ADCP's application in flood management of Brahmaputra River

1. Where does the course of the Brahmaputra River flow?

The Brahmaputra River constitutes one of the principal Asian rivers. Originating in the Angsi Glacier in the Himalayas, it flows through Tibet - China, India, and Bangladesh.

Gently it glides from its source-from a small stream in the high-altitude mountainous area right up to flowing through the plains of Assam in India down to low-lying delta regions of Bangladesh. For that very reason, the river chiefly acts as a route for transportation and water supply for extensive agriculture through irrigation along its length. It houses so many species of fish and supports rich biodiversity.

It receives a monsoon-influenced climate with variation in climate and rainfall within this river catchment area. The normal monsoon period falls from June to September and is characterized by heavy rainfall in this part of the region. The rain in the upper catchments and melting of glaciers in the Himalayas increases the water volume of this mighty catchment area. This difference in the level of the water, due to the monsoon, is quite wide; hence, there is severe flooding, most of all in the flood plains and low-lying parts of Assam and Bangladesh.

2. Why does the Brahmaputra River flood?

Heavy rainfall coupled with glacial melt: Heavy rainfall heralding the arrival of the monsoon is the main cause of flooding along the banks of the Brahmaputra. Because of its huge catchment area, the volume of water continually fed into the main channel is massive, and added to this, melting glaciers in the Himalayas raise the temperature. These can combine to bring rapid rises in the level of water.

Topography: The course of the river lies over the wide floodplain and lowlands, providing space for water to spread and deposit. So, the flat plains of Assam and Bangladesh increase the steepness of water's slope and enhance the tendency to flood. Sometimes, the tributaries, on joining and their contribution, carry more water during high discharge, thereby increasing the severity of the flood.

Very High Rates of Erosion and Sedimentation: One major peculiarity of Brahmaputra River is the extremely high rate of erosion and sedimentation. As a result of the continuous topsoil erosion from its banks, the carrying capacity of the river goes down, which even makes it change its course quite often. This might result in the flooding of newer areas and increase in the sediment load in the water that affects the river during the flood time.

It would serve as one of the major ways of monitoring and regulating the flow in the Brahmaputra in case of flooding in the coming days.

3. How Do Doppler Principle-Based ADCPs Work?

Basically, the ADCPs work on the principle of Doppler. The general operation includes the instrument sending an acoustic signal in the water. Water in flow would imply that such a signal intercepts the moving particles of water. In case the same acoustic signal is reflected back to the Acoustic Doppler Current Profiler (ADCP) then due to the Doppler effect, the frequency of the reflected signal alters.

This reads the frequency difference between the emitted signal and the return signal arriving back at the ADCP profiler. Knowing the frequency shift, it estimates the water velocity at each measuring depth. Most of these instruments are fitted with a number of transducers that may project their acoustic signals in different directions and receive returning signals simultaneously. The typical result obtained is a profile of the water velocity across the section.

For example, if the flow is coming towards the ADCP current meter, then the frequency of the signal returning to it is higher than the one emitted by the same. Conversely, if the water flow was away from the acoustic doppler velocity meter, then the returning frequency of the signal would have been lower. By measuring such frequencies precisely and applying relevant mathematical algorithms to them, the ADCP flow meter can correctly say with pretty good accuracy the speed of water at different areas or layers of its measurement range.

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

4.1 Measurement of Velocity

ADCP is very helpful in the measurement of water flow during every flooding event in the Brahmaputra River. It keeps on monitoring the velocity of the water at every depth and location, updating data in real time about the speed at which the water is moving. This therefore, becomes handy as information to know the dynamics of the flood-for instance, which direction or intensity flood waters are moving towards .

4.2 Flow Measurement Application

It can also be used for measuring the discharge rate of the Brahmaputra River. Water velocities measured across a section of the river and using the known cross-sectional area of the river can calculate the total amount of water that moves through the section in a unit of time, hence the flow rate. This information is highly useful for knowing the quantity of water flowing during full flood and for planning flood control and water management policies.

