Why We Measure River Flow in Assam?

1. Where is Assam?

Geographic Background

Assam is located in the north-eastern part of India, between approximately 24.79°N - 27.56°N latitude and 89.49°E - 96.79°E longitude. The state of Assam has a diverse topography comprising the Brahmaputra Valley in the north and the Barak Valley in the south with the Karbi Anglong and North Cachar Hills separating them. Brahmaputra Valley, dominated by the strong Brahmaputra River, is a vast, flat alluvial plain, formed by the loads of sediments carried by the river and deposited over centuries. It provides fine land for agriculture and is well suited for cultivation. Assam experiences a tropical monsoonal climate, and summers are warm and humid from March to June, with the highest temperature going up to 38°C. Winters, from November to February, are gentle with a temperature ranging from 10 - 18°C. The monsoon season, from June to September, is characterized by heavy rainfall, which is a prime source of the region's water resources and regulates the flow of its rivers.

Humanities and culture

The cultural heritage of Assam is rich and unique, woven strongly into its natural environment. It is home to different indigenous tribes, each of which has its own unique traditions, language, and art. Bihu festival is the biggest event in Assam and celebrates the agricultural cycles and is celebrated with great enthusiasm three times a year - Bohag Bihu (Rongali Bihu) in April, Kati Bihu (Kongali Bihu) in October, and Magh Bihu (Bhogali Bihu) in January. The festival is characterized by vibrant folk dances like the Bihu dance, where men and women dressed in multicolored traditional attire do energetic steps succeeded by traditional musical instruments such as dhol, pepa, and taal. Assam is also renowned for its production of tea; the tea gardens that sprawl across the state not only contribute a great deal to the economy but also form a part of the culture of the region. The state has a long tradition of handicrafts, where products like cigarette wrapping papers (khair), Assamese traditional silk (Muga, Pat, and Eri silk), etc., are highly valued.

Hydrology and River Overview

The Brahmaputra River is the backbone of Assam and is the hub of the state's hydrology, ecology, and human habitation. It arises in the Chemayungdung Glacier of Tibet near Manasarovar Lake in Tibet, runs through Tibet, India, and Bangladesh, to drain into the Bay of Bengal. The Brahmaputra River in Assam has numerous tributaries like the Subansiri, Jia Bharali, Dhansiri, and Kopili rivers. The other significant river system, the Barak River, flows through the state's southern part and is fed by tributaries like the Jatinga and Sonai rivers.

These rivers are most important to Assam. Ecologically, they support a rich biodiversity with the Brahmaputra River basin being home to an array of aquatic life such as the endangered Ganges River dolphin and numerous species of fish, birds, and other faunal elements. The rivers and associated wetlands also play a crucial part in maintaining the overall balance of the ecosystem. For domestic use, the rivers provide a primary source of freshwater for residential, agricultural, and industrial use. Agriculture, especially rice, tea, and oilseeds cultivation, is heavily dependent on water from these rivers for irrigation. Transportation is also convenient via the rivers, with most areas relying on riverine channels for transportation of goods and people. However, the Assam rivers also experience regular floods and erosion, which are the main problems of the state.

2. What is the River Flow Between Assam?

Controlling Factors

Rainfall and Runoff

Monsoon is the largest controlling factor of river flow in Assam. Heavy and normally unpredictable monsoon rains lead to sudden increases in river levels and flow velocities. The amount of rainfall varies significantly from one year to another, resulting in variability in river flows. In years with heavy monsoons, the rivers overflow and lead to extensive flooding. Urbanization and deforestation in catchment areas of rivers have also disrupted the natural flow regimes. The increase in urban impervious surfaces and deforestation reduce the amount of rainwater infiltration, leading to more surface runoff and elevating the risk of flooding.

Topography and River Morphology

The low-lying topography of the Brahmaputra and Barak Valleys contributes to relatively slow river flow in most areas. However, the presence of enormous amounts of sediment carried by the rivers during monsoon may result in the change of the elevation of the riverbed and the formation of sandbars and braided channels. The Brahmaputra River, being extremely dynamic, often changes its course due to erosion and deposition over time. The rivers may be much wider and deeper along their courses and are susceptible to erosion by the soft alluvial soils of the riverbanks, particularly during high-flow events.

Reservoir Operations

There are some dams and reservoirs in the higher reaches of the rivers that drain into Assam, primarily in the Himalayas. These are utilized for the generation of hydroelectric power, irrigation, and flood control. For example, the dams of the Subansiri River regulate water release downstream. But the operation of these reservoirs can have both beneficial and harmful impacts on the river's flow in Assam. Proper regulation is to be credited with reducing flood risk during the monsoon season and providing a minimum flow in the lean season. But unregulated discharge of reservoir water, especially during heavy rainfall, can aggravate flooding in the state.

Historical Hydrological Events

Assam is regularly affected by severe floods due to the excessive monsoon rainfall and the hydrological characteristics of the Brahmaputra and its tributaries. There was a severe flood in 2019 which affected major regions of Assam. According to the Assam State Disaster Management Authority, over 28 lakh were affected in 28 districts, thousands of hectares of crops were flooded, and many villages were inundated. The flood also caused huge damage to infrastructure, roads, bridges, and buildings. Observing such historical hydrological events is essential to plan efficient flood - control measures, improve early - warning systems, and construct the state's resistance to future disasters.

