ADCP for the N'Mai River Flood Management Application

1. Where is the N'Mai River?

The N'Mai River is located in Myanmar. It is a significant tributary within the northern part of the nation. Its origin is near the Chinese border in the mountain area, and the terrain with which it descends contains dense forests, small valleys, and rural villages.

It passes through a number of local villages and settlements in its course. The climate in the region where the N'Mai River is located has been influenced by the monsoon climate patterns: There is a marked season with high rainfall and another season that is dry. From around May to October is the wet season with heavy rainfall; this can often be very intense, as it plays a great part in replenishing the river's water levels. Conversely, during the dry season, it has much less rainfall. At the same time, during transition seasons, snowmelt from its higher elevations in its upper reaches also adds to the water volume.

2. What are the reasons for floods in the N'Mai River?

There are multiple factors contributing to floods in the N'Mai River. Firstly, the monsoon rainfall is a key factor. The heavy and prolonged rains during the wet season can quickly increase the water level of the river. As the tributaries feed into the main river channel during this time, the combined inflow of water can overwhelm the river's natural carrying capacity, leading to flooding.

Second, it's related to the topography of the site. The river runs down on an area with steeply rising slopes and deep valleys. When the area seems flat, the water flows relatively slower, and when much volume of water comes from the top, it overflows banks and inundates surrounding dry land.

Besides that, changes in land use in its catchment area, such as deforestation and agricultural expansion, have reduced the land's ability to absorb water. This means that more rainwater runs off directly into the river, increasing the likelihood of flooding. In addition, sediment deposition over time can raise the riverbed, reducing the river's capacity to handle large volumes of water and making it more prone to overflowing its banks.

In the presence of these flood-causing factors, the Acoustic Doppler Current Profiler (ADCP) is a more advanced and practical measurement tool than traditional methods, which allows for better flood management efforts.

3. How do ADCPs using the Doppler principle work?

The principle of operation for ADCPs is based on the Doppler effect. ADCPs transmit acoustic pulses into the water. These pulses scatter off moving particles in the water, such as sediment grains, small aquatic organisms, and other suspended material. When the acoustic waves return after reflecting off these moving objects, a frequency shift takes place.

This frequency shift is directly related to the velocity of the moving objects and, by extension, the velocity of the water itself. For instance, if an acoustic doppler velocity meter is placed in the N'Mai River, perhaps mounted on a small boat or a fixed structure close to the riverbank, it sends out sound waves that penetrate the water column. These signals received from the flowing water and its components, reflected back to the instrument as the waves. After receiving the reflected signals, the ADCP works on those to calculate speed and direction of the water flow at different depths.

4. Application of ADCP in flood studies of the N'Mai River:

Flow velocity measurement: ADCP current profiler can measure the exact velocity of the water flow in the N'Mai River. In the flood events, this information is valuable to understand how fast the floodwaters are moving. This helps in predicting the advancement of the flood, including how fast the water would reach the downstream areas, and which areas might be more prone to erosion due to high-velocity flows.

Flow rate measurement: Together with the cross-sectional area of the river-which may also be measured with ADCP or by separate surveying-measured velocities will deliver the flow rate of the river. The flow rate is a very important factor in flood conditions for assessing the amount of water passing through at any one point and thereby providing an indication of the total effect on the lower regions.

Sediment transport study: ADCP flow meter is able to detect sediment particles moving along with water. This becomes crucial at the time of flooding due to the quantification of sediments being transported by a river. The sediment transport study helps in simulating variations in the riverbed where sediment deposition or erosion will occur, thus informing on stable riverbanks and infrastructures along the course of a river.

5. How would the ADCP-measured data be utilized for the issuance of flood warnings and dealing with risks concerning the N'Mai River?

Flood warning

Monitoring flow velocity and flow rate data: ADCP provides for continuous monitoring of flow velocities and flow rates to enable early detection of changes that might indicate an imminent flood. For instance, if the velocities or flow rates start to increase beyond the normal levels, this may trigger an alert. In such a case, it enables the authorities to issue timely warnings to communities living along the riverbanks.

Water level prediction and warning: By analyzing the relationship between flow rates, velocities, and water levels-which can be established through historical data and continuous ADCP measurements-future water levels can be predicted. This helps in the provision of timely and accurate warnings to residents for the necessary precautions, such as evacuation or securing properties.

Risk management

Water Conservancy Project Scheduling Decision-Support: The value of the data from the ADCP profiler is highly valuable in supporting a decision to operate any of the water conservancy projects, such as small dams or water diversion structures, along the N'Mai River. For instance, if a flood is somewhat forecasted, then water release from reservoirs might be managed based on data measured by ADCP so as to avoid flooding further downstream and also for structural safety.

Flood disaster assessment and emergency response: After a flood event, ADCP data can be used to assess the extent of damage, such as the degree of erosion, changes in the riverbed, and the impact on infrastructure. This information guides emergency response efforts and helps in formulating future flood mitigation strategies.

6. What’s needed for high-quality measurement of the N'Mai River currents?

For high-quality measurement of the N'Mai River currents, several aspects are of essence. The equipment must be made of reliable materials. It is preferable for it to be small in size and lightweight so that it can easily be deployed in various locations along the river, either from boats or fixed positions on bridges or riverbanks. Low power consumption is essential, especially for long-term monitoring, to ensure continuous operation without frequent battery replacements or excessive energy requirements. Cost-effectiveness is also crucial to enable widespread deployment across different sections of the river for comprehensive monitoring.

The casing of ADCP is preferably made of titanium alloy. Titanium alloy has several outstanding advantages. It has excellent corrosion resistance, which is vital when dealing with river water that may contain various minerals and impurities over time. It also has a good strength-to-weight ratio, allowing for a durable yet relatively lightweight construction. This makes it more convenient to install and maintain the ADCP in different measurement scenarios along the river.

7. Selection of appropriate equipment for the measurement of current

When selecting equipment for current measurement in the N'Mai River, certain considerations come into play. Firstly, it depends on the purpose of measurement, for horizontal cross-section measurements, the Horizontal ADCP (HADCP) is suitable. It is designed to measure the flow velocities across a horizontal plane with accuracy, which helps in understanding the overall flow patterns in a particular reach of the river. On the other hand, for vertical cross-section measurements, the Vertical ADCP is more appropriate as it focuses on profiling the velocities from the water surface to the riverbed along a vertical line.

Secondly, different frequencies of ADCPs apply to different ranges of water depth. For instance, an ADCP with 600 kHz frequency is best suitable for water depth within approximately 70 meters. In stretches where the water depth of the N'Mai River ranges within this, it gives accurate and good resolution velocity records. For deeper sections, up to about 110 meters, an ADCP with a frequency of 300 kHz would be more appropriate, as it can penetrate further into the water column to effectively obtain reliable data on the flow velocities.

In the market, there are some famous ADCP brands like Teledyne RDI, Nortek, and Sontek. But if one wants a good combination of quality and affordability, the ADCP from China Sonar PandaADCP is suggested. Material-all-titanium alloy construction makes it tough enough for the river environment, with an impressive 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.