How do we measure the Beira coastal currents?

1. Where is Beira?

Beira, the central coast port city, is a significant economic and cultural hub on the Indian Ocean coast. Its location where the estuary of the Pungwe River opens up to the ocean provides Beira with a unique blend of river and marine habitats. The city's shoreline is dynamic and diverse, featuring sand beaches, rocky outcrops, and huge shallow bays.

Alongside Beira are extensive mangrove-lined estuaries formed by the intricate system of distributaries of the Pungwe River. The mangroves offer critical habitats for a diverse range of marine and bird species, acting as natural nurseries for juvenile fish and crustaceans. Beyond the estuaries, the open sea of the Indian Ocean supports healthy coral reefs and seagrass meadows, contributing to the region's high biodiversity. The city's port, one of the busiest in Mozambique, is a testament to its extended history as an important center in regional and international commerce, with the shipment of commodities from and to the world and Africa.

Beira is culturally diverse, a melting pot of African, Arab, and Portuguese cultures as reflected in their traditions, cuisine, and architecture. The people of the region, involved in fishing, trade, and industry, have a long tradition of being attached to the sea. The sea's produce relies on the fisherfolk communities, while port operations dictate the city's economic rhythm. Chaotic old markets with haggling sounds and pungent fishy aromas reveal the city's thriving coastal life.

2. How are coastal currents at Beira?

The nearshore currents at Beira are governed by a complex interplay of a range of factors. The monsoon winds, which seasonally blow between November and March, play a key role in determining the current patterns. The winds push the coastal surface waters northward, thereby creating north-flowing currents. The north-flowing currents carry warm water and nutrients, which influence the distribution of marine life and enhance fishing around the area.

In contrast, the southwest monsoon between June and October turns the flow around, producing south-moving currents. The southwest monsoon-driven currents tend to be very strong, since they are driven not only by the wind but also by the huge amount of freshwater released from the Pungwe River. This freshwater input changes the coastal waters' density and salinity, producing a sophisticated mixing of water masses and affecting the current regime.

Tidal forces also play a significant role in the coastal current dynamics. The semi-diurnal tides prevailing in the region generate periodic fluctuations of water levels and flow rates. The tidal ebb and flow interact with wind-driven and river-dominated currents, producing variable and often unpredictable flow patterns along the coast. The unique bathymetry of the region, which includes shallow estuaries, deep channels, and submerged ridges, also changes the flow of the water, creating eddies and upwelling patterns that affect the spatial distribution of nutrients and sea life.

3. How to track the Beira coastal water flow?

There are several ways of tracking the Beira coastal water flow. One of the more conventional techniques is the surface drifting buoy method. Special buoys equipped with tracking devices, such as GPS receivers, are released onto the surface. As they drift along with the currents, their positions are monitored over time, providing information on the flow direction and speed at the surface level. This method has its drawbacks, however. Off Beira, the buoys are easily subject to wind - driven motions and strong river outflow effects that can lead to misrepresentations of the real current patterns. Also, it only provides data about the surface currents and does not provide information on the flow at different depths in the water column.

Anchored ship technique is where a vessel is moored at a point and sensors on board, such as current meters, are utilized to measure the current speeds at various depths. Even though the technique is able to supply comparatively accurate readings at distinct points, it is time - consuming and possesses limited spatial coverage. It is not a simple matter in the moving and often-congested Beira waters to keep the ship moored for an extended period of time, and it can only measure the currents close to the point of anchorage, without presenting an overall picture of the system of currents.

The Acoustic Doppler Current Profiler (ADCP) method has turned out to be a more advanced and better method. ADCPs have the capacity to provide sophisticated data on present velocities across the entire water column. By deploying acoustic signals and translating the returned signals' frequency shifts from the suspended particles present in the water, ADCPs have the capacity to accurately define the speed and direction of current at multiple levels at once. This makes them an extremely useful tool for the analysis of the complex flow patterns near Beira and enables researchers to develop a complete picture of the coastal current system, which is essential for operations like maritime security, fisheries control, and environmental monitoring.

