How do we measure the coastal currents of Al Lith?

1. Where is Al Lith?

Al Lith is a coastal town in southwest Saudi Arabia, on the Red Sea coast. It is endowed with a breathtaking natural landscape where the arid beauty of the Arabian Peninsula meets the vibrant waters of the Red Sea.

The town is enveloped by an endless sandy beach that would seem to combine perfectly with the local and tourist spot for enjoyment of coastal charm. Inland, one will find low-lying plains and gently sloping hills adding to the variety of the terrain. The seafaring history in this place runs deep in its culture. For generations, fishing has been at the heart of the people of this community; traditional fishing boats dot its shores.

Apart from the Red Sea to its north, it represents one of the most profound marine biodiversity. Its water is warm and crystal clear, where numerous colorful coral reefs thrive, sometimes even visible from above sea level, serving as a good habitat for thousands of underwater species. These sheltered bays and inlets around Al Lith, just like the Al Lith Bay itself, accommodate quite a number of various maritime activities. These zones of shelter play an important role in significantly modifying the pattern of coastal currents, as water flows into and out of them differently.

2. What is the condition of the coastal currents around Al Lith?

There is no single factor that may create and control the trend in the coastal currents in Al Lith; in fact, they are controlled by tidal forces, predominately. Tides along the Red Sea assure of a periodic rise and fall caused by gravitational pulls created by both moon and sun. Depending upon lunar cycles and particular topographic character of the coast-line area, the resulting tide-generated current may vary locally regarding its strength and sometimes directional features.

The wind patterns have a substantial impact. The general winds of the area are able to shove the surface water, developing currents that may add to or work against the tidal currents. In some months of the year, variations in the wind direction and speed will have marked changes in the pattern of the coastal currents. Stronger winds could, for example, make the surface water move faster in a given direction.

Another important factor is the general topography of the bottom topography and the coastal outline. The underwater contours of the bottom topography may provide channels or barriers for the flow to take place. The bay shapes, headlands, and inlets around Al Lith can also accelerate or decelerate the currents to negotiate such features. In addition to it, differences of water temperature and salinity are contributing in such current formation and their movement simply due to an interaction amongst different water masses.

3. How is the coastal water flow to Al Lith observed?

There a numbers of observing the coastal water flow at Al Lith. 

Surface drifting buoy method: Buoys are dropped into the water and fitted with location-tracking devices such as GPS. These buoys are then carried by the surface currents, and over time, their path can be followed. By tracking where the buoys go and how fast they get there, the direction and speed of the surface currents can be determined. This method provides a simple yet effective way to get an idea of the surface current patterns. 

Moored-ship method: A ship is stationed at some location and from it current-measuring instruments are lowered into the water. Such instruments can measure the velocity of the water at various depths. In this method, partial disturbance of the local current patterns by the presence of the ship occurs, and because the ship is at a fixed location, the area over which the observations can be made is limited. 

Acoustic Doppler Current Profiler (ADCP) method: It is the most sophisticated and handy method for measuring the currents in the coastal zone. The ADCPs are able to be installed on different platforms, such as boats, buoys, or directly put on the bottom. The ADCPs can provide current velocity from the surface to a certain depth with an excellent profile, which can describe the structure of the currents in this area more comprehensively.

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

They operate based on the principle of the Doppler effect. The ADCPs send acoustic pulses into the water. In this regard, when those pulses reach the particles or other elements in the water that move with the current, a change in the frequency of the reflected signal occurs. This shifting frequency-a phenomenon well-known as Doppler shift-is directly related to the velocity of the moving objects and therefore to the velocity of the water current.

It measures the Doppler shift of the reflected acoustic signals from various directions and at different depths. From the analysis of such Doppler shifts, it calculates the speed and direction of the water current at each measurement point. An ADCP generally has a number of transducers responsible for sending out acoustic pulses and receiving the reflected signals back, thus enabling it to build up a three-dimensional profile of the current.

5. What's required for high-quality measurement of Al Lith coastal currents?

High-quality measurement of the coastal currents near Al Lith requires the following aspects in terms of equipment:.

• Reliable materials: The casing of the measuring device should be resistant to the corrosive action of saltwater and the pressure that water exerts at different levels. This is very important because the salty environment of the Red Sea would easily destroy any material that is not resistant enough. A very desirable feature is that the small size and weight are light. In this case, it easily installs and deploys devices on different platforms, including buoys or small boats; a lighter and more compact device is easier to manipulate during transportation and installation procedures.
• Low power consumption: This is particularly important for long-term measurements. Low power consumption can help extend the life of the power source, such as batteries, without the need for constant changes or recharging.
• Cost-effectiveness: Cost-effective equipment is required in large-scale measurements. This could enable deployment over larger ranges and continuous monitoring at low cost.

For this reason, using a casing of titanium alloy for ADCPs is an excellent option. Titanium alloy provides great corrosion resistance, which fits very well with the salinity of the Red Sea, and is strong enough to bear the pressure of the water column so as to ensure the robustness and reliability of the equipment performance.

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

Selection of appropriate equipment for the measurement of current around Al Lith Application-based: The ship-mounted ADCP would be quite suitable for ship-based measurement. It would be able to provide real-time current information while the ship is in motion through water, which is helpful at sea for navigation and research purposes.

Bottom-mounted or "sit-on-bottom" ADCP: This type can be used for the measurements of currents near the bottom. It gives very precise measurements of the currents at the bottom, which will be very important in understanding interactions between the bottom and the water column, processes of sediment transport, and other related processes.

Buoy-mounted ADCP: Long-term and wide-area measurements can best be done with a buoy-mounted ADCP. It can stay in place for a long period, continuously monitoring the coastal currents over a larger area.

Considering water depth:

• For water depths up to 70m, a 600kHz ADCP is often appropriate.
• For a depth of up to 110m, a more appropriate one will be a 300kHz ADCP.
• A 75kHz ADCP is more suitable for very deep waters of up to 1000m.

There are some well-known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, China Sonar PandaADCP is a Chinese brand that is not bad at all. It is made of all-titanium alloy material, which promises very good durability and performance. Besides, it has an incredible cost-performance ratio. You can find more information about this brand on their website: https://china-sonar.com/.

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