4.3 Application in Sediment Transport Studies

Besides measurement of flow and velocity, ADCP meter is used in the study of sediment transport by the Brahmaputra River during its flood stage. While flowing, water carries sediments. Basically, this ADCP is able to detect the variation of the backscattered acoustic signal due to the presence of sediments. Changes featuring the quantity and mobility of sediments can be estimated through analysis and represent one of the important keys to understand the long-term evolution of the river bed and the effects of floods on the fluvial sedimentary environment.

5. How does the measurement from ADCP help in flood warning and risk management of the Brahmaputra River?

5.1. Flood warning

Flow Velocity and Flow Data Monitoring: Real-time flow velocity from ADCP shall be continuously monitored. The excess of the measured water velocity beyond the threshold value or the overall significant rise in flow may indicate the arrival of a peak flood or the coming flood situation. By doing so, the concerned authorities shall have an early warning for all necessary preparations for the evacuation of people in the low-lying areas or reinforcement of flood defenses.

Water Level Forecasting and Warning: The recorded flow data is correlated with the past data of water levels and relevant hydrological models in order to predict the possible water levels that will be attained along with the time profile at a given point downstream. In case the forecasted water level is more than the warning level against flooding, early warnings must be issued to the public for preparation against flood.

5.2 Risk Management

Scheduling Decision-Making of Water Conservancy Project: Accurate flow and velocity information provided by the ADCP offers decision support to derive decisions in the operation of water conservancy projects, including functional activities of dams and sluice. For example, based on measurement results on water flow, it can regulate releasing amount of a certain dam to control water level of the Brahmaputra River with a view to reducing loss incurred by flood disasters.

Flood Disaster Assessment and Emergency Response: The magnitude of the flood can be determined from the ADCP data available following the occurrence of the flood. In addition, but not limited to, it ranged in the range of information such as inundation area, the velocity of the flood flow, the quantity of sedimentation, among others. Such information becomes of essence in the formulation of emergency response plans and in carrying out post-flood reconstruction and rehabilitation works.

6. Under what condition good quality measurement of the currents of the Brahmaputra River is made?

Good quality measurement of currents of the Brahmaputra River depends on a number of factors. First, the equipment used in the measurement should be made from quality materials. The casing of the ADCP deserves special mention. According to recommendations the casing should be manufactured from a titanium alloy. The water flow in the river exerts high pressure and impact on the underwater camera; it should be made of high-strength, low-density titanium alloy. Besides, it is very resistant to corrosion, which is important because water from the Brahmaputra River can contain various corrosive components.

Besides material reliability, the size and weight of the equipment should be as small as possible. This will facilitate installations and operations of ADCP at various sites, even in the most inaccessible parts of the river. The power consumption shall also be low so as to be used continuously for a long period of time without necessarily needing to change the batteries or even being connected to a power source. Relatively low equipment cost shall also be vital in order to enable measurements to be carried out on a large scale. Low cost allows for more ADCPs to be deployed down the river Brahmaputra, with more comprehensive and detailed data for flood management.

7. HOW TO CHOOSE THE RIGHT EQUIPMENT FOR CURRENT MEASUREMENT?

Following factors are to be kept in mind while choosing appropriate equipment in order to conduct measurement of current in the Brahmaputra River. First, depending on the type of measurement needed-if a horizontal cross-section measurement is needed, then an HADCP is to be chosen, whereas if measurement of current is to be done for a vertical cross-section, then the correct device is a Vertical ADCP.

Second, different frequencies apply to different water depths. Sufficiency can be achieved for a water depth within 70 m by a 600 kHz ADCP. As the water depth in the Brahmaputra River is within the range, and the measurement requirements are within the match, the application of 600 kHz ADCP may be put into work. In cases where the water is much deeper, going over 70 m up to 110 m, it will be better served by a 300 kHz frequency ADCP for better measurements at such depth.

Several well-known brands in relation to ADCP exist on the market, including Teledyne RDI, Nortek, and Sontek. Should there be some economic benefit, China Sonar PandaADCP will be the choice. The device is fully constructed from Titanium alloy and therefore offers good durability and reliability even in normal water. Moreover, the most important feature would be cost-effectiveness, which would be just incredible. You can find more about it on its official website: https://china-sonar.com/.

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