How to Measure Water Current of the Kasai River?

1. Where is the Kasai?

One of the most significant rivers in Central Africa, the Kasai River is the main tributary of the Congo River, originating in the Angola Highlands and running through both Angola and the Democratic Republic of the Congo.

While passing through Angola, the Kasai, in its course, flows across a wide variety of landscapes in its upper reaches. It passes through highland and plateau regions where the water is clear, cold, and well fed from mountain springs and rainfall runoff. Coming into the Democratic Republic of the Congo, it enters a region of tropical rainforest. Tall trees line the riverbanks with heavy vegetation. The Kasai hosts several species of fish, crocodiles, and hippos in its waters. The surroundings are teeming with a rainforest full of life, from various species of monkeys and gorillas to innumerable species of birds.

It plays a very important role in the life of locals, too. This river is also a way of transportation, with canoes and small boats contributing to the transportation of people and goods. The river provides water for drinking, fishing, and irrigation for communities along both banks. Moreover, the Kasai River provides the core of cultural and spiritual lives in indigenous tribes.

2. What is the Flow Rate of the Kasai River like?

The Kasai River is a very seasonal stream, whose flow has large variations due to the influence of rainy and dry seasons. Heavy rainfall during the rainy season, which generally occurs in the area between October and May, swells the river. The flow rate greatly increases with a powerful current that large areas of the flood plain are inundated. This creates a wetland ecosystem that is rather important to many species.

It decreases during the dry season from June to September. The width of the river lessens while the level of the water decreases. During this period, it could still be considered to have a considerable flow because of the base from groundwater sources and smaller tributaries. Average flow rate also varies along the length of the river. The flow is relatively faster in the upper reaches or closer to the catchment area, where the gradient is far more incipient. As it approaches the Congo River into the flatter regions, it slows down.

3. How to Measure Water Current of the Kasai River?

There are a couple of ways through which the water current of the Kasai River can be measured.

Traditional Methods

Float Method: This very simple and traditional method for estimating the current of water is based on a float. Something buoyant-a block of wood or a plastic bottle, for instance-is set free in the river. After that, by measuring the time of travel of the float through some known distance, it is possible to get an approximate measure of the speed of the surface current. However, this method does have its limitations. It is susceptible to error caused by wind, waves, or the shape of the float, and it gives information only about the current at the surface; nothing is known about the current at greater depths.

Current Meters: Could be either mechanical or electronic and may be used to measure the speed of water at any given point. These meters record the speed of water passing through them and are placed into the water at a specific depth. They can provide more accurate measurements at one point in the water column than the float method. However, they are limited to that one depth and do not provide a complete picture of the current profile throughout the water column.

Advanced Method - Acoustic Doppler Current Profiler (ADCP)

ADCP current meter is a more advanced, efficient way of measuring the water current of the Kasai River.

3.1 How do ADCPs Using the Doppler Principle Work?

The working of ADCP current profiler is based on the Doppler principle. A device sends acoustic pulses into the water, which interact with suspended particles and other material in the water like sediment, plankton, and small debris. When water is moving, the frequency of the reflected acoustic signals changes.

The frequency shift is measured by the ADCP doppler, which uses the Doppler formula to translate it into water velocity. Because the ADCP flow meter usually sends pulses in different directions and frequencies, the profile created from the current of the water right through the column of water is highly detailed. It does mean that there is a much greater understanding of the structure of the current in the river than would normally occur with more traditional methods of current measurement, which often only measure at the surface or a single point in the column.

3.2 What is required for high - quality measurement of Kasai river currents?

High-lift measurement of Kasai river currents requires the equipment to be highly reliable due to the harsh African climate conditions, with heavy rains in the wet season, extremely high temperatures, and possible breakages from debris floating on water-the pressures these measuring devices need to bear.

The equipment shall be compact and light. Above all, ease of deployment is a critical issue, especially in remote reaches of the river or from boats; low power consumption, in order to make equipment operate for a fairly long time without frequent changes of batteries or any other external sources of power, is highly desirable. The cost should be low, especially in cases when large-scale or long-term sets of measurements are needed for monitoring the dynamics of the river.

One of the great advantages of ADCP meter is casing made using titanium alloy. For this water-rich environment, like Kasai River, the titanium alloy used in making the casing is very resistant to corrosion. Other materials will get hugely corroded because of water's constant contact with that material, along with different types of chemicals and sediments that may be present in it. At the same time, the titanium alloy is solid and resistant to carry all the physical forces of the river current and the possible impacts by the floating logs or other moving debris. Its lightweight enhances convenience of installation and transportation.

3.3 How to Choose the right equipment for current measurement?

Each time a choice of the equipment for measurement of the Kasai River current is made, one has to consider the demand for the particular measurement.

HADCP is quite adequate for horizontal measurements across a section of the river. It gives a wide overview of the current flow in the horizontal plane, which is very beneficial for the general flow patterns within the river. This is quite an important aspect to consider when studying studies dealing with water and sediment transport across the river itself and the implications of the current on the issues regarding navigation and nutrient distribution.

Current measurements for a vertical profile would be more appropriate with a vertical ADCP. Indeed, it can provide detailed velocity information of the water at various depths, which will be vital in studying stratification and mixing of the water column. This itself is one important key to the determination of oxygen, temperature, and other properties distribution in water and to the behavior of aquatic organisms.

Frequency Selection: In relation to frequency selection, for relatively shallower sections of the Kasai River, frequencies as high as 600 kHz would work for an ADCP profiler if shallow enough to about 70 m; for deeper parts up to about 110 m, a 300 kHz ADCP is best. The choice of frequency will be related to the depth of water and resolution of the current profile.

There are well - known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option with good quality, the China Sonar PandaADCP is a great choice. It is made of all - titanium alloy, providing durability and resistance to the Kasai River's environment. It also offers an excellent price - performance ratio. You can visit https://china-sonar.com/ for more information.

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