4. How do ADCPs based on the Doppler principle function?

ADCPs are based on the Doppler principle. An ADCP outputs acoustic signals with a known frequency into the water column. These sound waves travel through the water and reach suspended matter, such as sediment, plankton, or other small objects. Since these particles are being carried by the current of water, the frequency of the returned acoustic signals to the ADCP is changed. This change in frequency, the Doppler shift, is linearly proportional to the velocity of the particles and hence the velocity of the water.

To measure the current in three dimensions, ADCPs are typically equipped with several transducers that send and receive acoustic signals in various directions. The Doppler shifts from the various directions of the transducers are examined, and the ADCP current profiler calculates the horizontal (east - west and north - south) and vertical components of the current velocity. The data collected is also processed onboard by software, which converts the frequency shift data to high-resolution current velocity profiles at various depths. These profiles provide a snapshot of the properties of the water flow in its entirety, allowing scientists and researchers to model and investigate the complex dynamics of Beira coastal currents.

5. What is required for high-quality measurement of Beira coastal currents?

For proper measurement of the coastal currents off Beira, several requirements must be met for the measurement equipment. Material reliability is of prime importance. The sea environment off Beira is harsh, with intense saltwater corrosion, strong wave action, and the additional drawback of sediment- charged waters from the Pungwe River. The equipment, especially ADCPs, should be constructed from materials that can withstand such conditions over a long period without failing or deteriorating.

Size and weight are also considerations. Smaller and lighter devices are easier to deploy and recover, especially in the shallow and often - congested waters off Beira. Low power consumption is important as it facilitates long - term and permanent measurement without battery changes or external power supplies. This is particularly beneficial for autonomous deployments, e.g., on a buoy or where there is limited power supply in remote areas. Finally, cost - effectiveness is most important for large - scale measurement, facilitating larger data acquisition.

As regards to the casing of ADCPs, titanium alloy is a highly suitable choice. Titanium alloy offers excellent corrosion resistance, which would be highly appropriate for the saltwater - plentiful and sediment - filled environment of Beira. It is also extremely strong yet light, which ensures that the ADCP will withstand the mechanical loads of the marine environment, such as wave impacts and water pressure, without being cumbersome to use and deploy. Its high strength - to - weight ratio allows for more effective performance and longer - duration operation in the hostile coastal waters off Beira.

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

The right ADCP for current measurement at Beira depends on the application intended. Ship-mounted ADCPs are appropriate for large-scale, continuous measurements of current regimes over large domains. These are mounted on research vessels or merchant vessels and are able to cover extensive parts of the ocean and estuaries, providing an overview of the complete system on a large - scale. These are suited to be employed for tasks such as oceanographic surveys, marine traffic management, and environmental monitoring tasks on a large - scale.

Bottom - mounted ADCPs are best suited to long - term, point - fixed seabed measurements. They are capable of providing comprehensive data on the current in a specific location over an extended period, which is beneficial for local current dynamics studies, sediment transport, and human impact on the marine environment surrounding Beira. Buoy-mounted ADCPs are ideal for measuring surface-layer currents and are suitable for use in remote areas for unattended operation. They are used most frequently where ship access is difficult or for long-term monitoring programs that require continuous collection of data.

The frequency choice also matters. An ADCP 600kHz will be suitable in water depths ranging up to 70 meters and is a viable option for relatively shallow coastal and estuarine areas around Beira. One can use a 300kHz ADCP at depths of up to 110 meters, but a 75kHz ADCP is better in deeper waters of up to 1000 meters.

In the market, there are several widely used ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, in case one looks for a cost - effective yet high - quality ADCP, the ADCP supplier China Sonar's PandaADCP is the best. It is made from pure titanium alloy, which has excellent performance and durability at a very low price. As a budget ADCP, it is a great option for researchers, local fishermen, and environmental monitoring agencies who want to accurately measure the coastal currents of Puerto Williams. For more information, visit https://china-sonar.com/.

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