Deployment Notes: Kolkata Port, West Bengal, October 2023
We hit the docks at the Kolkata Dock System just as the morning fog started to lift, though the air remained thick and humid. The smell of diesel and wet silt was overwhelming. My first look at the water told me everything I needed to know: this isn't your typical open-ocean deployment. The Hooghly River is a beast. It's a tidal estuary where the Bay of Bengal pushes salt wedges deep inland, fighting against the massive freshwater discharge of the Ganges system. This creates a chaotic, stratified environment that makes current prediction a nightmare for any pilot trying to bring a vessel into the berths.
The water was a murky, opaque brown—typical for this time of year. We were operating in a high-energy zone where the river's narrow geometry accelerates flow, and the sediment load is staggering. Visibility was effectively zero. I spent the first hour checking the bathymetry charts, but in this part of the Kolkata Port, the riverbed shifts almost daily due to heavy siltation. We had to be incredibly careful about where we dropped the gear to avoid burying the sensor in a fresh sandbank within the first twelve hours.
What We Found
The data came back noisier than I expected, but one thing jumped out immediately: the tidal asymmetry is aggressive here. We saw flood currents that were significantly shorter in duration but far more intense than the ebb tides. This is the primary driver for the siltation issues the port authorities fight daily. The incoming tide essentially 'shoves' sediment up-river, and the receding tide isn't strong enough to pull it back out. It's a conveyor belt of mud. I noticed a massive shear layer around the mid-column; the surface water was racing toward the city while the bottom layers were barely moving or even reversing. (This kind of vertical decoupling is exactly why ship captains struggle with docking during the transition tides).
We also caught some weird spikes in the velocity data during the peak flood. I suspect we were seeing the influence of localized eddies created by the port's infrastructure and the dredging channels. The 'bins' near the seabed were particularly messy. We had significant bin contamination because the water is so thick with suspended solids that the acoustic signal bounces off the silt clouds rather than the actual water movement. I had to spend three hours in the lab scrubbing the data to get a clean signal. Still, the raw magnitude of the current spikes was eye-opening. We recorded peaks that would make any tugboat captain sweat.
Equipment Performance
I opted for a 600kHz ADCP for this run, and honestly, it was the only right choice. A higher frequency would have been swallowed by the turbidity, and a lower frequency wouldn't have given me the vertical resolution needed to see that shear layer. The unit held its position well on the riverbed, though the bio-fouling started almost instantly. The silt acts like glue for organic growth. I did notice some drift in the compass calibration—likely due to the heavy metallic infrastructure of the nearby berths interfering with the magnetic field. It wasn't a deal-breaker, but it meant I had to perform a manual sanity check against the known tidal constants for the Kolkata region to ensure the vectors were pointing the right way.
Recommendations for Future Deployments
If you're heading back into the Hooghly, don't trust the charts. Ground-truthing is mandatory. I suggest the following for the next window:
- Use heavy-duty tripod mounts with oversized mud-plates to stop the ADCP from sinking into the silt.
- Increase the ping rate to capture the rapid acceleration of the flood tide, but keep an eye on battery life.
- Deploy a secondary CTD sensor to correlate velocity spikes with salinity changes; it'll help distinguish between tide-driven flow and freshwater pulses.
- Schedule a mid-deployment cleaning to scrub the transducers, or you'll lose signal quality to biofilm within two weeks.
The port is a logistical marvel, but the hydraulics are a mess. Between the monsoon runoff and the tidal push, the water column is in constant conflict. We got the data we needed, but this site reminds me that the ocean doesn't always follow the textbook, especially when it's mixed with a few million tons of Himalayan silt.
Field report by Sarah Jenkins. