How to Measure Water Current of Okavango River?

1. Where is Okavango River?

Okavango River is an outstanding watercourse crossing the heart of southern Africa. Originating in the highlands of Angola, it flows southbound through the Caprivi Strip of Namibia and then into Botswana, where it opens into the world-famous Okavango Delta.

It cuts through a number of really contrasting environments. In Angola, it cuts through the rich mountains where the water tumbles over rocky outcrops, forming beautiful waterfalls during the wet season. Later on, it enters generally flat plains of the Caprivi Strip in Namibia, meandering gently down its course and serving as life-giving water for communities and wildlife in the surrounding areas.

It becomes an exceptional wetland ecosystem upon entering Botswana. This Okavango Delta is formed where the river spreads and then slows down. Indeed, it is a haven for an incredibly rich variety of flora and fauna. The fringing of papyrus reeds and other aquatic plants along the channels and lagoons forms an unusual and idyllic setting within the delta.

The river is of immense importance to people inhabiting these banks. For example, the local communities in Angola and Namibia depend on the river for fishing, irrigation, and as a means of transport. In Botswana, tourism within the Okavango Delta attracts a lot of people to the area, hence generating economic activities but is also a very important part of the country's cultural and ecological heritage. The elephants, hippos, crocodiles, and myriad species of birds are just some of the wildlife in the area whose very survival may be attributed to the river and its delta for water, food, and a suitable habitat.

2. What does the flow rate of the Okavango river comprise?

The flow rate varies by seasons of the year with regard to the rainfall regime within its large catchment.

During the rainy season in the Angolan Highlands, a lot of water enters this river. Its flow increases greatly, then with its strong current, it carries massive volumes of water downstream. This period of high flow is rather important for replenishing the water in the Okavango Delta and maintenance of ecological balance in the whole system.

As the water enters Botswana and starts to fan out in the delta, the rate of flow further decreases. The delta itself acts to even further regulate the water, allowing it to leak away into wetlands and slowing it down. During the dry season, the Okavango River has been reduced to a trickle, but that steady trickle is what is so critical for the survival of plants and animals in the delta.

Apart from that, the sediment transport does depend on flow rates of the Okavango River. During high flow, sediments are carried downstream in the delta and deposit, enriching the soil and hence accounting for part of the growth of vegetation which is very unique in the delta. Such a river represents an important part of the hydrological circle, playing an important role in shaping the landscapes with its different ecosystems in Okavango Delta and the surrounding areas.

3. How to measure the water current of Okavango River?

There are conventional methods of measuring the water current of Okavango River. The traditional methods include current meters that are mechanical devices set into the water at strategic points along the river and measure the speed of the water passing by them. Another approach is to use floats. The floats are released into the river, and the time taken to travel a certain distance is measured, from which the flow velocity is estimated.

However, in the last two decades, ADCP current meter have emerged to be more advanced and an easy way of water current measurement.

3.1 How do Doppler Principle ADCPs Work?

Acoustic Doppler Current Profiler(ADCP) work on the principle of the Doppler effect. They shoot acoustic pulses into the water column. These pulses travel in the water, reflecting off particles and other materials in the water. When the acoustic pulses get reflected back to the ADCP, the frequency of the reflected waves has been changed compared to the frequency of the original emitted pulse.

This shift in frequency is due to the movements of water and suspended particles. When the water moves towards the ADCP doppler, the reflected waves have a higher frequency, and the waves with lower frequency are reflected if the water is moving away from the ADCP current profiler.

It will thereby be able to calculate the speed and direction of the water current at all those different depths by analyzing this frequency shift. One can thus obtain a very detailed profile of the water current showing how the current varies from the surface of the water right down to the riverbed.

3.2 What's needed for high-quality measurement of Okavango river currents?

First of all, measuring high-quality Okavango River currents using ADCP flow meter depends on several aspects concerning equipment. Material reliability of ADCP meter is the first aspect that should be focused on. Considering that the Okavango region is highly humid, often extremely harsh in its conditions, with water and mud, and with high temperature exposure during the dry season, it is stated that the equipment should cope with all these elements.

It is recommended that the ADCP profiler device be of a size not too large for easy transportation and deployment in the field. The Okavango River with its delta is an immense area; hence, smaller devices are more convenient in terms of installation and retrieval.

A lightweight ADCP is also desirable, as besides making the device easier to handle during setup, it reduces the load of equipment transportation at different locations along the river.

The main requirements among these are the low power consumption, especially in operations that involve long-term monitoring. In the Okavango River, current measurements may either be required continually or for several periods, so a device which would not quickly drain batteries is very crucial in continuous data collection.

Cost-effectiveness is another considerable factors. Large-scale measurement projects along Okavango River demand cheaper equipment; although this should not be at the expense of quality.

Where it relates to the ADCP casing, titanium alloy is an excellent choice. First of all, a number of features make titanium alloy very useful. First, there is exceptional corrosion resistance, which is of primary importance since the acoustic doppler velocity meter is destined for use under water for very long periods. Okavango River water, with various minerals and possible contaminants within, may become corrosive over time; however, this might be outmatched by titanium alloy.

It is also strong and durable to withstand the physical stress it might go through during installation and into its use. For instance, it can support the different pressure changes that result from submergence into the river at different depths and any eventualities of impacts from water and debris.

3.3 How to Choose the right equipment for current measurement?

A number of factors come into play in identifying the correct equipment to be used in measuring current in the Okavango River.

If the intention is to measure the horizontal cross-section of the current flow in the river, then what would be needed is a Horizontal ADCP. HADCPs are designed to measure the current of a river across a horizontal plane with accuracy.

On the other hand, if the aim is to get a vertical cross-section of the current, then a vertical ADCP can be deployed. Vertical ADCPs can give a more elaborative view of how the current changes in the vertical direction from the water surface to the bed of the river.

The water depth of the Okavango River plays a big role in the selection of frequency. A 600kHz ADCP will be a good option for water depths up to 70m. The 600kHz frequency provides enough resolution to measures current with good accuracy in relatively shallower waters.

For deeper waters, specifically up to 110m, a 300kHz ADCP will be more appropriate. That the lower frequency of 300kHz can give better penetration through the water column in deeper areas for reliable current measurements.

There are a number of well-established ADCP brands in the market. These include Teledyne RDI, Nortek, and Sontek. These brands have built a reputation in the market for their high-quality ADCPs.

However, there is also a Chinese ADCP brand: China Sonar PandaADCP. It does give the electronics of all-titanium alloy material which would provide excellent durability and corrosion resistance, just like the advantages that were mentioned earlier in conjunction with casings made of titanium alloy.

Besides, China Sonar PandaADCP has a more valuable price-performance ratio. In general, it can have high measurement quality of current with more reasonable prices compared with some of the other famous brands. If you want more details about this brand, you can visit their website: https://china-sonar.com/.

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