How to Measure the Water Current of the Zangbo River

1. Where is the Zangbo River?

The Zangbo River, in Tibetan called the Yarlung Tsangpo River, is an important Asian river. It originates on the Tibetan Plateau near Mount Kailash and Lake Manasarovar.

In its course through Tibet in China, the river flows through alpine meadows, deep valleys, and stretches of land that are sparsely covered by forests. The river serves as an important part of this local ecosystem and culture, providing irrigation water to these high-altitude regions, therefore supporting crops such as barley. There are many monasteries and religious sites of high importance in Tibetan Buddhism lined along its banks.

After flowing through Tibet, it flows into a famous deep gorge and flows uninterruptedly until, at length, it falls in other regions and plays an important part in the water system of South Asia, where, far downstream, it goes under different names.

2. What is the nature of the flow in the Zangbo river?

The flow rate of the Zangbo River has big differences in relation to different seasons and geographical aspects.

In spring and at the beginning of summer, during snow melting from surrounding high mountains and glaciers, it strongly increases the flow of the river. The water level rises, and the current becomes rather powerful. This period is vital for transporting sediments and nutrients downstream, enriching the floodplains and the ecosystems it traverses.

During the monsoon season-usually from June to September in the parts relevant to its downstream regions-heavy rainfall further augments its flow. The flow rate peaks, and in some cases, it can cause flooding in low-lying areas. In the dry season, which usually ranges from October to May, the flow rate decreases. The average flow rate is determined by the level of snowfall and glacial melt in the catchments, the nature of topography in the river basin regarding catchment and drainage, and the pattern of water use both in the local Tibetan areas and the areas downstream.

3. How to measure water current of the Zangbo River?

Velocity Meter Method

This conventional technique relies on mechanical or electronic velocity meters. Meters are placed at selected points in the water to record the speed of the water rushing past them. However, for a proper interpretation of the current, multiple placements at various locations and depths are required. This is somewhat time-consuming and does not always provide a continuous profile of the current throughout the full water column.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a more advanced and efficient way of measuring water current. It utilizes sound waves to measure the velocity of water at different depths simultaneously. 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 enables a more accurate and comprehensive measurement of the water current across various sections of the river.

Buoy Method

A few buoys are introduced to the stream and their position is plotted over time. The displaced movement of the buoys by the current of the water gives a rough measure of the speed and direction. However, this method suffers from a lot of errors. It will not accurately be able to measure the exact velocity at every depth while it can also be biased by other sources such as wind and wave that move the buoys.

Of all these methods, the ADCP current meter method is generally more advanced and convenient in measuring the water current of the Zangbo River.

4. How do ADCPs using the Doppler principle work?

The working of ADCPs is based on the principle of the Doppler effect. When an acoustic signal emitted from the ADCP transducer into the water, the sound waves will interact with moving water particles. Due to the flow of water, the frequency of the reflected sound waves back to the transducer changes; this is the so-called 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, then the frequency will be lower. Precise measurement of this frequency shift at several angles and over various depths enables the ADCP flow meter to compute the water velocity in various directions and at multiple levels within the water column. This enables it to make a fine profile of the water current, thereby providing much-valued information about the flow's speed and direction from surface to riverbed.

5. What is required for high-quality measurement of Zangbo River currents?

The measuring equipment, for high-quality measurement of the Zangbo River currents, must possess certain characteristics.

Materials of construction should be such that the measurement it provides is accurate and consistent. Small size would help in easier deployment and retrieval in the river at sites where access is not easy.

The equipment should also not be too heavy since its installation requires a lot of effort for handling.

It needs low power consumption to ensure the equipment will operate for a long time without having to change the battery or do complex adjustments in the power supply. Cost-effectiveness, which allows its widespread use and larger-scale measurement campaigns, is another important aspect.

In regard to casing material for ADCP profiler, one of the best materials that could be used is a Titanium alloy. The benefits of a Titanium alloy are many:. The material has excellent resistance against corrosion, which is indispensable as the equipment is all the time in contact with flowing water containing a variety of minerals, sediments, and other pollutants. Similarly, the good strength-to-weight ratio makes for a strong, lightweight casing. In addition, it will be under the mechanical stresses occurring during both the deployment and operation when faced by flowing water in Zangbo River.

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

When it comes to selecting the right equipment for measuring the current of the Zangbo River, two major factors should be considered.

Depending on the purpose of usage, for horizontal cross-section measurement, the Horizontal ADCP (HADCP) is the suitable option. It is designed to measure the flow velocities across a horizontal plane in the river with high accuracy for the purpose of understanding the lateral distribution of the current. In the case of vertical cross-section measurement, the Vertical ADCP would be more appropriate because it focuses on profiling the current from the surface to the bottom along a vertical line in the water column.

Different frequencies of ADCPs apply to different water depths: an ADCP with 600 kHz is appropriate for a water depth within 70 m. That can deliver accurate current measurement in relatively shallower parts of the Zangbo River. An ADCP meter with a frequency of 300 kHz is more appropriate for deeper waters up to 110 m, useful in areas where the river has greater depth.

Teledyne RDI, Nortek, and Sontek are well-known brands within ADCPs. But for the ones who seek a cheaper option with good quality, the China Sonar PandaADCP is very recommended; made of all-titanium alloy material, it's incredible for its price/ performance ratio. More about this can be read on their website: https://china-sonar.com/.

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