm is a picturesque island city that is part of the Persian Gulf, lying to the south of Iran. The island is a rare geological and cultural treasure. Different landscapes are found, such as arid deserts and rugged mountains, turning into sandy beaches and mangrove forests along the shores.

Qeshm's local culture has been a unique and dazzling amalgamation of the age-old, quintessential Iranian values along with a shade of its sea-trading heritage. People on this island have had great proximity to the sea for very long, and traditional employment in fishing and diving has persisted here. The waters around Qeshm in the Persian Gulf are warm and shallow in most places, providing a haven for various species of fish, crabs, and mollusks. The island borders a number of important shipping lanes and is also in proximity to several smaller islands and peninsulas, thus representing a very complex system of coastlines and marine ecosystems.

From an ecological viewpoint, the mangrove forests along the Qeshm coast are of utter importance. They serve as a natural barrier to the coast against erosion and are also nurseries for various types of fish and other marine animals. The waters here are rich in nutrients owing to the interaction between tidal currents and terrestrial input.

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

A number of factors influence the coastal currents near Qeshm. Tidal forces, a dominant influence, act to stir the tides in the Persian Gulf due to the gravitational pull of the moon and the sun. During high tide, water rushes in, and during low tide, it recedes. Such tidal currents vary in strength and direction on the basis of the lunar cycle and local topography.

The contribution of the wind-driven currents is also great. Regional prevailing winds can push the water surface to generate a surface current. The winds vary in direction and speed seasonally, which will again affect the coastal circulation patterns. In addition, the bottom topography around Qeshm plays an important role in determining the currents. The underwater ridges, channels, and shoals will deflect and alter the course of the flow of water. Shallow areas make the currents slower, whereas deeper channels hasten it.

3. How to observe the Qeshm coastal water flow?

The surface drift buoy method utilizes buoys deployed at the surface, which are left to drift with the currents. These buoys are commonly fitted out with GPS or any other location-finding instruments. In this way, by tracking the buoy movement over time, one will be able to find the direction and speed of the currents on the surface. However, this is a method that gives very limited information about the vertical structure of the currents and mainly covers the surface layer.

Anchor Ship Method: This involves mooring a ship in one fixed position in the water close to the Qeshm coast. The ship would carry measuring equipment, including current meters, to record the flow of water around the ship. This can give quite good measurements close to the ship; however, this is not very effective to cover a wide area and at different depths at the same time.

Acoustic Doppler Current Profiler: It is a more advanced and proficient method for measuring the coastal current. By using the Doppler effect, it measures the water's velocity at different depths in the column of water. The instrument emits an acoustic pulse in the water and then analyzes the frequency shift of the reflected signals from the moving particles of water. In that way, good profiling in the vertical sense right from the surface down to the seabed has been able to understand the currents.

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

ADCPs work on the principle of the Doppler. Transducers, which are part of ADCPs, send acoustic pulses into the water. Due to the Doppler effect, when pulses of sound come across the water particles that happen to be in motion, the frequency of the waves that bounce back changes. The frequency shift is proportional to the velocity of the water particles relative to the ADCP meter.

Usually, ADCPs are designed with several transducers that are able to measure the water velocity in various directions. By sending pulses in different directions and analyzing the frequency shifts of the reflected signals, the ADCP current meter calculates the three-dimensional velocity of the water-that is, the east-west, north-south, and vertical directions. The time difference for the sound waves to travel from the transducer to the water particles and back is also calculated. By this information and by frequency shift, the speed and direction of the water flow at various depths are determined with very high accuracy.

5. What's needed for high-quality measurement of Qeshm coastal currents?

The equipment to be deployed for measuring the coastal currents near Qeshm needs to have some essential qualities. First of all, the equipment has to be made from a material which is reliable and can bear the harsh marine environment. The device must be small in size and lightweight for easy deployment and handling. Low power consumption is very important so that it will enable continuous operation, especially in long-term monitoring.

The casing of the ADCP current profiler is preferably made from titanium alloy. The corrosion-resistant properties of the titanium alloy are excellent, which is very important in the saline environment of the Persian Gulf. Besides this, it has a high strength-to-weight ratio, meaning it can provide durability while keeping the equipment relatively light. This material will be able to resist pressure changes associated with different water depths.

6. Selection of equipment for current measurement?

In choosing equipment for current measurement at Qeshm, the following can be considered.

Depending on the purpose of usage, the types of ADCPs to be selected will vary. For example, ship-borne ADCP could be installed on a vessel and measure currents while the ship is moving along. This is useful in covering a wide area of the coastal waters. A bottom-mounted ADCP fixed on the seabed provides stable measurements of the currents passing over it. It is ideal for long-term monitoring at a specific location. A buoy-mounted ADCP enjoys the advantage of mobility and easiness in deployment, therefore allowing measurements in different areas.

Regarding the frequency of ADCP profiler, it is related to water depth. For water up to 70m deep, a 600kHz ADCP is appropriate because it gives highly resolved data in shallow waters. For water depths up to 110m, 300kHz is more suitable. In deeper waters, such as those near 1000m, the 75kHz is recommended as it allows for accurate measurements over a large vertical range.

There are well-known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option with good quality, the China Sonar PandaADCP is worth considering. It is made of all-titanium alloy material and offers an excellent cost-performance ratio. You can find more information on its website: https://china-sonar.com/.

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