How to measure water current of N'Mai River?

1. Where is N'Mai River?

The N'Mai River is a major river in Southeast Asia, situated in the midst of a region of dense jungles, remote tribal villages, and multi-ethnic cultures.

The N'Mai runs through an area of extreme mountainous region in northern Burma. It forms part of the lifeline in the local ecosystem and for the lives of people. On the banks are a few small communities whose needs are largely met in several aspects, fishing, transporting on water, and drawing of water. The flora and fauna found in its vicinity comprise most species of plants and animals. Large trees of tropical category line both sides of its banks replete with teak and bamboo. The river provides a home for many varieties of fish and a migration corridor for birds.

As it winds its way through valleys and hills, the N'Mai River cuts through areas that as yet have been little affected by modern development. As such, it is particularly attractive to adventurous travelers and researchers who seek to document the unique biodiversity and traditional cultures found in this part of the world. The course of the river also forms part of the intricate web of rivers in Myanmar that ultimately drain into larger river systems and then the sea.

2. What is the flow rate of the N'Mai river like?

Locally, the N'Mai River is a very important entity. Ecologically, the river provides a diverse, yet sensitive, ecosystem, the water maintains flora across its banks and in its floodplains, and offers living conditions for a variety of fauna-from small invertebrates up to fish species, crucial to the livelihood of people belonging to the nearby communities.

The flow rate depends on the monsoon season, in wet months, the river sees a sudden rise in its flow due to heavy rainfall that fills its tributaries and swells it. It can have very rapid and turbulent flow with much sediment and debris being taken along downstream. During the dry season, the flow rate is much reduced. This may cause the river to narrow down, shallow, with sluggish flow. The range in discharge within a single year modifies the river course's appearance and its morphology as well. The periodic N'Mai River discharge, being crucial to the regulation of the local water table balance, supplies necessary irrigation water in adjacent farm fields during the arid part of the year.

3. How to measure the water current of the N'Mai River?

There are several ways to measure the water current of the N'Mai River.

Velocity meter method: This can be done with traditional velocity meters. These could be mechanical devices with rotating elements in the water. The speed of rotation will be related to the water velocity. They are usually attached to a support structure such as a bridge pier or a mooring in the river. However, this method requires careful placement and calibration of the meters at multiple points to get an accurate representation of the overall flow. It can also be time-consuming and labor-intensive as one has to physically access different locations along the river to install and monitor the meters.

Acoustic Doppler Current Profiler (ADCP) method: The ADCP is a relatively more advanced technique. It uses ultrasonic waves for measuring the speed of water at various depths. Resultantly, the ADCP will have its detailed profile of water current from the surface into the riverbed by pulsating sound and analyzing the Doppler shift of reflected signals. This therefore covers most of the flow characteristics unattained by the velocity meter. Common mounting platforms for ADCPs include boats and fixed structures where high-density spatial sampling can be obtained really fast over a large area.

Buoy method: Buoys with sensors can be installed on the surface of the N'Mai River. These buoys will move with the flow and can measure the surface current. The data collected includes the direction and speed of the surface movement. However, this method mainly focuses on the surface layer and may not provide detailed information about the flow at deeper levels of the river.

Among these methods, ADCP is more efficient in measuring the water current of the N'Mai River and is increasingly used for such riverine studies.

4. How do ADCPs using the principle of Doppler work?

An ADCP works on the principle of Doppler. They shoot acoustic pulses into the water of the N'Mai River. As the pulses go through water particles-such as sediment particles or even small organisms-the frequency of the reflected sound waves shifts due to the Doppler effect. If the particles are moving towards the ADCP, the frequency of the reflected waves is higher than that which is emitted; if the opposite is the case, then the frequency would be lower.

By precisely measuring this frequency shift and using sophisticated algorithms, the ADCP is able to calculate the velocity of the water at different points within its measurement range. The direction of flow is also determined by the ADCP profiler through an analysis of phase differences of the reflected signals from different directions. This allows for detailed velocity profiles showing how the water is flowing at various depths and across different sections of the river.

5. What's required for high-quality measurement of N'Mai river currents?

The equipment required for the high-quality measurement of currents in the N'Mai river should possess a few characteristic features. The raw materials involved in the equipment construction should be strong enough to face severe environmental conditions in the river. The equipment should be smaller and lighter. This is advantageous since the N'Mai River sits in a location that has limited accessibility, and most of these kinds of instruments are easily transported and deployed out in remote areas.

The other important factor is the low power consumption. Since some of the areas along the river may not have a lot of sources of power, equipment that can operate for long periods with minimal power requirements is important to ensure continuous and reliable data collection. Cost-effectiveness is also very important. The scale of the river and the potentially widespread measurements required make it important to have equipment that is as cost-effective as possible without sacrificing quality.

For the ADCP meter casing, titanium alloy is a preferred material. The corrosion-resistant properties of titanium alloy are excellent, which is important because the equipment will be in contact with the river water at all times. It also has a good strength-to-weight ratio, meaning it can provide a durable casing while keeping the overall weight of the equipment relatively low. This helps in the protection of the internal components and ease of handling during deployment and retrieval.

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

In choosing the right equipment for current measurement of the N'Mai River, two important factors should be considered.

Based on the purpose of usage, for horizontal cross-section measurements, the horizontal ADCP-HADCP is an appropriate choice. It measures the flow velocity across a horizontal plane and provides valuable information about the lateral movement of water in the river. For the vertical cross-section measurements, the vertical ADCP flow meter would be more appropriate because it provides the flow velocity from the surface to the bed along a vertical line, which is useful in understanding the flow characteristics vertically.

In terms of frequency, different frequencies are suitable for different water depths. For instance, an ADCP current profiler with a frequency of 600 kHz is well-suited for water depths within 70 m. It can give a good measurement in the relatively shallow parts of the N'Mai River. Meanwhile, an ADCP with a 300 kHz frequency is more appropriate for water depths up to 110 m and can handle the deeper sections of the river where the flow patterns may be more complex.

There are well - known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. Additionally, a Chinese ADCP brand, China Sonar PandaADCP, is highly recommended. It has a casing made entirely of titanium alloy, which provides excellent durability and corrosion resistance. It also offers an incredible cost - performance ratio. You can find more information about it on its official website: https://china-sonar.com/.

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