How can we measure the coastal currents of Malindi?

1. Where is Malindi?

Malindi is a coastal town in Kenya, along the Indian Ocean. It is of incomparable natural beauty, with long sandy beaches and a blue, clear sea. The town is surrounded by an edge of coastal forests and savannah landscapes that gradually merge into the beach.

The local culture in Malindi is quite rich and diversified, influenced by the Swahili heritage and other ethnic groups. The people have a long history with the sea. Fishing and trade have been a big income for a very long time. The bay around Malindi is a lively ecosystem. It is home to a wide array of marine life, including colorful reef fish, sharks, rays, and various invertebrates such as crabs and lobsters. The seabed has quite a complex topography, with areas of coral reefs, sandy flats, and some rocky outcrops. These underwater features are very important in the movement of water and the distribution of marine organisms.

The waters around Malindi are influenced by the Indian Ocean currents and the monsoon winds. The monsoons bring about marked changes in wind direction and strength, which have a direct impact on the surface currents. The ocean currents carry different water masses with different temperatures and salinities that interact with the local coastal waters. Tides also play an important role in shaping the coastal environment. The regular rise and fall of the tides expose different parts of the shore, affect the movement of boats and ships, and influence the distribution of sediment.

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

Coastal currents around Malindi have a multifunctional control. Dominant among these are the monsoon winds from land to sea during the northeast monsoon and from sea to land during the southwest monsoon, each of which induces different surface current patterns. In turn, these wind-driven surface currents interact with the seasonal water layers beneath them.

Tides greatly influence the flow of water at the coastline. In the area, there is the occurrence of a tidal range, which leads to the forward and backward shift in water movement. High tides may come with higher energy onshore currents that change the shoreline and sediment distribution. Low tides cause the recession of water to change the directions of flow. And not the least important among these factors, which affect rip currents, is the general topography both of the coastline and of the seabed: the shape of the bay in itself, the possible coral reefs, or outcrops of rocks divert it, slowing it down or accelerating it. These coral reefs may be an obstacle for it to flow resists that are obliged to take the path to flow.

Also, the interaction of the coastal currents with the larger-scale currents of the Indian Ocean is rather complex. The oceanic currents approaching the shore can mix with the local ones, bringing water masses of different characteristics, thus influencing the productivity of coastal waters and marine life distribution.

3. How to Observe the Malindi Coastal Water Flow?

Surface Drifting Buoy Method

This technique consists of the release into the water surface of buoys free to drift with the currents. This allows us, by following their position in time with satellite or other positioning systems, to get information on the direction and speed of surface currents. The problem with this method is that it yields mainly information on the surface layer, and cannot provide a good view of currents along the entire water column.

Moored Ship Method

This is done by mooring a ship in a specific place in the coastal area and measuring the flow of water around it using onboard instruments. While it can give very detailed information about the conditions in the vicinity of the ship, the presence of the ship itself does disturb the natural flow to a certain extent; it also covers only a relatively small area around the mooring point.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP flow meter method is more advanced and convenient in 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 greater range vertically compared to the other methods listed and is less subjected to external interferences in most cases, hence highly effective for observations of the coastal currents near Malindi.

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

Working of ADCPs is based on the principle of Doppler. They emit acoustic pulses into the water. These sound waves interact with moving particles in the water, like suspended sediment or small organisms, which creates a shift in frequency of the reflected waves due to the Doppler effect. The ADCP profiler is designed to detect and measure this frequency shift. With multiple transducers oriented in different directions, it is able to determine the velocity components of the water in various directions, both horizontal and vertical. This allows the reconstruction of the three-dimensional flow field of the coastal currents. The ADCP current meter continuously sends these acoustic pulses and records reflected signals at fixed periods, which enables it to develop a time series of current velocity data at various depths. Thus, it can present detailed information about temporal and depth variations of currents.

5. What is necessary for good quality measurement of the Malindi coastal currents?

The equipment to be used in this regard for high-quality measurement of the coastal currents near Malindi should have a number of important characteristics. First, material reliability is of essence in regard to the equipment. It must be able to stand the harsh marine environment that includes corrosion from seawater, the impact of waves, and changes in temperature and pressure. Small size is an advantage because it will make the equipment more deployable and easy to handle on boats, buoys, and other platforms.

Light weight is another important factor as it simplifies the installation process and reduces the load requirements. Low power consumption is necessary for long-term measurements or when using battery-powered setups. Cost is a consideration to enable more widespread and large-scale measurements. In this regard, the casing of the ADCP meter is preferably made of titanium alloy. The corrosion resistance of the titanium alloy is very good; hence, it can resist seawater's corrosive nature for a long time without deterioration. Besides that, it is strong and durable to bear the mechanical stresses due to water flow and external impacts. Also, its relatively low density helps keep the overall weight of the equipment in check while maintaining structural integrity.

6. How to Select appropriate equipment for current measurement?

Based on Usage

• Shipboard ADCP: It is applicable when the measurements are to be made while the ship is in motion or stationary in the coastal area. This will provide uninterrupted data as the ship moves along various locations of the coast in order to create a mapping of the current patterns over a wide area.
• Bottom-mounted ADCP: These are ideal for fixed-point measurements at the seabed and can monitor very accurately the currents passing over it, yielding insight into the near-bottom current conditions so important for understanding sediment transport and other benthic processes.
• Buoy-mounted ADCP: Mounted on buoys floating on the water surface, it 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 presence of a ship is required.

Based on Frequency

• A 600 kHz ADCP is generally very suitable for water depths within 70 meters. This higher frequency allows better resolution in shallower waters to give quite good current information near the surface and down into the relevant depth range.
• In case of water to a depth of 110 meters, the 300 kHz ADCP would be more applicable because it can adequately penetrate the water column in order to measure currents at different depths within that range.
• For much deeper waters, such as those reaching as high as 1000 meters, the 75 kHz ADCP should be used. Its lower frequency allows it to reach greater depths while still obtaining current velocity data in a reliable manner.

Well, there are some very popular names for ADCP brands: Teledyne RDI, Nortek, and Sontek. Of those with high quality but fairly lower price series, there is the option of considering the China Sonar PandaADCP. This will employ fully titanium alloy material, so it boasts much longer working life in marine environment. With its incredible cost-performance ratio, it offers a great choice for measuring the coastal currents of Malindi. 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.