ADCP's Application in Flood Management of São Francisco River

1. Where is the São Francisco River?

São Francisco River is one of the major rivers in Brazil. It is one of the longest wholly Brazilian rivers, stretching for approximately 2,914 kilometers.

It originates in the Serra da Canastra region of Minas Gerais state. The river runs through many states in Brazil, including the states of Minas Gerais, Bahia, Pernambuco, Alagoas, and Sergipe. It passes through a lot of cities and towns that are important urban centers in the region, like Juazeiro and Petrolina. These cities depend on the river for water supply, transportation, and to develop economic activities such as fishing and small trade.

The São Francisco River basin is semi-arid for the most part, with a well-defined wet and dry season. The wet season is usually from December to March when the rainfall is more considerable due to the influence of the South Atlantic Convergence Zone. During this period, its tributaries feed an enormous amount of water into the main channel. On the other hand, during the dry season, the river's flow decreases, and the water level may go low. However, throughout the year, the river serves as a lifeline for the communities and ecosystems in its basin.

2. Why does São Francisco River flood?

High Intensity of Rainfall in the Wet Season: The main reason for flooding in the São Francisco River is the heavy and continuous rainfall throughout the wet season. Due to the big catchment area of the river basin, a huge amount of water comes from the surrounding areas. When the intensity of rainfall surpasses the natural capacity of the river to drain the water further downstream, the water level starts to rise very rapidly. The many tributaries contribute to the flow of water into the river, which then bursts its banks and causes floods in adjacent areas.

Topography and Drainage: Topography also plays a role in flood occurrence. It is about how the topography in the river basin has sections with relatively flat terrain or poor natural drainage systems whereby water accumulates rather than flows away. In some areas, the soil is of low permeability, resulting in more surface runoff during heavy rainfall. This excess water eventually reaches the river, adding to the volume and height of the water, raising the flood risk.

Deforestation and Land Use Changes: Human activities, mainly deforestation in the São Francisco River basin, have interfered with the hydrologic cycle, increasing the chances of flooding. Trees are very important in intercepting rainfall and allowing water to infiltrate the soil gradually. Once forests are cleared for agriculture, expansion of urban areas, or other purposes, the ability of the land to absorb water becomes reduced. This results in more water running directly into the river and thus increases the likelihood of flooding in cases of heavy rainfall.

In the context of the São Francisco River for flood management, Acoustic Doppler Current Profiler (ADCP) has become a more advanced and convenient measurement tool than conventional techniques.

3. How do ADCPs using the Doppler principle work?

The operation of ADCPs is based on the Doppler principle. Transducers of ADCP current profiler emit acoustic signals into the water-sound waves, actually. These interact with moving particles in the water, including sediment particles, organic matter, and water molecules flowing with the current. When those moving objects reflect back the emitted waves, a frequency shift occurs.

The shift in frequency, resulting from all motions-the so-called Doppler shift-is directly proportional to the velocity of the moving objects. ADCPs are also configured with multiple transducers, each sending acoustic pulses in different directions. The example of a downward-looking ADCP meter, already mentioned, will follow the velocity profile from the water surface down to the riverbed. The ADCP current meter can calculate, by analyzing the Doppler shifts of the reflected signals from different depth layers in the water column, the velocity of the flow at each depth.

This will then enable the finding of the actual velocity at several depths and, with knowledge about the cross-sectional area, which is usually measured or estimated based on other means, it will now be possible to calculate more important parameters such as the flow of the river.

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

Velocity Measurement: During the flood events at the São Francisco River, ADCP current profiler can measure the water velocity flow at several locations and different depths with accuracy. The recorded velocity is very helpful to calculate how fast the flood water is moving. For instance, it is able to predict when the floodwaters will reach the most vulnerable riverside communities, agricultural fields, or infrastructure such as bridges and dams. With a continuous velocity record, for instance, authorities can take proper precautionary measures in good time, such as evacuation of people or strengthening embankments.

Flow Measurement Application: The ADCP flow meter calculates the flow rate of the river by integrating velocity data over the cross-sectional area of the river. This helps in estimating the total volume of water passing through a particular section of the river in case of a flood. The information is important in flood forecasting and understanding the magnitude of the flood. It can also assist in assessing the capacity of flood-control structures and in planning for effective flood mitigation strategies.

