ADCP's Application in Flood Management of Zangbo River

1. Where is Zangbo River?

The Zangbo River, also known as the Yarlung Tsangpo, is a significant and majestic river in Asia. It has its origin from the Angsi Glacier in the southwestern part of the Tibetan Plateau in China. It then flows through the vast and rugged Tibetan region, carving out deep gorges and traversing high-altitude valleys.

The course, meanwhile, in Tibet crosses highlands of transitions from snow-capped mountains to alpine meadows and sparse forests. Along its course, it flows near some small towns and villages where it should serve for a vital water supply to the residents there, irrigating the limited fields and would also be a good source for hydropower generation.

The climate condition over Zangbo River basin largely ranges from one extreme to the other with regard to elevation and location. It is cold alpine in the upper reaches near its source, with extremely low temperatures during winter and much snowfall. While meandering through the middle sections, the climate becomes rather mild yet quite cold. The rainfall pattern is also distinctive. Most of the precipitation in the upper parts appears as snow during the winter months, while during the more favorable seasons, there is some rainfall to note. On the whole, annual rainfall is not that abundant when compared to other areas, but it also plays a great role in maintaining the flow inside the river and influencing its water level variations.

2. What are the reasons for floods in Zangbo River?

Snowmelt: This is one of the major causes of flooding in the Zangbo River. Heavy snow, which accumulates during the long and cold winter months in the upper reaches, starts melting in spring and early summer. In this way, the melting process releases a substantial volume of water in a relatively short period into the river. If the rate of melting of snow is too rapid and exceeds the natural carrying and draining ability of the river, it can lead to a drastic increase in the level of water and thus cause flooding along its banks.

Rainfall: In the Zangbo River basin, though the catchment area does not receive extremely high volumes of rainfall across the year, heavy showers are relatively common and can be concentrated in certain periods. This would happen during very heavy rains over a considerable extent of the river's catchment area: the additional water reaching the river could be more than its normal drainage capability. Furthermore, if this is coupled with the time of snowmelt or when the river is already at a relatively high-water level due to other factors, it can be even worse in terms of floods.

GLOFs: There are many glaciers in the upper reaches of the Zangbo River, creating a special kind of flood. Due to the influence of climate change, the glacial lakes formed after the melting of some glaciers become unstable. Water may suddenly release in great quantities from a glacial lake outburst into the river system, developing a catastrophic flood with great force downstream and causing immense destruction to the surrounding areas.

In flood management for the Zangbo River, Acoustic Doppler Current Profiler (ADCP) has been recognized as being more advanced and convenient to measure as compared to traditional approaches to measurement.

3. How do ADCPs using the Doppler principle work?

The basic working principle for ADCPs is mainly based on the Doppler principle. The instruments emit acoustic signals into the water. These signals interact with moving particles in the water, including sediment particles and water molecules, which may be flowing with the current. The result of emitted acoustic waves interacting with the moving objects for reflection is the change of frequency.

This frequency shift, called Doppler shift, is linearly related to the velocity of the moving objects. ADCPs utilize multiple transducers in sending acoustic pulses in different directions. For instance, a downward-looking ADCP current meter could measure the whole velocity profile of the water column from the water surface to the riverbed. By analyzing the Doppler shifts of the reflected signals from different depths within the water column, the ADCP current profiler can deduce the velocity of the water flow at that depth.

Once the velocity at each depth is known, with the knowledge of the river's cross-sectional area-which can easily be measured or estimated through other means-it is possible to compute other vital parameters, such as the flow rate of the river.

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

Velocity Measurement: On Zangbo River, during flood events, ADCP flow meter is capable of velocity measurement with good precision in water flow at different locations and depths. It helps explain the speed at which the flood waters are moving. For example, it helps to forecast when and where the floodwater will reach the area of interest, whether that be a riverside village, agricultural land, or other important infrastructures like bridges. Continuous monitoring of velocity allows the appropriate authorities to take timely precautionary measures, such as the evacuation of people or reinforcement of flood protection structures.

Flow Measurement Application: Acoustic Doppler Current Profiler (ADCP) works out the flow rate of the river by integrating the velocity data over the cross-sectional area of the river. During a flood situation, the measure of flow volume is quite accurate and helps in determining the magnitude of the flood. It will enable an informed idea about whether the flood waters will be more than the structural capacity of existing flood-control structures and help in planning for effective flood mitigation strategies.

