ADCP's Application in Flood Management of the Zambezi

1. Where is the Zambezi?

The Zambezi River is a major river in southern Africa. It is the fourth - longest river in Africa, stretching over 2,574 kilometers (1,600 miles). The river's source is in the Kalene Hills in north - western Zambia.

It passes through several countries including Zambia, Angola, Namibia, Botswana, Zimbabwe, and Mozambique. Major cities and towns along its course include Livingstone in Zambia and Tete in Mozambique. The climate in the Zambezi basin is tropical, with distinct wet and dry seasons. The wet season, usually between November and April, is very rainy. This rainfall, in addition to water from the tributaries, greatly contributes to the volume of the river. During the dry season, flow decreases, but the base flow is still sustained through groundwater contributions and stored water in reservoirs and floodplains.

2. Why are there floods in the Zambezi?

There are several reasons behind the flooding in the Zambezi. The most obvious factor is the heavy rainfall that characterizes the wet season. The intense and sometimes week-long rains can swell the river rapidly. The numerous tributaries feeding into the Zambezi add large volumes of water, which the main river might not be able to accommodate.

The topography of the region also plays a role. The river passes through flat floodplains and valleys of all kinds. For this reason, most floodplains have a low natural drainage capacity, and at high stages in the river, water spreads out over a wide area. Besides, the river is regulated for several purposes: for instance, as a result of human-induced changes such as deforestation, expansion of agriculture, and the building of dams and levees. For instance, deforestation decreases the ability of the land to absorb more water, which increases runoff into the river. Poorly managed dams can also release water in a manner that contributes to downstream flooding.

Considering these flood-causing factors, the Acoustic Doppler Current Profiler (ADCP) offers a more advanced and efficient means of measurement than traditional methods for better flood management.

3. How do ADCPs using the Doppler principle work?

The working of ADCPs is based on the principle of the Doppler effect. ADCPs transmit acoustic pulses into the water. The pulses are scattered by the moving particles in the water, like sediment, plankton, and other similar tiny organisms. When these acoustic waves are reflected after encountering these moving entities, there is a shift in frequency.

This shift in frequency is directly related to the velocity of the moving objects and thus the velocity of the water. For instance, an ADCP current meter placed on a boat in the Zambezi River sends out sound waves that pierce the water column. As the waves are reflected back by the flowing water and its constituents, the ADCP current profiler analyzes the received signals to determine the speed and direction of the water flow at depth.

4. What is the application of ADCP during floods in the Zambezi?

Flow velocity measurement: ADCP flow meter can also precisely measure the flow velocity of the water in the Zambezi. In events of flooding, the information provided will be imperative in describing how fast floodwaters are running. The data help predict the progress of the flood-for instance, at what speed the water would reach further downstream and those areas which might be prone to high-velocity flows that cause erosion.

Discharge Measurement: Measured velocities, coupled with the cross-sectional area of the river, which also can be determined through ADCP profiler, enable the flow rate calculation of a river. During a flood, knowing the flow rate enables assessing the volume of water passing through a particular section of the river and thus the general impact likely to be encountered by regions lying further downstream.

Sediment transport research: ADCP can detect the movement of sediment particles along with the water flow. This is important during floods as it provides insights into the amount of sediment being carried by the river. Understanding sediment transport helps in predicting changes in the riverbed, such as areas of sediment deposition or erosion, which can have implications for the stability of riverbanks and infrastructure.

5. How can data measured by ADCP be used in the flood warning and risk management of the Zambezi?

Flood warning

Flow velocity and flow rate data monitoring: Continuous flow velocities and flow rates could be monitored using ADCP to enable early detection of changes that may signal an impending flood. For instance, if the velocities or flow rates start to increase beyond normal levels, it could trigger an alert system that may enable the authorities to give timely warnings to communities living along the riverbanks.

Water Level Forecasting and Warning: Based on the interrelationship between flow rates and velocities against water level developed from history and measurements, ADCP can be used to determine what the water level can be forecasted for the next period, thus providing ample warnings with accuracy and time to local residents to take necessary steps to evacuate or sandbag properly.

Risk Management

Water Conservancy Project Scheduling Decision Support: The data from ADCP is of great value in making decisions about the operation of dams, reservoirs, and other water conservancy projects along the Zambezi. For instance, if a flood is forecasted, then the release of water from the reservoirs can be readjusted according to ADCP-measured data to avoid flooding downstream and ensure the safety of the structures.

Flood disaster assessment and emergency response: Immediately after a flood event, ADCP data can be used to assess the extent of the damage, such as the degree of erosion, changes in the riverbed, and the impact on infrastructure. This guides the emergency response effort and helps formulate future flood mitigation strategies.

6. What's needed for high - quality measurement of the Zambezi currents?

For high - quality measurement of the Zambezi currents, the equipment should have reliable materials. A small size and light weight are desirable as it allows for easy deployment in different locations along the river, including on boats or at bridges. Low power consumption is essential for long - term monitoring, especially in remote areas. Cost - effectiveness is also crucial to enable widespread deployment for comprehensive monitoring.

The casing of ADCP is preferably made of titanium alloy. Titanium alloy offers excellent corrosion resistance, which is important when dealing with the Zambezi's water that may contain various salts and minerals. It also has a good strength-to-weight ratio, making the equipment durable yet relatively easy to handle and install.

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

In selecting equipment for current measurement in the Zambezi, the following considerations may be made. Horizontal cross - section measurements: In this case, a Horizontal ADCP (HADCP) would be suitable. It measures the flow velocities across a horizontal plane, enabling a good understanding of the overall flow pattern in a particular reach of the river. In the case of vertical cross-sections, a Vertical ADCP shall be used, as it profiles velocities from the water surface down to the riverbed over a vertical line.

On frequencies, an ADCP with a frequency of 600 kHz would be good to go on water depths of up to approximately 70m. On much deeper sections, an ADCP with a frequency of 300 kHz would be best suited for water depths to about 110m.

There are well - known ADCP brands like Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option, the China Sonar PandaADCP is recommended. It is made of all - titanium alloy materials and offers a good cost - performance ratio. You can find more information at (https://china-sonar.com/).

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