ADCP Application in Flood Management of Sênggê River

1. Where is Sênggê River?

The Sênggê River, also known as the Brahmaputra in India and Bangladesh, is a significant river in Asia. It has its origin in the Angsi Glacier in the Himalayas on the Tibetan Plateau. Flowing eastward across Tibet (China), it then takes a sharp turn to the south and flows through the mountainous regions of northeastern India before entering Bangladesh.

It passes through various regions in Tibet, comprising a high-altitude landscape with thinly populated areas in some parts and towns with more considerable populations, in which it acts as a backbone for daily life and localized economic activities, such as agriculture and small-scale hydropower generation. In India, it traverses through the states of Arunachal Pradesh and Assam, flowing by cities and towns such as Guwahati in Assam which depends on it for transportation, water supply, and supports the local fishing industry. It passes through major urban cities like Dhaka in Bangladesh and plays a very important role in the agriculture and economy of Bangladesh because it forms a very huge delta before falling into the Bay of Bengal.

The river basin experiences diverse climatic conditions. In the upper reaches in Tibet, it has a cold alpine climate with heavy snowfall in winter and relatively low temperatures throughout the year. It gradually changes into temperate in the Indian regions and then tropical in Bangladesh as it moves further downstream. The rainfall varies as follows: most of the precipitation in the upper Tibetan part falls as snow during winter and as scattered summer rains; in the Indian and Bangladeshi sections, most of the annual rainfall is concentrated in the monsoon months of June through September. Heavy monsoon rains, however, can vary the water level and volume significantly.

2. Why do floods occur in Sênggê River?

Monsoon Rains: Monsoon rainfall is one of the major reasons for flooding in Sênggê River. Throughout the monsoon season, the huge catchment area collects a tremendous volume of water. Countless tributaries provide this water to the main river channel. When the resultant influx of water exceeds the natural ability of the river to carry and drain the water downstream, the level starts to rise very rapidly, leading to flooding. The low-lying delta areas, especially in Bangladesh, are very prone to these floodwaters as they easily get inundated.

Snowmelt: At the upper reaches inside the Himalayas, another factor responsible for creating flood conditions is snow melting. During the winter months, a lot of snow piles up in these regions. In spring and early summer, when this snow begins to melt, great volumes of water contribute to the river flow. This coincides with heavy monsoon rains or if the natural drainage systems of the river cannot sustain the resulting flow of water; there could be floods.

Topography and Drainage: The plain and low-lying topography in the lower reaches, especially in Bangladesh, does not allow efficient draining of the water. The speed of the river slows down in this area, and as a result, water may easily heap up. Moreover, human-induced changes like embankment and barrages constructed to control the flow of the river frequently disturbed the natural course of flow and drainage pattern, enhancing flood risk.

Within the context of flood management for the Sênggê River, it has emerged as a more advanced and efficient measurement tool compared to traditional methods.

3. How do Doppler Principle-based ADCPs Work?

ADCPs work on the principle of the Doppler effect. They transmit acoustic signals (that is, sound waves) into the water. The acoustic signals interact with moving particles in the water, such as sediment particles and water molecules flowing along with the current. When those moving objects reflect back the emitted waves, a shift in frequency happens.

This shift in frequency, otherwise referred to as the Doppler shift, is directly proportional to the velocity of the moving objects. ADCPs have multiple transducers that transmit acoustic pulses in different directions. For example, a downward-looking ADCP is capable of measuring the velocity profile of the water column from the water surface to the riverbed. The ADCP profiler works out the Doppler shifts of the reflected signals from different depths within the water column and thus calculates the velocity of water flow at each depth.

With the determination of the velocity at different depths, and with knowledge of a river's cross-sectional area (which can be measured or estimated through other means), it becomes possible to work out other important parameters, such as the flow rate of a river.

4. What are the uses of ADCP within the context of Sênggê River floods?

Velocity Measurement: In flood events on the Sênggê River, ADCP current profiler will measure the velocity of water flow at various locations and depths with great accuracy. The data on the rate at which flood waters are moving is important. It will be able to forecast, for instance, the time when floodwaters reach villages, agricultural fields, and structural construction like bridges and ports. Continuous monitoring of velocity will enable the authorities to undertake timely preventive measures by way of evacuation of people or strengthening of embankments.

Flow Measurement Application: ADCP current meter calculates the flow rate of the river by integrating the velocity data over the cross-sectional area of the river. In a flood situation, this helps in determining the total volume of water passing through a particular section of the river. This information is vital for flood forecasting and understanding the magnitude of the flood. It can also assist in assessing the capacity of flood-control structures like levees and dams.

