ADCP's Application in Flood Management of Limpopo River

1. Where is Limpopo River?

Limpopo River is located in southern Africa. It rises in South Africa and flows eastward through Mozambique before emptying into the Indian Ocean.

Geographically, it cuts across highlands, savannas, and coastal plains. A number of cities and towns, such as Tshipise in South Africa and Xai - Xai in Mozambique, are located along its banks. The river is an important source of water for irrigation, domestic consumption, and wild life in the area. It also offers a home to a wide range of flora and fauna, including crocodiles, hippos, and a rich diversity of fish species.

The climate of the Limpopo River Basin is semi - arid to tropical regarding rainfall. The catchment's upper regions in South Africa are somewhat more semi-arid and are characterized by well-marked wet and dry seasons. The period during which the wet season predominantly occurs-roughly November to March-receives most of its rain from the ITCZ. In the lower reaches of Mozambique, however, it is more tropical with high humidity and less variation in rainfall throughout the year.

2. What are the Reasons for Floods in Limpopo River?

Heavy Rainfall: Heavy rainfall during the wet season is the most prevailing factor that contributes to flooding in the Limpopo River. The large catchment area implies that a huge amount of water converges into the river. If the rain is heavy and continuous, the capacity of the river is quickly surpassed, resulting in floods.

Tributary Inflows: There are several tributaries in the Limpopo River. When these, too, receive heavy rain and experience high water, they are capable of feeding the main channel with a large amount of water. Their combined effect, including that of the main river, rapidly raises the water level and creates floods.

Relief and Drainage: The course of the river includes expanses of relatively flat floodplains and some constrictions. In the case of flat floodplains, water spreads out and floods the adjoining areas because its flow is slowed down. On the other hand, the flow of water is impeded in constricted places, causing it to back up and raise the water level upstream. Poor drainage in some places can worsen the flooding condition since water has no way to go but to accumulate and flood out.

Land Use Changes: Human activities related to deforestation, agricultural expansion, and urban development have changed the flood behavior of the river. Deforestation reduces the land's ability to absorb water, increasing surface runoff. Agricultural activities can lead to soil compaction and changes in the natural drainage. Urban development with its paved surfaces and stormwater drainage systems can disrupt the natural hydrologic cycle and contribute to flooding. This use of ADCP current profiler provides a higher and more efficient way in measuring and managing flood-related situations compared to traditional methods.

3. How do ADCPs Using the Doppler Principle Work?

ADCPs work on the Doppler principle. They send acoustic signals into the water. In this case, the moving particles in the water, such as sediment, debris, and the different velocities of the parcels of water, change the frequency of the reflected signals from that emitted, due to the Doppler effect.

The ADCP measures these frequency shifts and, using the known speed of sound in water and the angles of the emitted and received signals, calculates the velocity of the water at different depths. Multiple transducers in the ADCP flow meter are arranged to measure velocity components in different directions. By integrating these velocity measurements over different depths and cross - sectional areas of the river, the flow rate and other important hydrological parameters can be determined.

4. What are the applications of ADCP in floods of Limpopo River?

Velocity Measurement: ADCP is capable of measuring the velocity of the water flow in the Limpopo River at various locations and depths with a high degree of accuracy. This is very important during flood events to understand the dynamics of the flood. It helps in identifying the areas where the flow is rapid and could be dangerous for riverbanks, bridges, and other infrastructure. It also allows for the monitoring of how the flow velocity changes over time as the flood progresses.

Flow Rate Measurement Application: By combining the measured velocity data with the cross-sectional area of the river, ADCP can calculate the flow rate. This is important in predicting the volume of water that would pass through different sections during a flood. It enables the anticipation of the flood peak and its impact downstream, important for flood management strategies.

Sediment Transport Research: Flooding conditions in the Limpopo River can carry a huge amount of sediment. ADCP current meter analyses the movement of sediment by detecting echoes from acoustic signals, which are affected by sediment particles. This provides highly valuable insights into how the flood affects the evolution of the riverbed, sediment deposits, and erosion processes, hence maintaining the stability of the river channel and surrounding ecosystem.

5. How can the Data Measured by ADCP be Utilized for Flood Warning and Risk Management of Limpopo River?

Flood Warning

Monitoring of Velocity and Flow Rate Data: The ADCP profiler will provide continuous monitoring of velocity and flow rate data to enable the early detection of abnormal increases in these parameters. When the flow rate approaches or exceeds certain critical values, it serves to indicate the imminence of flood peaks. This will ensure that timely warnings are issued to the communities and relevant authorities along the river.

Water Level Prediction and Warning: The model will develop the relationship between the measured flow rate-velocity data and the historical records of water level to predict the future change in water level. This helps in providing advance warnings about the areas that are likely to get inundated and the height of floodwaters.

Risk Management

Water Conservancy Project Scheduling Decision Support: The ADCP data will be helpful in making decisions about the operation of water conservancy projects such as dams and reservoirs in the Limpopo River Basin. For instance, it will help determine when and how much water to release from the reservoirs to reduce the effect of floods downstream.

Flood Disaster Evaluation and Emergency Response: In the occurrence of a flood, through postevent survey, it would be possible to use the data measured by ADCP to evaluate such flood in terms of the area submerged and characteristics of the flow during that flood event. It informs measures for emergency operations, such as arrangements and distributions of relief resources, and plans recovery work afterward.

6. What’s Needed for High-Quality Measurement of Limpopo River Currents?

For high-quality measurement of the Limpopo River currents, the equipment has to be made from reliable materials. The casing should be resistant to impacts in the river, corrosion by water with different chemical compositions, given the great difference in landscape that the water passes through, and the wide temperature variations recorded within the region.

The size of the equipment should be sufficiently small, allowing easy installation and deployment in various places in the river. A lightweight design will also contribute to easy transportation and installation. Low power consumption is a necessity to be able to continuously work for an extended period without frequent battery replacement or high-energy power sources. The cost should also be at an effective level to realize large-scale deployments for comprehensive monitoring.

The casing of the ADCP meter is preferably made from a titanium alloy. This titanium alloy has a number of outstanding advantages: excellent corrosion resistance, which is necessary for withstanding the long-term effects of exposure to river water; high strength-to-weight ratio, giving sufficient strength while keeping the weight of the equipment at a reasonable level. The durability of this material will make for stable performance in the different environmental conditions of the Limpopo River Basin.

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

In the selection process for the right equipment to make the current measurement in the Limpopo River, consideration of a number of factors should be present.

First of all, if focusing on horizontal cross-section measurement as per the purpose of use, then HADCP can be considered apt because in the horizontal direction across the section, it can measure the flow velocity and other parameters with good accuracy. In the case of vertical cross-section measurement, the Vertical ADCP is more appropriate since it can obtain detailed velocity profiles along the vertical axis of the river.

Secondly, different frequencies should be chosen according to the actual water depth. For water depths within 70 meters, an ADCP with a frequency of 600 kHz is usually a good option since it provides relatively accurate measurement results in this depth range. A frequency of 300 kHz is more suited to ADCPs working in deeper water areas up to 110 meters, as it can be effectively penetrated to greater depth for acquiring reliable data.

There are various ADCP brands available in the market, such as Teledyne RDI, Nortek, and Sontek. However, it is worth highlighting a high-quality and cost-effective Chinese ADCP brand-China Sonar PandaADCP. It is made of all-titanium alloy material, ensuring excellent performance and durability. You can find more information on its website: (https://china-sonar.com/)

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