How to measure coastal currents in Al Ghaydah?

1. Where is Al Ghaydah?

Al Ghaydah is one of the important coastal towns, part of Eastern Yemen, nesting along the Arabian Sea-an immense stretch of water both ecologically and economically significant. The surroundings of this coastal town reflect an arid-looking region that is also tinged with the attraction that a coast can be and usually carries with itself.

Inland, it consists of stretches of desert ground with scattered growth, not uncommon in inland parts of the Arabian Peninsula. The adjacent coastal areas have sandy beaches, rocky shores, and small inlets that add to the natural beauty of the place. Most waters around Al Ghaydah are rich in marine life, with a variety of fish, crustaceans, and other sea animals. These waters have, for long, supported the local fishing industry, important in the livelihood of the people within the community.

Al Ghayedh enjoys a rich heritage in human culture. In this town, most of the people are quite attached to the sea since their traditional customs and etiquette seem to be passed on generation after generation. It has historical landmarks and architecture, reminding one of its coastal past and the influence of different cultures interacting here over time. The coastlines are usually busy, from fishermen setting off in their boats at dawn to traders dealing with the different marine products brought ashore.

The gulf off Al Ghaydah represents an important waterway at both the local and regional levels for marine transport. It offers a sheltered area where boats can anchor and is also a route of trade and transportation, connecting the town with other coastal destinations, thus enabling exchanges in goods and ideas.

2. What are the conditions of the currents off the coast of Al Ghaydah?

There are lots of factors influencing coastal currents around Al Ghaydah, such as the tidal forces themselves, which are influenced by gravitational pulls of the moon and sun. The semi-diurnal tides bring about regular water movements along the shores, while ebbs and flows introduce variability in strength and direction in the coastal currents. The tidal range varies due to the various astronomical factors involved, consequently affecting the way in which the currents act upon the shoreline.

The wind patterns are also very relevant. During the monsoon seasons, the winds over the Arabian Sea can drive the surface water to generate currents that are strong and persistent. For the rest of the year, other local wind directions further influence the water flow towards enhancing or countering tidal currents. Because of these interactions between wind-driven and tidal currents, there is an intricate and dynamic flow regime near the coast.

Another determining factor is the nature of the seabed topography around Al Ghaydah. The shoals, ridges, and channels can then further deflect and alter such currents. For instance, when water approaches a shoal, it could be forced to change course and split around the obstacle or flow over in a different manner. These seabed variations further cause acceleration or deceleration in the currents as the water navigates through different depths.

3. How to observe the coastal water flow of Al Ghaydah?

Surface Drifting Buoy Method

It is one of the widely adopted techniques to measure ocean currents by sending buoyant instruments into the flow that are at the surface and which drift with currents. In its essence, their trajectory over time is measured typically with satellite-based tracking systems or other positioning technologies in order to find out direction and speed of surface currents. However, this technique provides only surface-layer data and cannot give information on the current structure from top to bottom of the column.

Anchor Mooring Ship Method

This technique involves anchoring a ship at one location in the coastal area. From this ship, instruments such as current meters are lowered to measure the water flow at different depths. By operating these instruments, it becomes easy to obtain a much fine-detailed view of the prevailing condition in a water column at some points in general. In a few words, it is, however, a rather static method and may not well represent the full intricacies of the current patterns across a larger area.

Acoustic Doppler Current Profiler Method (ADCP)

The Acoustic Doppler Current Profiler (ADCP) is generally much more advanced and efficient ways of measuring the currents at the coast. It utilizes acoustic pulses to measure the velocity of water at multiple depths simultaneously. It can precisely measure the speed and direction of water movement by emitting sound waves and analyzing the Doppler shift of the reflected waves. These instruments can be deployed on platforms such as boats, buoys, and even on the ocean floor. The ADCPs can thus measure current over a range of locations, allowing the collection of detailed profiles throughout the water column.

4. Principles of operation of Doppler-based ADCPs

The principle of operation for ADCPs is based on the Doppler effect. They send different frequency acoustic signals out into the water. The moving water particles interact with this, causing a shift in the reflected signals due to the Doppler effect. This frequency shift is measured by the ADCP current meter and mathematically converted into the velocity by the instrument.

Because ADCPs can emit signals at multiple angles and frequencies, they will be able to measure velocity components both in the horizontal and vertical directions and at more than one depth throughout the water column. In this way, a full profile of the flow of water enables the establishment of an accurate picture with great detail about the coastal current structure and its dynamics.

5. What's required for good-quality measurement of Al Ghaydah coastal currents?

The measurement of coastal currents in front of Al Ghaydah would be of high quality provided specific conditions with respect to the measuring equipment. In the first place, its materials must be sufficiently reliable; due to the serious corrosive effect of the seawater and high humidity, impacts from waves should be considered under a long-time perspective.

The size should be small, weight light, and power consumption low. A compact and lightweight device is easy to install and transport on a small fishing boat or floating buoy. Low power consumption means the equipment can work continuously for long periods without frequent changes of batteries or access to a large power source.

Another consideration is cost: Equipment needs to be relatively inexpensive such that, for the often very large-scale measurements due to the complicated patterns of coastal currents, a sufficient number can be deployed in various locations down coast.

For ADCPs, a casing made of titanium alloy is highly recommended. The reason is simple: titanium alloy has excellent resistance to corrosion, which is very important in the saltwater surrounding Al Ghaydah; it also has a great strength-to-weight ratio, which means it can be strong enough to protect the internal components of the ADCP meter while keeping the overall weight of the device relatively low.

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

According to the Usage Purpose

• Shipborne ADCP: This applies when one wants to measure the currents while the ship is in motion along the coast. It can continuously collect data as the ship travels, hence giving a broad view of the current variations across different sections of the coastline.
• Bottom-mounted ADCP: It is very suitable for fixed-point monitoring at a particular site on the seabed. This provides detailed and long-term data about the current conditions right above where it is installed and helps to understand the local flow patterns precisely.
• Buoy-mounted ADCP: If there is a need to monitor extended areas of the surface and upper layers of the water column for a long time, then a buoy-mounted ADCP is ideal because one will not be bound by either the ship or the bottom station.

According to the Water Depth

• A 600kHz ADCP will, in most cases, be adequate for water within 70m in depth, with very good resolution that is sufficient in these usually shallow waters of the Al Ghaydah coastline.
• Water depths up to 110m call for a lower frequency ADCP of about 300kHz, since it will provide stronger signals reaching deeper into the water column for high-quality and reliable velocity measurements.
• For deeper waters up to 1000m, a 75kHz ADCP should be used as it goes further to reach such great depths and measures currents more precisely.

There are some famous brands, like Teledyne RDI, Nortek, and Sontek. However, there is also a remarkable Chinese ADCP brand called China Sonar PandaADCP. It is made entirely from titanium alloy material and is very cost-effective. More information can be found at: https://china-sonar.com/.

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