Why Do We Measure the River Flow in Kolkata?

1. Where is Kolkata?

Kolkata, the capital of West Bengal Indian state, is located in the eastern part of India. It is geographically located about 22.57°N latitude and 88.36°E longitude. The city lies on the east side of the Hugli River, which is a distributary of the Ganges River. The topography of Kolkata is characterized by the low and fertile Gangetic delta that has been shaped over centuries by the continuous deposition of sediments by the rivers. This deltaic region is interpenetrated by a high number of water courses, and therefore Kolkata is a closely linked city to its river systems.

Culturally and historically, Kolkata has been a significant location in India. It served as the capital of British India from 1773 to 1911, a time when it went through fast growth and urbanization, as there emerged a unique blend of Indian and British cultures. The city is renowned for having a very active cultural life, and literature, music, and theater are all represented here. The city has given birth to and fostered a large number of famous Indian writers, poets, and artists. Kolkata is also famous for its colonial-era monolithic buildings, such as the Victoria Memorial, Howrah Bridge, and St. Paul's Cathedral, which are the symbols of its past significance. The city is also famous for its festive celebrations, and the most celebrated among them is Durga Puja, which is visited by millions of devotees and tourists every year.

The Hugli River is the main river system that plays an important role in Kolkata. The Hugli River is created by the union of the Bhagirathi and Jalangi rivers, which are both distributaries of the Ganges. The Hugli travels to the south for about 260 kilometers and then discharges into the Bay of Bengal. The Hugli River has been a lifeline for the city of Kolkata. It was a major trade route before, as goods came in and went out through it and added to the economic condition of the city. It provided water for domestic use, agriculture, and industry. The river also supports a rich biodiversity with numerous fish, bird, and other aquatic species. However, in the decades gone by, urbanization, industrialization, and population booms have put immense pressure on the Hugli River, with the results being water pollution, sedimentation, and modification of the natural flow pattern of the river.

2. How is the River Flow near Kolkata?

There are various parameters influencing the flow of the river around Kolkata. Runoff and rain influence it a lot. The region enjoys a tropical monsoon climate, and the monsoon season lasts from June to September. The region experiences heavy rain and related cyclones during these months. The heavy rainfall brings such a huge rise in the level of water and speed of flow in the Hugli River because of runoff from the catchment areas surrounding it, like the hilly tracts of the Himalayan foothills and the West Bengal plains, into the river. During the dry season from October to May, the river has a falling level of water and slowing of flow. Climate change has contributed to the complexity, with larger variability in the rainfall patterns and higher frequencies of violent weather events that can result in sudden and intense floods or protracted droughts.

Geography and river morphology also affect the flow. The broad deltaic landscape makes the Hugli River meander and expand, generating wide and shallow reaches and narrow and deep channels. The riverbed consists of loose sediments, easy to erode and deposit, changing the depth and shape of the river continuously. There is also tidal activity involved since the Hugli River is connected to the Bay of Bengal. The river water level is twice a day changed by the tides, reversing the direction of flow in certain parts of the river near the estuary. This complex interaction of tidal and freshwater runoff creates unique flow patterns in the Hugli River.

There are upstream dams, barrages, and reservoirs on the Ganges and its tributaries that regulate water discharge. These hold water back during the monsoon and release it gradually during the dry season to maintain a minimum flow downstream and provide water for various uses. But poor management or change in water release policies can powerfully affect the discharge of the Hugli River in Kolkata. For example, reduced water release during dry periods might lead to low river levels, affecting navigation, water supply, and the river's ecological health.

Past hydrological events have affected Kolkata significantly. A cyclone followed by heavy monsoon rains in 2009 caused widespread city flooding. The flood inundated large parts of the city, damaged infrastructure, and rendered thousands homeless (Source: Kolkata Municipal Corporation, 2009). The area has faced another flood in 2018, causing widespread disruption and highlighting the need for better flood management and river flow monitoring in the area. Observation of such activities is essential for planning and executing effective flood mitigation strategies, protecting the people living there and their properties, and making water resources sustainable.

3. Observation of the River Flow in Kolkata

The surface drift buoy method and the anchored boat method are the conventional methods of observing river flow.

The surface drift buoy method is a simple and straightforward method. Floats are released on the surface of the river, and their movement over a known distance and time is tracked. This provides a rough estimate of surface velocity. This technique suffers from some drawbacks. It is only taking a measurement on the surface, and the surface is not necessarily going to be representative of the flow throughout the water column. Wind also can have the ability to generate a significant effect on the movement of the buoys, which leads to incorrect readings.