3. How is the River Flow in Assam Observed?

Traditional Methods

Surface Drift Float Buoy Method

Surface drift float buoy method is a very simple method of measurement of river flow. One suspends a buoy, such as a plastic bottle or a cork, on the surface of a river and gauges its movement for a specific distance and time interval. The velocity of the buoy is taken to be the river's surface velocity. The method is easy to employ and requires minimal apparatus but can not be recommended. It measures only the surface flow and is highly affected by wind and other surface disturbances. In turbid rivers of Assam, the floating debris also affects the movement of the buoy, i.e., there can be measurement errors.

Anchored Boat Method (Staff Gauge/Current Meter)

A boat is moored at a fixed location in the river in the anchored boat method. A current meter is lowered at different depths to record the velocity, and a staff gauge is used to measure the level of water. The method provides a true velocity profile at a location but is time - consuming, labor - intensive, and unsafe, especially in rapidly flowing or contaminated rivers. The need for qualified personnel and the risks of working in the river environment are a serious limitation.

ADCP Introduction: Acoustic Doppler Current Profiler (ADCP)

The Acoustic Doppler Current Profiler (ADCP) offers a more advanced and efficient method of measuring river flow in Assam. Unlike traditional measurement methods, ADCP can generate continuous velocity profiles from surface to river bed non-invasively. It can quickly survey extensive areas of the river, required to understand the complex flow regimes in the Brahmaputra and its tributaries, especially in terms of sedimentation, pollution, and anthropogenic modification. ADCPs are also safer for operation because there is no direct human contact with the potentially polluted river water.

4. How Does an ADCP Work Based on the Doppler Principle?

An ADCP flow meter operates on the Doppler principle. It emits acoustic signals of a specific frequency. Upon collision with moving particles in the water, i.e., sediment, plankton, or small organisms, the frequency of the reflected signals depends on the relative movement of the particles. The frequency of the returned signal is greater if the particles are approaching the ADCP and lower if departing from it. By analyzing these changes in frequency, the ADCP can establish the velocity of the water at different depths. Due to the presence of more than one transducer set to different directions, it can even measure the components of velocity in three directions, facilitating accurate establishment of the composite flow pattern of the water.

5. What is Required for High-Quality Measurement of River Flow in Assam?

Equipment Requirements

Reliability of Materials

Because of the muddy and often sediment - rich condition of Assamese rivers, ADCP gear has to be made from extremely strong materials. Corrosion - resistant metals like stainless steel or special alloys and hard plastics must withstand the hostile river environment, including exposure to sediment, agricultural runoff chemicals, and sporadic factory contamination.

Small Size and Lightweight

Assam's diversified riverine profiles like narrow channels in some locations and inaccessible zones make small and portable ADCPs more handy. They are easy to transport and deploy on different boats or in shoal water sections, allowing for comprehensive flow measurements on diverse stretches of rivers.

Cost - Effectiveness

For long - term and large - scale monitoring of the numerous rivers in Assam, cost - effective ADCPs are highly crucial. Instruments offering quality performance at a reasonable cost allow mass deployment, which makes total data collection possible, necessary for efficient water resource management, flood control, and environmental protection efforts in the state.

6. How to Choose the Best Current Measurement Equipment?

Deployment Methods

Boat - Mounted (Moving Boat) ADCP

A boat-mounted ADCP can be utilized for preparing detailed surveys of the rivers. It can measure the discharge and chart the flow characteristics across the river width quickly, which can be used in flood-monitoring, pollution-assessment, and overall river-flow studies. It can be used to survey the Brahmaputra, Barak, and other big rivers of Assam to gather valuable data for water management and disaster prevention.

Bottom - Mounted (Fixed) ADCP

For observing long - term trends in river flow over extended periods at all times, an ADCP may be installed on the riverbed through a bottom - mounted ADCP. This provides continuous long - term trends in river flow records, which is of utmost importance to the study of ecological health of the rivers, flood prediction, and developing strategy for sustainable water use.

Cableway ADCP

Where cableways exist in some sections of rivers, there can be the use of a cableway ADCP. This is suitable where boat deployment is not possible or where fixed - bottom installation is not possible, providing precise flow measurement in challenging river conditions, such as in areas with high currents or in tight gorges.

Working Frequency

For Assam rivers, a 600 kHz ADCP would be a proper choice because it is a satisfactory compromise between range (as high as 70 meters in the horizontal plane) and resolution, which is adequate to measure flow in relatively wide and shallow sections of the main rivers. For deeper or wider sections of the rivers, a 300 kHz ADCP, with a greater range of up to 110 meters but lower resolution, can be considered.

Brand Recommendations

Top-of-the-line world-known ADCP current meter brands such as Teledyne RDI, Nortek, and SonTek are well known for their first - class and reliable products. For Chinese products, the ADCP manufacturer Chinese company’s "China Sonar Panda ADCP" is an economical one. Made of all-titanium alloy, it has higher strength and anticorrosive properties and is capable of enduring the rugged environment of the rivers in Assam. It is also very cost-effective, making it economical for most monitoring projects in the state. You may find more information by visiting their website at https://china-sonar.com/.

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