How do we measure the Tanga coastal currents?

1. Where is Tanga?

Tanga is a lively city located in northeast Tanzania along the banks of the Indian Ocean. It is the capital city of Tanga Region and an important Tanzanian economic and cultural hub. The climate in Tanga is tropical with extremely hot temperatures throughout the year, with very wet and dry conditions. Being a beach town, wedged in between green hills and the wide ocean, it has a beautiful and picturesque landscape.

Geographically, Tanga is located at the mouth of Pangani River, which empties into the Indian Ocean. Saltwater and freshwater mixing form a rich, dense estuarine environment with a variety of distinct plant and animal life. All the surrounding waters belong to the broader Western Indian Ocean, which is famous for rich coral reefs, diversity of marine organisms, and complex oceanic processes. The coasts of Tanga have a mix of sandy beaches, mangrove forests, and rocky outcrops and is inhabited by a mix of birds, fish, and other sea creatures.

Tanga is a culturally an ethnic melting pot with several ethnic groups including the Swahili, Pare, and Shambaa, among others. This can be seen in the cuisine of the city, languages, and customs. Swahili also exercises enormous influence, visible in indigenous houses, music, and bustling spice, textile, and craft fairs. They enjoy fine annual festivals such as the Mwaka Kogwa festival celebrated by the Shambaa. It is to usher in the start of the new year and a celebration marking the festive season disguised as traditional ceremonies, dancing, and music. The city's history is tied to the ancient Indian Ocean trade routes, and remnants of its colonial history still exist in the old buildings and infrastructure that are scattered around the city.

2. What are the coastal currents off Tanga?

The coastal currents off Tanga are influenced by a variety of different factors, and this is the reason why the marine environment there is dynamic and complex. One of the primary causes is the monsoon winds. The Indian Ocean experiences seasonal monsoons, with the southwest monsoon during May to September and the northeast monsoon from November to March. These strong winds have a strong impact on the surface currents, forcing the water in different directions and affecting the movement of sea animals. During the southwest monsoon, the winds push the surface water off the coast, and it results in upwelling of the cold, nutrient-rich water of the deeper layers of the ocean. The mechanism of upwelling maintains an active ecosystem, and it is receiving a dense concentration of fish and other marine animals.

The second significant influence is the influence of the Indian Ocean on the Pangani River. The river deposits a great amount of freshwater into the coastal waters, altering the salinity and density of the water. This alteration of water properties has the potential to create estuarine circulation patterns where freshwater travels on top and heavier saltwater travels in the opposite direction at the bottom. These circulation patterns are of great significance in the transportation of sediments, nutrients, and contaminants in the coastal area.

The bottom topography on the seafloor of the ocean beneath Tanga also controls the currents' patterns. Slopes, ridges, and valleys on the seafloor deflect and change the course of water flow and create eddies and regions of differing speeds and directions of currents. In addition, the effect of larger-scale oceanic circulation in the Western Indian Ocean, such as the Agulhas Current and its tributaries, can have a secondary effect on the coastal currents off Tanga, adding to the overall complexity of the marine environment.

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

There are many ways that can be employed to observe the coastal water flow of Tanga. The surface-drifting-buoy is a traditional technique. Researchers deploy floating buoys with tracking devices, such as GPS, onto the sea surface. The buoys ride the currents, and researchers monitor their locations over time. Researchers infer the size and direction of the surface currents from the path and velocity of the buoys. But this method provides information for the surface layer of the water column alone and cannot provide real indications of the currents below.

Ship-anchored method is where a vessel is anchored and equipment like current meters are thereafter used to determine the velocity of water at different depths. The method is able to quantify more accurate readings of the vertical current structure. However, it is limited by the presence of appropriate ships, operation cost, and feasibility of obtaining a long-term mooring, especially in turbulent seas and high currents.

