Why Do We Measure River Flow in West Bengal?

1. Where is West Bengal?

Geographic Background

West Bengal lies in the eastern part of India, between approximately 21.31°N - 27.17°N latitude and 85.53°E - 89.51°E longitude. It has a diverse geography. North of the state is characterized by the great Himalayan foothills, part of the eastern Himalayas, and cover regions like Darjeeling, which hosts tea gardens and natural scenery. Central and southern parts of the state are characterized by the rich Ganges-Brahmaputra delta, which is one of the world's largest river deltas. This deltaic area is a low, alluvial plain built up by sediments carried by the Ganges, the Brahmaputra, and their numerous tributaries over the centuries. West Bengal's tropical monsoon climate features hot and humid summers from March to June, with temperatures regularly crossing 35°C, and temperate winters from November to February. The monsoon season, June to September, experiences heavy rains, which are vital for the state's water supply as well as agricultural yields.

Humanities and culture

West Bengal has a rich and varied cultural background. For centuries, it has been a center of art, literature, and music. The state is famous for its poets and writers, including Rabindranath Tagore, the first non-European Nobel Prize in Literature laureate. The capital city Kolkata is also referred to as the "Cultural Capital of India" and has a large number of theaters, galleries, and literary societies. Bengali cinema has also made its own contribution to Indian cinema. The state festivals, such as Durga Puja, are observed with great enthusiasm and fanfare, and are visited by millions of country and foreign tourists. Durga Puja is not only a religious festival but also a display of Bengali art, culture, and craftsmanship in terms of grandly decorated pandals (temporary structures containing the idols of goddess Durga) and colorful processions.

Hydrology and River Overview

Ganges-Brahmaputra river system is the largest water system exerting a highly dominant control over West Bengal. The Ganges River, after flowing through the northern part of India, enters West Bengal and diverges into different distributaries, the most important among them being the Hooghly River. The Hooghly River flows southwards through Kolkata and is excessively important in state economy, transport, and water supply. The Brahmaputra River, also with its source in Tibet, is equally significant in the eastern West Bengal, more so in the Sundarbans. The Sundarbans is the world's largest mangrove forest and delta, evolved where the Ganges, Brahmaputra, and Meghna rivers confluence.

The rivers are of vital importance to West Bengal. Economically, they support a thriving and diverse biodiversity of fauna and flora. Sundarbans, for example, houses the Royal Bengal Tiger, birds, reptiles, and fish of different species. The rivers also have vital duties of maintaining overall environmental balance, like groundwater recharging as well as flood protection. The rivers are utilized for domestic, agricultural, as well as industrial uses of fresh water. Agri-production, especially rice, jute, and tea farming, is heavily reliant on the rivers' water for irrigation. The Hooghly River is also a principal transportation artery, carrying goods as well as persons, and Kolkata port, located on its bank, is India's leading port. Though, as with most river systems in India, these rivers also suffer from the problems of pollution, erosion, and the effects of climate change.

2. How is the River Flow Near West Bengal?

Factors of Influence

Runoff and Precipitation

The monsoon is by far the most important factor controlling river flow in West Bengal. The heavy and occasionally erratic rainfall during the monsoon months causes a sudden rise in river levels and river flow velocities. The rains are highly unpredictable from one year to another, and this introduces fluctuations in river flows. In the heavy monsoons, there is a possibility of excess water, and this may lead to floods, especially in the low-lying deltas. A poor monsoon year leads to low levels of river flows, and this leads to water scarcity for various purposes. Urban growth and deforestation within catchment areas of the rivers have also altered runoff patterns. Urban and town hard surfaces' expansion reduces rainwater infiltration, leading to faster surface flow and increased flood chances.

Topography and River Morphology

The flat topography of the Ganges - Brahmaputra delta determines river flow by creating a relatively slow current in most of the area. But north in the foothills of Himalayas, the streams are swift due to the slopes. The width and depth of the rivers could vary significantly along their routes. In the delta, the rivers are shallow and broad with numerous distributaries and channels. The riverbeds are made of soft alluvial materials, which can be easily eroded and deposited and hence cause shifting of the course of the river over a time span. Man-made impediments such as dams, barrages, and embankments have also modified the natural river morphology and affected the flow and sediment transport.

Reservoir Operations

There are several dams and reservoirs in the upper catchments of the river that flow through West Bengal, within and outside the state. They are used for hydroelectric power, irrigation purposes, as well as flood management. For example, Himalayan area dams control river flow downstream, storing water during the monsoon season and releasing it gradually in the dry seasons. But there can be sporadic downstream flooding or water scarcity based on the timing and amount of releases if not optimally managed.

Historical Hydrological Events

There were large-scale floods in West Bengal in 2019 due to copious monsoon rains and Ganges and tributary rivers overflow. Thousands of villages were inundated, displacing millions, as per the West Bengal State Disaster Management Authority. Croplands, especially rice and jute, were very much affected. The flood also ruined infrastructure, including roads, bridges, and power lines. In addition, Sundarbans has the risk of cyclones and storm surges, which can trigger torrential flooding and devastation of the delicate mangrove forest. Observation of these phenomena is crucial for the development of effective flood - control and disaster - mitigation measures, improved water resource management, and protection of the people's life and livelihood.

