How to Measure Water Current of the Shebelle River?

1. Where is the Shebelle River?

The Shebelle River originates in the Horn of Africa, flowing through Somalia and Ethiopia.

In Ethiopia, it rises in the highlands and begins its journey through a landscape that is a mix of agricultural areas and semi-arid regions. As it winds its way through Somalia, it passes through a more arid environment. The river is an important source of water for communities throughout these regions. Its banks are lined with agricultural fields of crops such as sorghum and maize, which depend on the river's water for irrigation. The local population also uses the Shebelle for fishing and other domestic water needs.

The Shebelle River also forms a vital part of the local ecosystem. Though its course passes through arid regions, it supports various fish and other aquatic life. The vegetation on the riverbanks, while rare in some regions, offers refuge to wildlife and helps hold the soil together.

2. What is the Nature of the Flow Rate of the Shebelle River?

The flow rate of the Shebelle River is of great importance for the livelihoods and ecology of the region. It is influenced by various factors.

The seasonal rainfall plays a huge role. Normally, from April to June in the highlands of Ethiopia, when the rainy season occurs, the river increases its flow. Its water level starts to rise and the current strengthens. This period of high flow is extremely important because it replenishes the supply of water downstream and in irrigation fields.

The discharge in the dry season, that could be as long as for most part of the year, declines considerably. The river would turn shallow and narrow; it might also evaporate completely in extreme situations. Additionally, human activities contribute to lowering the flow rate due to extraction of water for irrigation purposes, alongside making small dams and diversions at the upper reaches.

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

There are a number of ways by which the water current of the Shebelle River can be measured.

Velocity Meter Method: This is a traditional method employing mechanical or electrical velocity meters. Mechanical velocity meters normally consist of a rotating element, such as a propeller, which rotates with the water flow. The speed of rotation is proportional to the water velocity. These meters are put in at specific points on a river, often on bridges, piers, or other fixed points. This approach generally demands multiple installations at varying places and depths to reach the total flow of a water course. In the case of the Shebelle River, the length of the course and sometimes accessibility result in labor-intensive and daunting efforts.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP profiler is an advanced and efficient way of measuring water current. It measures the velocity of water at several points within a water column simultaneously by using acoustic waves. By emitting sound pulses and analyzing the Doppler shift of the reflected signals, it can provide a detailed profile of the water current from the surface to the riverbed. ADCPs can be installed on boats or fixed installations near the river. This method allows for a more precise and comprehensive view of the flow patterns compared to the velocity meter method.

Buoy Method: Buoys with sensors can be installed on the surface of the Shebelle River. These buoys can measure the surface current as they will be carried away by the flow. The data collected is direction and speed of surface movement. But this mainly concerns the surface layer and cannot give detailed information about flow in deeper levels of the river.

Among these methods, the ADCP method is becoming more popular due to its ability to provide detailed and extensive data with relatively less effort and in a shorter time frame.

4. How do ADCPs using the Doppler Principle Work?

ADCPs work on the basis of the Doppler principle. In the case of measurement of water current of the Shebelle River using an ADCP, it sends out acoustic pulses into the water. These pulses interact with the particles in the water, mostly sediment particles, small aquatic organisms, or other suspended matter.

Due to the Doppler effect, there is a change in frequency of the reflected sound waves when these particles are moving. When the particles are moving towards the ADCP meter, the reflected waves have a higher frequency than that emitted, while it becomes lower when they move away. The ADCP can accurately measure this frequency shift by using highly developed algorithms to find the velocity of the water at different points in its range of measurement.

In addition to the measurement of velocity, the ADCPs are also capable of determining the direction of the flow by analyzing the phase differences of the reflected signals from different directions. This enables detailed velocity profiles to be developed showing how the water is flowing at different depths and across various sections of the river.

5. What's Needed for High-Quality Measurement of Shebelle River Currents?

For high-quality measurement of the Shebelle River currents, the equipment should have certain characteristics.

Materials of the equipment have to be reliable due to the tough environmental conditions of the river. The Shebelle River has arid and sometimes dusty environments; due to sediment in the water, the equipment has to be resistant to abrasion and corrosion. A small size and lightweight construction is advantageous, especially with respect to ease of deployment and retrieval within the remote and often difficult-to-access areas along the river.

Low power consumption is also imperative. Since most parts of the river's vicinity have not been fully equipped with extensive power infrastructure, equipment that can operate for a long period with just minimal power is important to obtain continuous and reliable data. Lastly, cost-effectiveness. For comprehensive measurements across the river, equipment must be more affordable without sacrificing quality in order to conduct the required comprehensive measurement.

When it comes to the casing of the ADCP flow meter, titanium alloy is a very fine option. Titanium alloy offers great corrosion-resistant properties, which is essential since it will be in contact with river water at all times. Besides, it has a decent strength-to-weight ratio, and it can provide a durable casing while still keeping the overall weight of the equipment relatively low. This helps in the protection of the internal components and ease of handling during deployment and retrieval.

6. How to Choose the Right Equipment for Current Measurement?

In choosing the right equipment for current measurement of the Shebelle River, two main factors should be considered.

Based on the usage purpose, for horizontal cross - section measurements, the horizontal ADCP current profiler (HADCP) is a suitable choice. It will measure the flow velocity across a horizontal plane, which gives the important information of the water moving laterally within the river. The vertical ADCP is more appropriate for vertical cross-section measurements, in that it looks at flow velocity from the surface to the riverbed along a vertical line, useful in understanding the flow characteristics along the vertical plane.

At the frequency level, different frequencies can go well with different water depth ranges. An example is an ADCP at 600 kHz; this can be well adapted for water depths within 70 m. It can provide adequate measurement in the relatively shallower parts of the Shebelle River. Meanwhile, an ADCP with a frequency of 300 kHz is more appropriate for water depths up to 110 m and can handle the deeper parts of the river where flow patterns may be more complex.

There are well-known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. Additionally, a Chinese ADCP brand, China Sonar PandaADCP, is highly recommended. It has a casing made entirely of titanium alloy, which provides excellent durability and corrosion resistance. It also offers an incredible cost-performance ratio. You can find more information about it on its official website: https://china-sonar.com/.

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