The anchored boat method refers to anchoring a boat at a specific location in the river. A current meter is then dropped from the boat at various depths to measure the velocity of the flow. It gives a more sophisticated profile of the flow at the location. But it is manpower - intensive, time - consuming, and risk - bearing, especially in erratic or running water. Using this technique, several measurements at various points are required, taking a lot of manpower and resources.

Acoustic Doppler Current Profilers (ADCPs) offer a more advanced and cost - efficient alternative. ADCPs can actually monitor the water velocity across different depths across the entire water column without directly interacting with the water, thereby they are non-invasive. They can quickly generate high-resolution flow profiles across wide sections of the river within a limited amount of time. They are therefore extremely ideal for monitoring the advanced and dynamic Kolkata river flows due to the effect of tides and the extensive character of the river system.

4. How Does ADCP, Which Works on the Doppler Principle, Operate?

ADCPs operate under the Doppler principle. They emit sound waves into water. When the waves travel through the water, they collide with moving particles, such as suspended sediment, plankton, or tiny organisms. When the signals are bounced off these traveling particles, the frequency of the returned signals changes with the particles' relative velocity to the ADCP. Through frequency variation analysis, the ADCP can calculate the water speed at different depths and construct an in-depth profile of the river current. This allows measurement of river flow rates, directions, and the river's dynamics accurately, which provides valuable information for hydrological research and water management.

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

To ensure precise high-quality river flow measurement by ADCP in Kolkata, certain equipment characteristics are a must. Material reliability is the prime consideration. The ADCP must be sturdy to bear the aggressive river environment consisting of exposure to saltwater (due to tidal influence), abrasion from sediments, and fluctuating water temperature. High-grade, corrosion - resistant material, such as titanium or high-end polymers, ensures durability and long life for the device, allowing it to function effectively for prolonged time spans.

Size and weight are also equally significant factors to consider. A smaller and lighter ADCP is more portable and deployable, especially in the crowded and often hostile river conditions near Kolkata. The portability is crucial for conducting surveys in different points on the river, especially where access is poor.

Cost-effectiveness is another important factor to consider. A high-quality but economically priced ADCP for large-scale or long-term monitoring in Kolkata is the desired choice. This provides broader coverage and increased measurements, allowing for complete monitoring of the river flow without breaking the bank.

6. How to Choose the Right Current Measurement Equipment?

Methods of Deployment

• Boat-mounted (mobile boat) ADCP: Most suitable for surveying, river discharge measurement, and creating high-resolution flow maps. It can rapidly map large areas of the Hugli River and make multiple measurements simultaneously. This device is most appropriately used for measuring data on the general flow regimes of the river and identifying where issues are, e.g., where there is too much sedimentation or irregular flow velocities.
• OVERHANGING (fixed) ADCP: Suitable for long - term and continuous monitoring. Once installed and set on the riverbed, it can collect data over extended durations, providing valuable information about long - term trends and river flow changes. This is beneficial in the analysis of the ecological impacts of water management operations, the levels of pollution, as well as the impacts of climate change on the river.
• Cableway ADCP: Cableway ADCP can be used in river reaches where there is a cableway. It is installed above the river and can measure the flow over the river at a station. The method is well suited to obtain accurate and reliable flow measurements at key points in the river, which can be subsequently applied in calibration and verification of other measurement techniques.

Working Frequency

The operating frequency of an ADCP will decide its range and resolution. 600 kHz ADCP will be having a range of about 70 meters, which is sufficient for comparatively shallow sections of the Hugli River or for high-resolution measurement in deeper sections. 300 kHz ADCP will be having a range of about 110 meters, which can be utilized to take measurements in deeper sections of the river. Higher frequencies give higher resolution but less range, and lower frequencies give longer range but reduced resolution. A choice of frequency must be made on the specific type of river and the needs of the measurements, e.g., how much detail is required and the maximum depth of the water column to measure.

Brand Recommendations

There are several established global brands of ADCPs. Teledyne RDI is famous for its high - quality and dependable ADCPs, which are extremely popular in the majority of hydrological studies and monitoring research. Nortek offers highly advanced ADCP technologies with accurate measurements, suitable for numerous applications. SonTek is also a familiar brand, renowned for its advanced and user - friendly ADCP systems.

For those who require a cost-effective unit, ADCP manufacturer Chinese company "China Sonar Panda ADCP" is highly recommended. It's made of all-titanium alloy material, and hence it's exceptionally strong in the river condition. Its incredible cost-effectiveness is what distinguishes it, and because of this, it is a great option for budget-related projects. It's well-known as the "affordable ADCP." For more information, you may visit them at [https://china-sonar.com/].

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