In the past few years, the newest and most efficient method to gauge coastal currents has been through the Acoustic Doppler Current Profiler (ADCP) technique. ADCPs monitor water velocity at different depths all at once through sound waves. ADCPs may be mounted on ships, on buoys, or on the seafloor. Ship-based ADCPs can sample the currents while the ship is passing through with a full description of the flow field over extensive regions. Buoy-based ADCPs can provide continuous measurement at one location, whereas bottom-mounted ADCPs are convenient when fixed point, long term measurements are being considered. This non - invasive method measures accurate and complex data, and it is therefore very convenient in understanding the complex coastal currents around Tanga.

4. What is the principle of ADCPs?

ADCPs are based on the Doppler effect principle. An ADCP has a transducer that emits acoustic pulses into the water. The pulses travel through the water column and are backscattered by suspended particles such as plankton, sediment, or water bubbles. In areas where water is in motion, the sound waves backscattered are frequency shifted, known as the Doppler shift.

The size of the Doppler shift is directly proportional to water speed. Through measurement of Doppler shift over a range of depths, the ADCP can determine the water speed of the water currents at any depth. Most ADCPs employ several transducers in an array. This configuration allows the device to measure the water velocity in a number of different directions, thus allowing the device to calculate the three-dimensional velocity vector of the water currents. From this information, the ADCP builds an accurate profile of the current flow, giving valuable information for oceanographic studies, environmental monitoring, and navigation.

5. What's required for quality measurement of Tanga coastal currents?

To obtain the high-quality measurement of the Tanga coastal currents, there are different parameters that have to be kept in view while choosing an ADCP current meter. First is the dependability of the equipment. The marine setting around Tanga, with subjected to saltwater, high monsoon-forced currents, and varying temperatures, necessitates the use of an ADCP which will be resilient enough to counter such aggressive circumstances. It needs to be mechanical and corrosion proof so that data can be gathered continuously with accuracy for a timeframe.

Size, weight, and power usage are also factors. For uses like long-term monitoring on buoys or in the field, a low-power, light, and compact ADCP is extremely preferable. This is easier to install and maintain, and long-term operation can be achieved without needing constant battery replacement or recharging.

Cost is a relevant factor, especially in cases of large-scale monitoring schemes or for low-cost organizations. While there are expensive ADCPs with extra features, there are also cheaper ones that can offer good performance. There is a requirement to find a balance between cost and the level of accuracy and operation required when selecting an ADCP.

The ADCP's casing material determines its lifespan. An appropriate material to use in making the casing of an ADCP to be mounted in the sea off Tanga is titanium alloy. It is very robust due to its light weight, and it is a very good protective material for the internal working mechanisms. It has high corrosion resistance to saltwater that guarantees the life of the ADCP so that it can last long enough and still function efficiently in the harsh marine environment of Tanga.

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

The choice of a proper ADCP to utilize for measuring currents off Tanga would be a function of many parameters like planned application, water depth, and affordability. Ship-mounted ADCPs are best for surveys and research with regard to broad-scale large oceanic coverage. They make multiple depths while on board a ship and yield accurate information about spatial and temporal current structure.

Bottom-mounted ADCPs are appropriate for long-term point monitoring. They can be installed on the ocean floor to observe the impact of tides, monsoons, and other environmental conditions on the currents over a long period of time.

Buoy-mounted ADCPs are best suited to measuring nearshore waters or places where it is not convenient to access with a boat. They are quite easy to install and can provide continuous data on the prevailing local current conditions.

Frequency of ADC is also a point of concern. For water depth not exceeding 70 meters, the best option would be a 600kHz ADCP, for a water depth of up to 110 meters, a 300kHz ADCP, and up to 1000 meters of water depth, a 75kHz ADCP. While choosing an ADCP, due care needs to be exercised while choosing the frequency in terms of water depth and accuracy.

Some of the most well known ADCP brands that are found in the market are Teledyne RDI, Nortek, and Sontek. However, for cost - effective users,the ADCP supplier China Sonar's PandaADCP is the most recommended. It is entirely constructed of titanium alloy, and it performs well with an affordable price. It is suitable for stingy users who require reliable ADCPs for coastal currents measurement. For more information, you can find it on their website at: https://china-sonar.com/.

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