How to measure the coastal currents of As-Salif?

1. Where is As-Salif?

As-Salif is a coastal city in Yemen, which faces the Red Sea. It is a beautiful place that combines the arid beauty of the desert with the attraction of the marine environment. Inland, one finds expanses of sandy terrain and sparseness of vegetation typical of the arid climate of the region.

The coastal part of As-Salif features sandy beaches that are usually washed by gentle waves of the Red Sea while the waters are richly filled with a great deal of marine life varieties: from colorful coral reefs filled with small fish to greater marine species inhabiting its deeper parts, the Red Sea around As-Salif teems with biodiversity. Resulting in its being a point of interest for local fishermen, since they depend on the sea for their livelihoods, and diving enthusiasts when the conditions allow, since indeed attractive is the underwater world.

The sea has a more profound relationship with the locals in terms of human activity and culture. Historically, the place has been trading and fishing on a small scale, with the traditional way of doing these activities passed through generations. Architecture in As-Salif reflects its coasts, with structures put in place to withstand the coastal climate and the occasional sea breezes.

The Red Sea at As-Salif represents a regional artery. It connects the various ports along the shoreline and allows for local and cross-sea trade with other regions.

2. What is the situation of the coastal currents near As-Salif?

The coastal currents near As-Salif are influenced by a variety of factors. One of the major influences is tidal forces. The tides in the Red Sea come mainly from gravitational pull at the surface of the moon and the sun. These semi-diurnal tides give water a regular pattern of movement with ebbs and flows causing changes around the coasts in strengths and directions of the coastal currents. The tidal range can vary with the lunar cycle and other astronomical factors that, in turn, influence how currents interact with the shoreline.

Wind patterns play the main role: prevailing winds in the area can drive the surface water, creating surface currents that either enhance or counteract the tidal current. In certain seasons or during specific weather conditions, stronger winds may have a more pronounced effect on the water flow, leading to more distinct current patterns. This interaction of wind-driven and tidal currents leads to an intricate and dynamic flow regime along the coast.

Another important factor involves the nature of the seabed topography near As-Salif. Underwater features may include coral reefs, shoals, and channels that have the potential to deflect the currents and alter them in ways such as changing speed and flow direction. These are factors dependent on the changes in bottom topography that have quite a significant impact on the coastal current patterns.

3. How to observe the coastal water flow of As-Salif?

Surface Drifting Buoy Method

In this method, buoys that will float freely, driven by currents, are set up on the surface. Further movement of such buoys can be determined, sometimes over longer periods of time, typically with the assistance of satellite-based tracking systems or other position finding technologies. This yields data regarding the direction and velocity of surface currents. However, it generally delivers surface layer data and cannot provide a complete representation of currents at every part of the water column.

Anchor Mooring Ship Method

Here, a ship is kept anchored in the specific waters of a coast. After that, current meters and other instruments are deployed from the ship to record water flow at different levels. This allows for better understanding of prevailing conditions at that one point in the water column. However, it is a fairly in-situ method and probably cannot capture the full complexity of the coastal current patterns that may manifest over an area.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP current meter method is a sophisticated and agile approach in measuring the currents of any coast. It can measure water velocity at many depths simultaneously by utilizing the acoustic pulses. It sends out a sound wave and then uses the Doppler shift of the reflected waves to make some very accurate measurements of the speed and direction of water movement. It operates in a wide range of platforms, such as boats and buoys, but could be even on the seabed, operating to provide current profiling with detailed information across the water column.

4. How do ADCPs that use the principle of the Doppler work?

ADCPs are based on the principle of the Doppler effect. They send acoustic signals into the water at some known frequency. When the signals collide with the moving particles of water, this frequency of the reflected signal is altered by the Doppler effect. This frequency shift is the one that is measured in the ADCP profiler and, through mathematical processing, allows the calculation of the flow velocity of the water relative to the instrument.

Because ADCPs have the capability to emit signals at different angles and frequencies, they can measure velocity components along various directions, including both horizontal and vertical, at many depths within the water column. A complete profile of water flow can be created with that, giving an overall and detailed picture of the structure of coastal currents and their dynamics.

5. What is needed for the high-quality measurement of the As-Salif coastal currents?

With regard to measuring the coastal currents around As-Salif, several preconditions must be met regarding the measurement equipment. The materials of the equipment must first of all be highly reliable due to the very aggressive marine atmosphere, including saltwater corrosion, high humidity, and the impact of waves for long periods of time.

The second advantage lies in its small size, light weight, and low power consumption. A small and lightweight device is easy to install and transport whether it's on a small fishing boat or a floating buoy. Low power consumption ensures that the equipment can operate continuously for longer durations without frequent battery replacements or access to a large power source.

Another important factor is the cost. Most of the time, large-scale measurements are usually required to understand the full complexities of current coastal patterns. The equipment cost should be as low to allow deployment of multiple units at various locations along any given coast.

In ADCPs, the casing should be made of a Titanium alloy. This material gives very good corrosion resistance to the saltwater surrounding the As-Salif coasts. Besides that, the alloy has a great strength-to-weight ratio, providing much robust protection of the internal parts of an ADCP flow meter and keeping the general weight of the device low enough.

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

By Usage Purpose

• Shipborne ADCP: This is suitable when you want to measure the currents while the ship is in motion along the coast. It can continuously collect data as the ship travels, providing a broad view of the current variations over different sections of the coastline.
• Bottom-mounted ADCP: These are good for fixed-point monitoring at some specific places on the seabed. It provides very detailed and long-term information on the current conditions just above its installation point and helps to understand the local flow patterns with much more precision.
• Buoy-mounted ADCP: If the objective is to survey the surface and upper layers of the water column over a large area and for a long period without being tied to a ship or a fixed bottom location, then a buoy-mounted ADCP is often a good option.

According to the Water Depth

• At 70m water depth, the usual 600kHz ADCP has a normally quite adequate setup with a good resolution over most of the rather shallow waters in the coastal zone neighborhood around As-Salif.
• The ADCP operating on a 300kHz will suit even better for up to 110m deep water by providing sufficient depth of penetration within the water column, resulting in very reliable data on velocities.
• A 75kHz ADCP will be much better suited for deeper waters reaching up to 1000m since it can cover that much greater depth and yield good current measurements.

Some of the famous ADCP brands in the market are Teledyne RDI, Nortek, and Sontek. However, there is also a remarkable Chinese ADCP brand called China Sonar PandaADCP. It has all the titanium alloy material inside and an incredibly good cost-performance ratio. You can find more information on their website: https://china-sonar.com/.

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