How would measurements of the Alappuzha Currents be taken?

1. Location of Alappuzha

Alappuzha, also known by the name of Alleppey, is a coastal city of India; it is in the westernly state of Kerala. As defined geographically, it happens to be a place with unique beauty: a considerably elongated and serpiginous coastline intermingled by lagoons, canals, and estuaries.

The town is an important part of the Kerala backwater network. These are a section of brackish water bodies that have a variety of flora and fauna in them. The coastal area has a number of fishing villages dotted all over, with the local economy dependent on fishing and tourism to a great extent. The adjacent Arabian Sea is a vast expanse of water that plays an important role in the climate and livelihood of the town.

Alappuzha is also rich culturally. Major attractions include snake boat races, especially the Nehru Trophy Boat Race, which is associated with festivals. Seafood dishes are widely available here and are prepared from the catches of the day in both the sea and backwaters. The inhabitants are essentially attached to water as their livelihood depends on it and it fluctuates according to tides.

The waters off Alappuzha are affected by monsoons. The heavy rainfall from the southwest monsoon during the months of June to September further affects the water level and current. The backwaters serve as a buffer zone, soothing the impact of the sea on the inland areas. There is a constant sea breeze adding to the pleasant climate of the town.

2. What is the condition of the coastal currents off Alappuzha?

At Alappuzha, several variables affect the nature of the coastal currents: the mighty influence of monsoon winds, for instance. Strong surface currents driven by the southwest monsoon winds during that season play an important role in sediment transport and nutrient advection that will impinge on the ecosystem. Northeast monsoon controls the characteristics of the current, too.

Tidal currents are a major constituent. The Arabian Sea off Alappuzha has semi-diurnal tides. Strong tidal currents due to the rise and fall of tides interact with the wind-driven currents. The topography of the seabed off Alappuzha, comprising sandbars, channels, and underwater ridges, may deflect and modify the currents.

Even fresh water flowing from the backwaters and the rivers into the sea would contribute to these coastal currents. Sometimes, the resultant flow will present complex circulation due to differences in density between the two types of waters. Still, indirectly influencing the local coastal currents might come from the larger-scale oceanic circulations in the Arabian Sea-the Somali Current and the Arabian Sea Gyre.

3. How to Watch the Flow of Water at Alappuzha Coastal Area?

Surface Drifting Buoy Method: A buoy is set adrift on the surface, while its movement is monitored with GPS or other position-fixing devices. From the buoy's displacement in time, the speed and direction of the surface current can be estimated. However, this method is limited to the surface layer and the buoy's movement can be affected by wind and waves.

Anchor-Boat Method: A boat is anchored in a given location near the coast, from which current meters are deployed to measure the current at different depths. This provides a more detailed view of the current profile at a particular point but is restricted to the area around the anchored boat and may not capture the full spatial variability of the currents.

ADCP Method: The ADCP current meter is a more advanced and efficient way of measuring the flow of water in the coast. It can measure simultaneously the velocity of water at several depths. By emitting sound waves and analyzing the Doppler shift of the reflected waves, ADCPs are capable of providing a comprehensive understanding of the current profile from the surface to the seabed.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They send acoustic pulses into the water. These pulses interact with moving particles in the water, including sediment, plankton, and other small organisms. When the particles are moving towards the ADCP meter, the frequency of the reflected wave is higher than the frequency emitted. Conversely, when the particles are moving away from the ADCP, the frequency of the reflected wave is lower.

By precisely measuring this frequency shift, the ADCP current profiler is able to calculate the velocity of the water at different depths. Most ADCPs have several transducer elements that send out beams in various directions. This allows them to measure the three-dimensional structure of the current and hence give a better representation of the pattern of flow of water.

5. What's needed for high-quality measurement of Alappuzha coastal currents?

For high - quality measurement of the coastal currents in Alappuzha, the equipment should have reliable materials. Given the corrosive nature of seawater, it needs to be able to withstand long - term exposure. The device should be small in size and light in weight to facilitate easy deployment and retrieval, especially in areas with limited access or difficult conditions.

Low power consumption is essential for long - term operation, especially for continuous monitoring. Cost - effectiveness is also a crucial factor to enable widespread and frequent measurements.

The casing of the ADCP profiler is preferably made of titanium alloy. Titanium alloy offers excellent corrosion resistance, which is so vital in the corrosive seawater environment. It is also strong and able to withstand the pressures exerted by the water column and the forces of waves and currents. Its relatively low density helps to keep the equipment's weight down, making it more manageable during installation and maintenance.

6. How to Choose the right equipment for current measurement?

The choice of equipment for current measurement depends on the application. 

For Ship-based Measurements: Ship-mounted ADCP is used, as it will allow one to take the measurements while the ship is in motion and this would be able to map the currents over a larger area.

For Near-Seabed Currents: An ADCP can be used in bottom-mounting. This is best in the study of currents at the seabed since these are important in understanding the dynamics of sediment transport and benthic ecosystems.

For Surface and Upper-Water-Column Measurements: A buoy-mounted ADCP will be a good alternative to get continuous measurements of surface and upper-layer currents.

As far as frequency of ADCP is considered, different frequencies suit water with different depths. With 600kHz ADCP, water can go as deep as 70m; similarly, in 300kHz ADCP, the water should be up to approximately 110m, whereas 75kHz ADCP supports up to around 1000m of water depth.

There are several well-known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand-China Sonar PandaADCP is also a great option. It is made of all-titanium alloy material and offers an excellent cost-performance ratio. You can visit their website at https://china-sonar.com/ for more details.

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