Sediment Transport Research Application: Since the sediment is transported by floodwaters in the São Francisco River, ADCP current meter will perform an analysis of the backscattered acoustic signals, which in turn study the motion of these sediment particles. Knowledge of sediment transport during a flood is crucial for understanding and assessing the changes in the riverbed. Deposition and erosion of sediments may alter the depth, width, and stability of the river, modifying the risks of future flooding and the quality of the fluvial ecosystem.

5. How can the data measured by ADCP be utilized for flood warning and risk management of São Francisco River?

Flood Warning

Velocity and Flow Data Monitoring: ADCP flow meter continuously monitors the flow velocity and flow data in order to quickly detect abnormal changes in the flow of the river. A sudden rise in velocity or a major change in the flow rate may indicate that a flood is imminent or that the flood situation is deteriorating. This information can be fed into flood-warning systems for the timely alerting of local communities, emergency services, and other concerned authorities.

Water Level Prediction and Warning: By combining ADCP-measured flow data with other factors such as the geometry of the river's cross-section and historical records in terms of water level, it is possible to do predictions of future water levels. If the level of water is predicted above the flood level mark, warnings can be issued so that people can evacuate or go into protective mode.

Risk Management

Water Conservancy Project Scheduling Decision Support: ADCP data will provide significant input into the decision-making processes of water-conservancy projects, including dams, reservoirs, and diversion channels along the São Francisco River. For instance, during a flood, the flow data would help to find the ideal release rate from a dam to minimize the impact of the flood further downstream.

Flood Disaster Assessment and Emergency Response: In cases of flood disasters, ADCP data should be used in post-flood assessments of the damage brought about by the flood, for example, areas subjected to high-velocity flows, the quantum of sediment deposition, and so on. This guides emergency response and recovery efforts in search and rescue operations, post-flood rehabilitation of infrastructure and ecosystems, and many more.

6. What is required for high-quality measurement of São Francisco River currents?

Several aspects are considered in the high-quality measurement of the São Francisco River currents.

Reliable Equipment Materials: The ADCP current profiler should be made from materials that can resist the harsh aquatic environment of the São Francisco River. It needs to be resistant to corrosion from the river water, which may contain various minerals and pollutants. It also needs to endure impacts from floating debris such as branches, logs, and other objects that can be present in the water.

Small Size, Light Weight, and Low Power Consumption: Smaller and lighter ADCP current meter will be easy to install and deploy at various locations along the river, especially in difficult-to-reach areas. Low power consumption allows for longer-term operations without frequent battery replacement or complex power-supply arrangements, which is advantageous for continuous and long-term monitoring.

Low Cost: A lower-cost ADCP flow meter device is desired to accommodate wide-scale deployment for monitoring at many points along the river.

For the casing material, one of the best options is titanium alloy. The strength of titanium alloy is very high, so it can bear the forces on it from the water with the ADCP profiler inside. Its resistance to corrosion is also excellent to ensure long-term functioning of the equipment in the water. Its relatively low density further helps in reducing the overall weight of the device with no compromise on structural integrity.

7. Selection of Equipment for Current Measurement

The following should be considered when choosing equipment for current measurement in the São Francisco River:.

Purpose of Use: If the focus is on horizontal cross-sectional flow measurement, a Horizontal Acoustic Doppler Current Profiler (HADCP) is suitable for this purpose. It measures the flow velocity and other parameters across a horizontal section of the river. If the measurement is for a vertical cross-section, a vertical ADCP is more appropriate as it can measure the velocity profile from the water surface to the riverbed along a vertical line.

Frequency Selection: Different frequencies of ADCPs correspond to different water-depth ranges. For instance, an ADCP current meter at a frequency of 600 kHz is generally suitable for water depths within 70 meters, while frequencies of 300 kHz are more appropriate for water depths up to 110 meters. Choosing the right frequency according to the actual water depth of the São Francisco River ensures accurate and reliable measurement results.

There are a few well-known brands in the market, such as Teledyne RDI, Nortek, and Sontek. There is also a Chinese brand called China Sonar PandaADCP, which has a casing made of titanium alloy and enjoys a good cost-performance ratio. You can visit its website: https://china-sonar.com/.

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