Why Do We Measure River Flow in Siliguri?

1. Where is Siliguri?

Siliguri is a large city located in the northern Indian state of West Bengal. It is geographically located at around 26.72°N latitude and 88.40°E longitude. Located at the base of the Himalayas, the city has a unique topography with the confluence of the plains and mountainous region. Siliguri geography is a mixture of flat alluvial plains that are fertile and cultivated for agricultural purposes and gently sloping land ascending to the Himalayan foothills. Siliguri, therefore, is a vital transit point connecting the Indian mainland with the northeastern states and countries such as Bhutan and Nepal.

Siliguri is a cosmopolitan melting pot culturally. It has been influenced by various ethnicities since it is an important trade and transportation hub. The city is well known for its multicultural celebrations, with Durga Puja, Diwali, and Bihu being celebrated all over the place. These celebrations show a mix of rich Bengali, Nepali, and other local cultures. Historically, Siliguri has witnessed the rise and downfall of kingdoms and has been a major commercial center, especially for commodities like tea, wood, and spices. Its ancient trade route and colonial-era infrastructure legacy adds to the historical charm.

The dominant river system is the Teesta River, which makes deep imprints on Siliguri. Traversing south for about 309 kilometers from its origin in the Pahunri glacier on the India-China border, the Teesta River flows into the Brahmaputra River. In Siliguri, the Teesta River has been a lifeline, providing water for irrigation, drinking purposes, and industries. It is also a vital component of the local ecosystem, which is full of varied aquatic life. In addition, the river has long been an important transportation site, supporting the transportation of goods in the region. However, the river is also prone to flooding, which has razed most of the city and its surroundings over the years.

2. What is the River Flow like near Siliguri?

Several factors regulate the flow of the river around Siliguri. Precipitation and runoff are the main drivers. The region has a monsoon-prevailing climate, with June to September being the monsoon months. Heavy rainfall, sometimes in the form of cyclonic disturbances, occurs during these months. The Himalayan catchment over the Teesta River is responsible for causing the water level and velocity to increase very quickly. Mountain runoff and the rains over the plains fill the river to huge capacity. On the other hand, the period between October and May is the dry season, which has a sharp decline in water levels, and the flow becomes much slower. Climate change has also contributed to the additional complexities with the patterns of rainfall becoming more erratic, leading to higher frequencies of occurrence of extreme flood and drought events.

Terrain and river morphology also play a strong role. The Teesta River flows through an area whose nature changes from deep mountain gorges to the upper reaches to broad, flat floodplains as it moves towards Siliguri. In the mountains, the river has a steep gradient, and hence the water is quick- flowing and very erosive. When it enters the plains along Siliguri, the river broadens into meanders and floodplains. The sand, gravel, and silt forming the riverbed are still sculpted by erosion and deposition processes that correspondingly alter the local flow patterns.

There are barrages and dams upstream of the Teesta River, which regulate water discharge. These are built for hydroelectric power generation, irrigation, and flood management. Although these dams can have some favorable as well as harmful influences on flow downstream. Even though they could help in checking the severity of floods, poorly managed water release could lead to low water levels during the dry season, affecting water supply for all uses in Siliguri.

A large number of hydrologic events with historic effects on Siliguri have occurred. A gigantic flood in the Teesta River was caused by heavy monsoon rainfall in 2017. According to the local disaster management reports, the flood covered the majority of Siliguri city, damaged infrastructure, and affected thousands of people (Source: Local Disaster Management Reports, 2017). It is essential to witness these events for devising effective flood - control measures, protecting the lives and property of the people, and enabling sustainable water resource management.

3. How to Observe the River Flow in Siliguri?

Traditional methods of river flow measurement include the anchored boat method and the surface drift buoy method.

Surface drift buoy method is a relatively simple approach. The floats are allowed to fall on the river surface, and their movement through a measured distance and a known time period are tracked. This provides a rough estimate of the speed of the surface flow. The method provides measurements of the flow at the surface only, which may be unlike the flow over the entire water column. Wind also has a significant effect on the movement of the buoys, leading to incorrect values.

Anchored boat technique is where the boat is anchored at a specific location on the river. A current meter is lowered down to record the velocity of flow at varying depths. This gives a complete profile of the flow at the location. However, it is time - consuming, labor - intensive, and poses safety risks, especially in turbulent or high - speed waters.

Acoustic Doppler Current Profilers (ADCPs) offer a more advanced alternative. ADCPs can simultaneously measure water velocity at multiple points down the entire body of water without contact, thus non-invasive. They can instantly deliver detailed flow profiles, scanning long sections of the river within a short time, which is very suitable for observing the complex river flows around Siliguri.

4. How Does ADCP, Which Utilizes the Doppler Principle, Work?

ADCPs operate based on the Doppler principle. ADCPs transmit acoustic waves into the water. Where such waves encounter moving particles, such as suspended solids or plankton, the returning signal frequency changes with the particle velocity relative to the ADCP. From a measurement of these frequency changes, the ADCP calculates water velocity at different depths and generates a detailed river flow profile.

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

To achieve high-quality river flow measurement using ADCP in Siliguri, there are some equipment attributes required. Material durability is essential since the ADCP needs to withstand the tough conditions of the river, which include exposure to sediment-load water, varying water temperatures, and the risk of damage from floating moving debris. Lightweight and compact nature allows it to be made portable so that it can be conveniently installed in any part of the river, especially in remote areas. Economical prices are also important because they allow extensive monitoring of the river flow without spending much money.

6. How to Select the Suitable Current Measurement Equipment?

Deployment Techniques

• Boat-mounted (mobile boat) ADCP: Ideal for conducting surveys and measuring river discharges. It can quickly survey the wide span of the river and give an overall impression of the pattern of flow.
• Bottom-mounted (fixed) ADCP: Ideal for long-term continuous monitoring over, giving useful data about the long-term river flow trends.
• Cableway ADCP: Handy in river reaches with cableway facilities, providing accurate and repeatable measurements at specified points.

Working Frequency

Working frequency selection is dependent on the depth of the river. For relatively shallow waters, a 600 kHz ADCP with a range of up to 70 meters would be suitable. For deeper parts of the river, a 300 kHz ADCP with a range of up to 110 meters would be best.

Brand Recommendations

World - leading ADCP brands include Teledyne RDI, Nortek, and SonTek, with high - quality products, advanced technology, and excellent reliability. For a cost - effective budget, ADCP manufacturer Chinese brand "China Sonar Panda ADCP" is also very good. Made of all-titanium alloy material, this "economy ADCP" is extremely durable and resistant to corrosion, well-suited to the harsh environment of the Soan River. Its high cost-effectiveness makes it within the budget of a range of users, from small-scale research projects in the vicinity to city-scale water management initiatives. To find out more, visit their website at https://china-sonar.com/.

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