ADCP Application in the Ganges Flood Management

1. Where is the Ganges?

The Ganges River is one of the most important and sacred rivers in Asia. It originates from the Gangotri Glacier in the Indian state of Uttarakhand. The river flows through northern India and into Bangladesh over a course of about 2,525 kilometers (1,569 miles).

It flows by the course of several major towns including Varanasi, Allahabad, and Kolkata in India. Primarily, the basin comprising the Ganges represents the typical tropical monsoon climate. Generally, there is a well-marked rainy and dry season. Heavy rainfall mostly pours during the wet part of the year from June through September. This rain added with the water contribution provided from its many tributaries and melting ice glaciers in the upper area course causes the river water level to rise. The dry season is from October to May, during which time the water level is relatively low, although the river still flows due to the contribution of groundwater and stored water in reservoirs and natural depressions.

2. What are the causes of flooding in the Ganges?

The Ganges is flooded because of many factors. One main cause is the rainfall during the monsoon. Heavy and sustained rains in the wet season cause a sudden rise in the level of water in the river. The vast catchment area of the Ganges, with its numerous tributaries, channels enormous quantities of water into the main river channel.

The flat topography of the Gangetic plains, through which the river flows, contributes to flooding. The floodplains have limited natural drainage, and as the water rises, it spreads out across a large area, covering agricultural land, villages, and cities.

The second is sedimentation: with time, the sediment brought along from the headwaters would eventually raise the bed level, which lowers the depth and carrying capacity of the stream, thus the likelihood of overflow of its banks at times of high flow.

Human-induced changes also contribute. The reduction in the land's water-absorbing capacity due to deforestation in the upper catchment area reduces the land's infiltration capacity, hence regulating runoff. Construction of dams and embankments not well designed or badly maintained increases flood risks. For instance, a breach in an embankment may result in sudden and severe flooding.

In this context, the Acoustic Doppler Current Profiler (ADCP) provides a more advanced and efficient measurement method compared to traditional techniques, which is essential for effective flood management.

3. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They transmit acoustic pulses into the water. These pulses scatter off moving particles in the water, like sediment grains, small aquatic organisms, and other suspended material. When acoustic waves reflect back after hitting these moving objects, a frequency shift occurs.

This frequency shift is directly related to the velocity of the moving objects and, by extension, the velocity of the water. For instance, an ADCP current meter placed in the Ganges-say, on a boat or some fixed structure near the riverbank-emits sound waves into the water column. As those waves are reflected back by the flowing water and its constituents, the ADCP analyzes the received signals to calculate the speed and direction of the water flow at different depths.

4. What are the applications of ADCP in floods of the Ganges?

Flow velocity measurement: ADCP current profiler is capable of measuring the velocity of water flow in the Ganges with high accuracy. In case of flood events, this information becomes vital in knowing the speed at which floodwaters are traveling. It helps in predicting the spread of the flood and identifying areas where high - velocity flows may cause erosion or other damage.

Discharge calculation: Measured velocities, when combined with the cross-sectional area of the river, which is again measurable with ADCP flow meter, enable the flow rate of the river to be calculated. Understanding the flow rate in relation to a flood is very crucial as this helps in determining the volume of water passing through a specific section of the river and further helps in prediction at a downstream location.

Sediment transport studies: The ADCP meter can detect the transport of sediment particles with water flow. During floods, this gives an indication of just how much sediment is in the river. Sediment transport understanding can predict changes on the riverbed by deposition of sediment or erosion, which eventually may alter the stability of the river and its infrastructure.

5. How can the data measured by ADCP be used for flood warning and risk management of the Ganges?

Flood Warning

Flow velocity and flow rate data monitoring: The ADCP profiler will continuously monitor the flow velocities and flow rates to promptly detect abnormal changes that may signal an imminent flood. If the velocities or flow rates increase beyond normal levels, an alert can be triggered to allow timely warnings to communities along the riverbanks.

Water level prediction and warning: By analyzing the relationship between flow rates, velocities, and water levels through historical data and ADCP profiler measurements, one can predict water levels for the future. This information is crucial for proper warnings to residents, where they can take necessary precautions for evacuation or flood-proofing.

Risk management

Water Conservancy Project Scheduling Decision - Support: ADCP-measured data are useful in decision-making on the operation of dams, reservoirs, and other water conservancy projects along the Ganges. For instance, if there is a prediction of flooding, water release from the reservoirs can be adjusted according to ADCP-measured data to avoid flooding downstream and to maintain the safety of the structures.

Flood disaster assessment and emergency response: ADCP meter data allows the estimation of the impact of the flood event regarding erosion, riverbed changes, and the impact on the infrastructure. This information supports emergency response efforts and allows the development of future flood mitigation strategies.

6. What's needed for high - quality measurement of the Ganges currents?

The equipment should be made of reliable materials for high-quality measurement of the Ganges currents. Compact size and light weight will help in easy deployment at different locations along the river, even on boats or at bridges. Low power consumption is necessary for long-term monitoring, especially in areas with limited power supply. Cost-effectiveness will enable extensive deployment for comprehensive monitoring.

The casing of ADCP flow meter is preferably made of titanium alloy. Titanium alloy provides excellent corrosion resistance, which is necessary when dealing with the Ganges' water that may contain various minerals and salts. It also has a good strength-to-weight ratio, making the equipment durable and relatively easy to handle and install.

7. How to choose the right equipment for current measurement?

Selection of Equipment: Criteria for current measurement in Ganges-For horizontal cross-section measurement, one can use HADCP; it will measure the velocities of flow across a horizontal plane. It provides, in fairly clear terms, the general pattern of flow within the reach being examined. In the case of vertical cross-section measurements, the Vertical ADCP current profiler is ideal because it profiles the velocities from the water surface to the riverbed along a vertical line.

For frequencies, an ADCP with 600 kHz is suitable for water depths up to approximately 70m, while frequencies of 300 kHz are more appropriate for deeper sections with water depth up to about 110m.

There are well - known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option, the China Sonar PandaADCP is recommended. It is made of all - titanium alloy materials and offers a good cost - performance ratio. You can find more information at (https://china-sonar.com/).

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