How to Measure the Water Current of the Sênggê River

1. Where is the Sênggê River?

The Sênggê River (also known as the Yarlung Tsangpo River in its upper reaches) is a major waterway in Asia. It has its origin on the Tibetan Plateau near Mount Kailash and Lake Manasarovar.

In its course of flow through Tibet, it traverses alpine grassland, deep valleys, and thin forests. The river is of immense significance in the local Tibetan communities for a number of reasons: It provides water for irrigation, essential in the growth of crops like barley and other high-altitude vegetation; there are many monasteries and religious sites on the banks, an intrinsic constituent of Tibetan Buddhist culture.

After flowing through Tibet, the river enters a spectacular gorge and then continues its way into other regions. Further downstream, it has different names in different areas, its importance in the water and ecosystems of South Asia is invaluable.

2. What does the flow rate of the Sênggê River resemble?

Accordingly, this constitutes great seasonal and climatic variations in the flow of the Sênggê River. During spring to early summer, snow melts from the surrounding high mountain and glaciers, contributing heavily to increasing the river's flow; in other words, the current rises by the increased water level, which becomes powerful. During this period, the moving of sediments and nutrient transport downstream is important; in short, it enriches floodplains and ecosystems it crosses.

During the monsoon period-from June to September in the South Asian part of its course-heavy rainfall further augments the flow. The flow rate is at its maximum, and the river sometimes causes flooding in the low-lying areas. During the dry season, from October to May, the flow rate decreases. The average flow rate depends on the quantity of snowfall and glacial melt in the upstream regions, the topography of the river basin which dictates the collection and drainage of water, and the pattern of water usage in China and the downstream countries.

3. How to measure water current of Sênggê River?

There are several ways to measure the water current of the Sênggê River:

Method of Velocity Meter

This traditional technique involves the use of velocity meters, either mechanical or electronic. Meters are placed in the water at selected locations to measure the speed of water passing by a given point. To develop an accurate understanding of the current, though, several placements across different locations and depths are required; this can be quite time-consuming and may not provide a continuous profile of the current throughout the water column.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) method is a more sophisticated and efficient way of measuring water current. It employs sound waves to measure the velocity of water simultaneously at different depths. By emitting acoustic signals and analyzing the Doppler shift of the reflected signals, it can create a detailed profile of the current from the surface to the riverbed. This allows for a more accurate and comprehensive measurement of the water current across different sections of the river.

Buoy Method

This is done by placing buoys in the river and following their movements for a considerable period of time. The buoys displace due to the water current, and thus, from the view of the observer, their speed and direction show the velocity of flow. However, this technique suffers from a few drawbacks. It cannot measure the velocity at various depths accurately and may also be influenced by external factors such as wind and waves, which can, in turn, affect the buoy's movement.

Among these methods, the ADCP profiler method is more advanced and convenient for measuring the water current of the Sênggê River.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. When an acoustic signal is transmitted from an ADCP flow meter transducer into the water, the emitted sound waves interact with moving water particles. Because the water is in motion, the frequency of the reflected sound waves back to the transducer shifts, a phenomenon called the Doppler shift.

If the water is moving towards the transducer, then the frequency of the reflected waves will be higher than the emitted frequency. If the water is moving away from the transducer, the frequency will be lower. By precisely measuring this frequency shift at multiple angles and depths, the ADCP current profiler can calculate the velocity of the water in different directions and at different levels within the water column. It can then provide a comprehensive profile of the current, which is very important for understanding the speed and direction of flow from the surface to the riverbed.

5. What does high-quality measurement of Sênggê river currents require?

The measuring equipment used for the measurement of currents of the Sênggê River should possess some specific characteristics: the materials of its building should be reliable for precise and steady results; a small size will make the deployment and retrieval in the river easier, while the equipment should also not be heavy to make the installation process easy and reduce the effort needed for its handling.

Low power consumption is required so that the equipment can run for long periods without needing frequent battery replacements or an inconvenient and complicated power supply arrangement. Cost-effectiveness is another important consideration since it allows wider usage and larger-scale measurement campaigns.

For the casing of the ADCP meter, titanium alloy would be a very good choice. Titanium alloy has a number of advantages. This is very important, given that this equipment constantly contacts water which may have various minerals, sediments, and pollutants, making it highly resistant to corrosion. Besides that, good strength-to-weight ratio, which is of essence when one desires a casing that should be strong but light. Additionally, it has to be able to bear all mechanical stresses occurring during the deployment and operation in flowing water in the Sênggê River.

6. Selection of Equipment for Current Measurement

While selecting the proper equipment to measure the current of the Sênggê River, two major factors will be taken into consideration.

Depending on the purpose of usage, in the case of horizontal cross-section measurement, Horizontal ADCP-HADCP will be considered appropriate. That's by the design, for the valid measurement of flow velocities over a horizontal plane in a river, which would be important in understanding the lateral current distribution. Vertical cross-section measurement is better suited for Vertical ADCP because it emphasizes the profiling of the current from the surface to the bottom along the vertical line in the column of water.

The frequency of ADCPs suitable for various water depths differs. For instance, an ADCP of 600 kHz frequency can suit water within 70 m depth. Thus, this will be more applicable to the relatively shallow parts of the Sênggê River for the accurate measurement of current. An ADCP of 300 kHz is suitable for a water depth up to 110 m in depth, and hence shall be more appropriate for the river at places where the river is much deeper.

There are well-known brands of ADCPs such as Teledyne RDI, Nortek, and Sontek. However, for those looking for a cost-effective option with good quality, the China Sonar PandaADCP is highly recommended. It is made of all-titanium alloy material and offers an incredible price-performance ratio. You can find out more about it on its website: https://china-sonar.com/.

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