3. What is the Observation Method of River Flow in West Bengal?

Traditional Methods

Surface Drift Float Buoy Method

The surface drift float buoy method is a simple method of measuring river flow. A cork or plastic bottle is floated on the river surface, and its path is followed for a known length and interval of time. The velocity of the buoy is then equated to the surface velocity of the river. Easy to do with little equipment, this method has limitations. It measures only surface flow and is significantly influenced by wind and other surface-level disturbances. In the dirty and often turbulent rivers of West Bengal, the movement of the buoy may be impacted by trash as well, leading to erroneous readings.

Anchored Boat Method (Staff Gauge/Current Meter)

A boat is typically positioned at a fixed point in the river in the anchored boat method. A current meter is allowed to fall from different depths in order to measure the velocity, while a staff gauge is used to measure the water level. This measurement provides an accurate velocity profile at a point but is time - consuming, labor - intensive, and exposes the staff to safety risks, especially in polluted or rapid - flow rivers. The need for skilled personnel and the risk to operation in the riverine environment are significant constraints.

ADCP Introduction: Acoustic Doppler Current Profiler (ADCP)

Acoustic Doppler Current Profiler (ADCP) is a new, more efficient, and advanced instrument for measuring river flow in West Bengal. Compared to traditional methods, ADCP current profiler can provide uninterrupted velocity profiles between the surface and river bed non - invasively. It is capable of quickly scanning large tracts of the river, which is paramount in understanding the very complex flow characteristics of the Ganges, Brahmaputra, and their tributaries, considering the pollution, sedimentation, and man-induced changes. ADCPs are also safer to handle since they don't entail direct human contact with the potentially polluted river water.

4. How Does an ADCP Work Using the Doppler Principle?

An ADCP operates based on the Doppler principle. It emits acoustic waves of a specific frequency. When these are bent by particles in the water, such as sediment, plankton, or small animals, the frequency of the reflected waves changes with the relative particle motion. The reflected signal will be higher in frequency if the particles are approaching the ADCP, and lower in frequency if the particles are moving away. These frequency changes can be analyzed by the ADCP to find the water velocity at varying depths. When there are multiple transducers oriented at various angles, it is able to measure the three-dimensional components of velocity, allowing to determine precisely the integrated flow pattern of the water.

5. What is Needed for High-Quality Measurement of River Flow in West Bengal?

Equipment Requirements

Material Reliability

Given the contaminated and often turbid state of West Bengal's rivers, especially urban and near-industrial rivers, ADCP flow meter equipment has to be of very reliable quality. Heavy-duty plastics and certain corrosion-resistant metals like stainless steel or specialty alloys will need to withstand the rigors of the harsh river environment, which may involve contact with chemicals from industrial effluent, agricultural runoff, and sewage.

Small Size and Lightweight

The water-supported and heterogeneous watercourse conditions of West Bengal, including the narrow water channels in the delta, and inaccessible water bodies in the Himalayan foothills, make light and portable ADCPs more handy. They can be carried and mounted on different types of boats, or even transported on foot in shallow waters, enabling complete flow measurement.

Cost-Effectiveness.

For large - scale and long - term observation of West Bengal 's numerous rivers and water courses, cost - effective ADCPs are a must. Cost - effective products offering satisfactory performance at reasonable cost enable wide - area deployment with full data coverage, a prerequisite for effective water resource management, flood prevention, and natural resource preservation.

6. How to Choose the Appropriate Current Measurement Instrument?

Methods of Deployment

Boat - Mounted (Moving Boat) ADCP

A boat-mounted ADCP is effectively suited to perform in-depth surveys of the rivers. It can quickly measure the discharge and profile the flow characteristics over the river width, which is useful for flood-monitoring, pollution-assessment, and general river-flow studies. It can be used in West Bengal to measure the Hooghly River, the major distributaries of the Ganges, and Sundarbans, providing valuable water management information.

Bottom - Mounted (Fixed) ADCP

For continuous long-term river flow monitoring, a bottom-mounted ADCP may be installed on the river bed. This provides continuous records of the long-term trends of river flow, which is critical in understanding the ecological state of the rivers, flood forecasting, and water use planning sustainably.

Cableway ADCP

Where cableways are installed in sections of the rivers, a cableway ADCP can be used. This method is suitable for river areas where boat deployment is difficult or where a fixed - bottom arrangement is not feasible, making accurate flow measurement in difficult river conditions, e.g., in narrow gorges or where currents are highly turbulent, possible.

Working Frequency

For West Bengal rivers, 600 kHz ADCP is usually a suitable choice because it provides a reasonable trade-off between range (up to 70 meters horizontally) and resolution, which is adequate for measurement of flow in the relatively wide and shallow sections of the main rivers and distributaries. A 300 kHz ADCP with extended range to 110 meters may be applied in deeper or wider sections of the rivers, such as in the main channels of the Ganges and Brahmaputra, but at low resolution.

Brand Recommendations

Very well-known ADCP brands of international fame such as Teledyne RDI, Nortek, and SonTek are well-known for their good quality and reliable products. Amongst Chinese brands, the ADCP manufacturer Chinese company’s "China Sonar Panda ADCP" is one of the economic options. Made of all - titanium alloy, it is very strong and corrosion - resistant, and therefore ideal for the unfavourable condition of the rivers of West Bengal. It is also very cost - effective, and therefore affordable for use in various projects of monitoring in the state. For more information, you can visit their website at https://china-sonar.com/.

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