How can we measure the coastal currents of Watamu?

1. Where is Watamu?

It's a paradise located on the coastline of the Indian Ocean. It has nice beaches, coral reefs full of life, and a generalized marine ecosystem. The town is set amidst coastal vegetation and sandy dunes that gradually descend into the sea.

The local people of Watamu have an attachment to the ocean. Tourism and fishing are the two major sectors in the local economy. The marine life is varied around Watamu Bay, ranging from brightly colored tropical fish flitting among the corals to large marine mammals that occasionally visit the shallow waters and show activity at any one time. The seabed topography is uniquely mixed; it consists of coral gardens, patches of sandy seabed, and small rocky outcrops in places. These features not only provide habitats for the marine life but also play a very important role in the movement of the coastal currents.

The waters around Watamu are influenced by the Indian Ocean currents and the seasonal monsoon winds. Changes in wind direction and strength brought about by the monsoons have a direct impact on the surface currents. The ocean currents carry water masses of different temperatures and salinities that interact with the local coastal waters. Tides also play an important part in shaping the coastal environment. The regular ebb and flow of the tides expose different parts of the shore, affect the movement of boats, and influence the distribution of sediment.

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

These currents along Watamu result from combined action and interplay of a variety of processes. A principle basis of the aforementioned may be presented by monsoon winds. In various monsoon periods, surface top wind-driven currents change. The northeast monsoon may push water in another direction than what results during the southwest monsoon. Also, interaction between these surface currents with the deeper layers is done in producing this flow relatively complicated pattern.

The tides importantly influence the coastal currents. Because of the tidal range, there is a regular backward and forward motion of water in the coast. Stronger onshore currents associated with high tides alter the shoreline and distribute sediments around the coast. With low tide, the receding water makes flow directions change. Notably, it also depends much on the topography of the general coastline and the seabed bottom topography. The shape of the bay and the coral reefs or rocky outcrops will be able to deflect, retard, or speed up the currents. Reefs may form dams that force the water to divide around them and create areas with turbulence and eddies.

The interaction of coastal currents with larger-scale currents in the Indian Ocean is rather complicated. In some areas, oceanic currents can come into contact with local ones, and different water masses are mixed concerning temperature, salinity, and nutrient content. This influences productivity and species distribution along the coast.

3. How to observe the coastal water flow of Watamu?

Surface Drifting Buoy Method

This technique consists of the deployment of buoys on the water surface, free to drift with the currents. The buoy's position, as determined by satellite or other positioning systems over time, will provide information on the direction and speed of the surface currents. This method mainly focuses on the surface layer, and it does not provide a full understanding of the currents throughout the water column.

Moored Ship Method

In this method, one vessel is moored at some position in the coastal zone; instruments on the ship measure the water flow around the vessel. Though it can give detailed information in the area surrounding the ship, the presence of the ship could disturb the natural flow partially, and only a small area around the mooring position can be covered.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP current meter is a more advanced and convenient way of measuring the coastal currents. It uses acoustic waves to measure simultaneously the velocity of water at different depths. This enables it to create a detailed profile of the current from the surface down to a certain depth, giving a comprehensive understanding of the coastal water flow structure. It can cover a larger vertical range compared to the other methods and is less affected by external interferences in many cases, making it highly effective for observing the coastal currents near Watamu.

4. How do ADCPs using the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. They send acoustic pulses into the water. These sound waves, interacting with moving particles in the water-like suspended sediment or small organisms-the frequency of the reflected waves shifts due to the Doppler effect. It is this frequency shift that the ADCP current profiler was designed to detect and measure. With more than one transducer oriented differently from each other, it would, therefore, be able to find components of water velocity in several directions and measure 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, thus enabling 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 needed to measure the currents of Watamu coastal currents with high resolution?

The equipment to be used for measurement has to have certain characteristics for high-quality measurement of the coastal currents around Watamu. First is the material reliability of the equipment; it has to be able to bear up against the harsh marine conditions, which include corrosion by seawater, the impact of waves, and changes in temperature and pressure. Small size is also an advantage since it enhances ease of deployment and handling on boats, buoys, and other platforms.

Other important factors to be considered relate to light weight, which easily installs the transducer and reduces its load requirements; low power consumption, especially over long-term measurements or from battery-powered outfits; and reduced cost to promote wider and increased measurement. In this latter respect, for example, titanium alloy is preferred for casings of the ADCP profiler. It is for this reason that titanium alloy reveals excellent corrosion resistance, being able to resist the corrosive nature of seawater for a long time without showing any significant degradation. Besides, it is strong enough and durable to resist mechanical stresses from water flow and external impacts. In addition, its relatively low density helps in keeping the overall weight of the equipment in check while maintaining its structural integrity.

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

Based on Usage

• Shipboard ADCP: This is suitable for when the measurement is required while the ship is in motion or stationary in the coastal area. It can provide continuous data as the ship traverses across different locations along the coast, thereby helping to map out current patterns over a larger area.
• Bottom-mounted ADCP: Best suited for fixed-point measurement from the seabed, enables the recording of currents that pass over it in providing a very good view of the current condition near the bottom and, hence, helping to assist the study of sediment transport and other benthic processes.
• Buoy-mounted ADCP: On buoys floating on the water surface, it is possible to measure currents from the surface downwards. It serves useful for observing surface and upper layer current variations, and finds common application where there is a requirement for long-term monitoring without the necessity of a ship's presence.

Based on Frequency

• A 600 kHz ADCP is generally the best choice when the water depths are within 70 meters; the higher frequency gives better resolution in shallower waters, supplying detailed current information close to the surface and down into the relevant depth range.
• For water depths up to 110 meters, a 300 kHz ADCP is more adequate. It can well penetrate the water column to measure currents at different depths within this depth range.
• For much deeper waters, like up to 1000 meters, a 75 kHz ADCP would be quite recommended. Its lower frequency can dive deeper and still get good data about the current's velocity.

Some of the well-known brands for ADCP are Teledyne RDI, Nortek, and Sontek. For those clients whose intention is to have excellent quality with fair prices, one can have the China Sonar PandaADCP. The hull is made of a titanium alloy material to ensure excellent durability and performance in the marine environment. Due to its incredible cost-performance ratio, it offers a great choice for measuring the coastal currents of Watamu. 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.