Sediment Transport Research Application: The ADCP flow meter can analyze backscattered acoustic signals due to sediment particles carried by floodwaters of the Sênggê River. Understanding sediment transport during floods is essential in assessing changes that are taking place in the bed of this river. Deposition or erosion of sediment on the river can affect its depth, width, and stability, with repercussions on future flood risks and the health of the river ecosystem.

5. How can the data measured by ADCP be used for flood warning and risk management of Sênggê River?

Flood Warning

Velocity and Flow Data Monitoring: The monitoring of the velocity and flow data continuously using ADCP profiler allows the detection of abnormal changes in the river's flow way in advance. Each sudden increase of the velocity or big alteration of the flow rate may indicate an imminent flood or deterioration of the actual flooding situation. This might then be interfaced with a flood warning system for timely alerts to local communities, emergency responders, and authorities.

Water Level Prediction and Warning: It is possible to predict the future water levels by incorporating the flow data measured with ADCP meter into other factors such as the geometry of the cross-section of the river and any historical records of water levels. In case the predicted water level is forecasted to exceed the flood level mark, warnings can be issued in advance for people to vacate the area or take protective measures.

Risk Management

Water Conservancy Project Scheduling Decision Support: ADCP data is an important input to decision-making for the operation of water-conservancy projects located along the Sênggê River, which include dams, reservoirs, and diversion channels. For instance, when a flood occurs, the flow information can be utilized in determining the most appropriate rate of release from a dam to alleviate the flood impact along a stream course section downstream.

Flood Disaster Assessment and Emergency Response: Upon the occurrence of a flood, ADCP data can be applied to determine the amount of damage that the flood has caused. This includes assessment of the areas affected by the high-velocity flow, the sediment deposition amount, among other factors. The information obtained is important in guiding the emergency response and recovery efforts such as rescue operations and post-flood rehabilitation of infrastructures and ecosystems.

6. What measurement would require good quality of the Sênggê River currents?

The equipment should be of appropriate material for quality measurement of currents of the Sênggê River. The ADCP current profiler should be resistant to corrosion from the water that may contain different minerals, sediment particles colliding due to the movement of floating debris such as branches, logs, and sheets of ice during the cold seasons in the upper catchment, and abrasion by sedimentary particles.

Secondly, smaller size, lighter weight, and lower power consumption are highly welcome. A small, lightweight ADCP flow meter is easier to deploy in operation, especially in locations with difficult access along the course of the river. Low power consumption will enable longer-term operation without the frequent replacement of batteries or complex power-supply arrangements in continuous and long-term monitoring. Additionally, a lower cost is preferred to facilitate large-scale deployment of the equipment for comprehensive monitoring of the river.

As for the material of casing, titanium alloy could be an excellent choice. Titanium alloy is very strong and enables the ADCP current meter to withstand some external forces from the flowing water or possible collision with floating objects in the river. It also has good corrosion resistance, allowing the equipment to function continuously in the water. Its relatively low density helps reduce the overall weight of the device while maintaining its structural integrity.

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

In the selection of equipment for current measurement in the Sênggê River, one has to consider the specific purpose of the measurement. When the focus is on horizontal cross-sectional flow measurements, then the Horizontal Acoustic Doppler Current Profiler (HADCP) would be appropriate. A HADCP is an instrument designed to make accurate flow velocity and related parameter measurements across a horizontal section of the river with valuable information about the lateral distribution of the flow.

In contrast, when the measurement is intended to understand the vertical cross-sectional flow, then a vertical ADCP is suitable. This will precisely measure the velocity profile from the water surface down to the riverbed along a vertical line, which would be useful in analyzing the flow of the river in its vertical variations.

Besides, different frequencies of ADCPs fit different ranges of water depth. For instance, a 600 kHz frequency ADCP meter shall be suitable for water within 70 meters in depth. However, an ADCP with a frequency of 300 kHz is more appropriate for water up to 110 meters deep. By selecting the right frequency according to the real water depth in the Sênggê River, measurement results can be accurate and reliable.

There are several famous brands of ADCP profiler in the market, such as Teledyne RDI, Nortek, and Sontek. But here is a fantastic Chinese ADCP brand-China Sonar PandaADCP. Its casing is of all titanium alloy, durable, and shows excellent performance. What's more, with an excellent price-performance ratio, you can go to its official website for more information: (https://china-sonar.com/).

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