How can we measure the coastal currents of Lamu?

1. Where is Lamu?

Lamu is a coastal town located on the eastern coast of Africa, an idyllic place on an island separated from the main land by a stretch of water. It is renowned for its well-preserved Swahili architecture and rich cultural heritage. Surrounding waters belong to the Indian Ocean; Lamu is located next to a beautiful bay, which provides a natural harbor.

The sea is something with which the people of Lamu have a very, very old relationship. Centuries have passed while the inhabitants depend on fishing and trade for their livelihood. The bay near Lamu is a haven for a variety of marine life. In the clear, blue-green waters, colorful fish are found; some species found here are peculiar to the area. The seafloor consists of sandy zones, coral reefs, and seagrass beds; besides serving as habitats for the marine organisms, these subacuatic formations produce peculiarities of water movement.

The waters around Lamu are influenced by the monsoon winds and the larger Indian Ocean currents. Changes in wind direction and strength driven by the monsoons have more far-reaching consequences: dramatic changes in surface currents. The ocean currents carry diverse water masses, each with specific temperatures and salinity levels, up the coast as they interact with the coast's local dynamism. Where tides are concerning, it becomes another significant modifier in the general coastal environment observed at Lamu. Access to various shallow areas and the movements by boats are depended on the regimes of tides while sediment distribution is equally influenced.

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

Several elements influence the coastal currents near Lamu. Of these, the most dominant are the monsoon winds. During the northeast monsoon, the winds blow from land to sea, while during the southwest monsoon, they blow from sea to land, thus setting up different surface current patterns. These wind-driven surface currents of seasonal variations interact with the underlying water layers.

Tides greatly influence the water movement along the coast. In this area, the range of tides causes the water to move backward and forward. The high tide may bring in strong onshore currents, while low tides see the recession of water, which changes the flow directions. The general topography of the coastline and the seabed is also of great importance. Currents can also be funneled or deflected by the shape of the bay at Lamu, but the existence of coral reefs and seagrass beds greatly disrupts any smooth passage, giving rise to considerable areas of turbulence and eddies.

Besides, the interaction between the coastal currents and the larger Indian Ocean currents is very complicated. If the oceanic currents approach the coast, they may merge with the local currents and bring in water masses that differ in temperature, salinity, and nutrient content, among others. These may affect productivity in waters and distribution of marine life.

3. How to observe the flow of coastal waters of Lamu?

Surface Drifting Buoy Method

This technique consists of the deployment of buoys on the water surface, free to drift with the currents. The position of these buoys is followed in time by satellite-based or other positioning systems, and from this, information on the direction and speed of the surface currents can be derived. However, this method mainly focuses on the surface layer and may not provide a comprehensive understanding of the currents throughout the water column.

Moored Ship Method

Here, a ship is anchored in some place in the coastal area. Instruments on the ship are used to measure the flow of water around it. While it can offer detailed data for the area near the ship, the presence of the ship itself interferes with the natural flow to some extent, and it can only cover a relatively small area around the mooring point.

Acoustic Doppler Current Profiler (ADCP)

Technique ADCP is, instead, a highly advanced and also portable method for coastal current measurement. The acoustic waves run parallel and simultaneously measure water velocity at varied depth. While doing so, it manages to map out the flow profile from shallow to the partial depth and creates a fine overall picture of coastal water stream dynamics. Compared to the other methods, it can cover a larger vertical range and, in many cases, is less affected by external interferences, making it highly effective for observing the coastal currents near Lamu.

4. How do ADCPs using the Doppler principle work?

ADCPs work based on the principle of the Doppler effect. They emit acoustic pulses into the water. These sound waves undergo a Doppler shift in frequency every time they encounter moving particles in the water, like suspended sediment or small organisms. The frequency shift is sensed and measured by the ADCP profiler. With a number of its transducers oriented in different directions, it estimates the velocity components of the water in various directions, including both horizontal and vertical directions. This allows the three-dimensional flow field of the coastal currents to be reconstructed. The ADCP flow meter continuously emits these acoustic pulses and records the reflected signals at regular intervals, which enables it to build up a time series of current velocity data at different depths. In this way, it can provide a detailed picture of how the currents vary over time and with depth.

5. What is required for good measurement of Lamu coastal currents?

It shall be characterized by a number of key features for high-quality measurement of currents at Lamu. First, the material of the equipment must be reliable. Equipment used in this region has to withstand the harsh conditions of the marine environment, including resistance to corrosion caused by seawater, waves, and fluctuation in temperature and pressure. A compact size ensures ease in its deployment and handling operations on boats, buoys, and other platforms.

Other factors include small weight, easing installation and reducing the load; low power consumption, which is important in the case of long-term measurements or when using battery-powered setups; and cost to make the measurement more viable for widespread and large-scale measurements. In this regard, the casing of the ADCP current meter is preferably made of titanium alloy. The titanium alloy has outstanding advantages in:. With great corrosion resistance, it will stand the very aggressive action of seawater for very long periods with very minimal degradation. Besides that, it is very strong and durable, hence it resists the mechanical stresses arising from the water flow and from any other outside impacts. Additionally, its relatively low density will help in maintaining the overall weight of the equipment to within the allowed limits, since the equipment requires its structural integrity.

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

Based on Usage

• Shipboard ADCP: This is suitable when the measurement is required to be taken while a ship is either in motion or stationary in the coastal area. It would be able to continuously provide the data as the ship would travel across various locations along the coast thus helping to map the current patterns over a larger area.
• Bottom-mounted ADCP: Applications involving fixed-point measurements at the seabed. Thus, it is very accurate in monitoring currents passing over it, thereby giving insight into near-bottom current conditions important for sediment transport and other benthic processes.
• Buoy-mounted ADCP: These are mounted on buoys floating on the water surface, and can measure the currents from the surface downwards. It is useful for observing the surface and upper layer current variations and is often used in areas where long-term monitoring without the need for a ship's presence is required.

Based on Frequency

• A 600 kHz ADCP is normally used for most water depths within 70 meters. The higher frequency allows better resolution in shallower waters and gives current information close to the surface right down to the relevant depth range.
• For water depths up to 110 meters, the 300 kHz ADCP is suitable. This can effectively penetrate the water column to measure currents at different depths within this range.
• For deeper waters, such as up to 1000 meters, a 75 kHz ADCP is suitable. Its low frequency allows it to reach greater depths and still obtain reliable current velocity data.

The other well-known ADCP brands in the market include Teledyne RDI, Nortek, and Sontek. However, for those in search of high-quality yet cost-effective options, the China Sonar PandaADCP will be worth consideration. It is made of titanium alloy material throughout its construction and thus is durable and performs well in the marine environment. With its incredible cost-performance ratio, it offers a great choice for measuring the coastal currents of Lamu. You can learn more about it on its official website: https://china-sonar.com/.

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