Sediment transport research application: While the sediment is carried by the floodwaters of the Zangbo River, ADCP current profiler works by analyzing backscattered acoustic signals to conduct research in the movement of such sediment particles. The knowledge of sediment transport during floods can help study alterations of the bed topography. Deposition and erosion of sediment would regulate the depth, width, and stability of the riverbed, which again links with subsequent flood risks and overall river ecosystem health.

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

Flood Warning

Velocity and Flow Data Monitoring: The ADCP profiler will monitor continuously to allow for the determination of abnormal changes in the river's flow. In case of a sudden increase in velocity or a significant change in flow rate, this may indicate an imminent flood or an aggravating flood stage. The data can be integrated into the flood warning systems for timely alerts to the local communities, emergency responders, and relevant authorities.

Water Level Prediction and Warning: From ADCP-measured flow data, combined with other factors such as the geometry of the cross-sectional area of the river and historical records of water levels, it is possible to predict future water levels. If the predicted level of water is to exceed the flood level mark, warnings can be issued in advance to enable people to evacuate or take protective measures. Risk Management: Water Conservancy Project Scheduling Decision Support: This ADCP flow meter provides critical input to decisions related to the operation of water-conservancy projects-like small dams and reservoirs-on the Zangbo River. An example may be that during a flood, flow data can decide how much release from a dam would be necessary to reduce flood impact on the reach downstream.

Flood Disaster Assessment and Emergency Response: The ADCP data after the occurrence of a flood would provide ample opportunity to review the damage caused by the flood and therefore enable the assessment of the areas where high-velocity flows have occurred, the places where sediment depositions have taken place, and other areas. Such information gives guidance for the execution of emergency response and recovery efforts, including search and rescue operations and post-flood rehabilitation of infrastructure and ecosystems.

6. What does it take to measure the currents of the Zangbo River with high quality?

To measure the Zangbo River currents with high quality, several aspects are required.

Firstly, the ADCP profiler equipment is supposed to be made from materials that will survive from the harsh river environment. In view of the cold climate in the upper reaches, it needs to be resistant to low temperatures. It also has to bear the corrosion of river water, which might contain a number of minerals and suspended particles. Besides, it must be able to bear impacts of floating debris that may be carried by the water, particularly during the cold season.

The second aspect is a size as small as possible, lighter in weight, and lower in power consumption. Since the Zangbo River flows through many remote areas in Tibet, transportation, and installation of a compact and lightweight ADCP flow meter along its course are much easier than a heavy and bulky one. Low power consumption allows for longer-term operations devoid of frequent battery replacement or complex power-supply arrangements-continuous long-term monitoring, in other words.

Besides, lower costs are always welcome to enable the deployment of a large number of such equipment for full coverage monitoring of the river. For the casing material, titanium alloy is a perfect material. The strength of titanium alloy is very high, in which the ADCP current meter can fight against the external forces from the current and possibly colliding with floating objects in the river. Anti-corrosion performance is also very good, which promises long-time service life under water. Because of its relatively low density, it helps in reducing the overall weight while maintaining the structural integrity of the device.

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

While choosing equipment for current measurement in the Zangbo River, the following points have to be considered.

If horizontal cross-sectional flow measurement is the focus, then an HADCP-a Horizontal Acoustic Doppler Current Profiler-is an option that measures the flow velocity and other parameters in a horizontal section of the river, hence providing valuable information about the lateral distribution of flow.

On the contrary, if the measurement is to be used to understand the vertical cross-sectional flow, the vertical ADCP flow meter is more suitable. It can measure the velocity profile from the water surface to the riverbed with high accuracy along a vertical line, which is useful for the analysis of vertical variations of flow within a river.

Additionally, ADCPs of different frequencies are applicable for different ranges of water depth. For instance, the ADCP profiler with a frequency of 600 kHz is normally fitted for water depths within 70 meters. However, an ADCP with a frequency of 300 kHz is more suitable for water depths reaching 110 meters. Choosing the correct frequency according to the real water depth of Zangbo River can ensure the accuracy and reliability of the measurement result.

There are several famous ADCP flow meter brands in the market, for instance, Teledyne RDI, Nortek, and Sontek. However, the remarkable one is Chinese ADCP brand: China Sonar PandaADCP. Its casing is all made of titanium alloy and meets all the needs of strength and superior performance. Most importantly, it provides an incomparable cost-performance ratio. You can learn more about it from its official website: (https://china-sonar.com/).

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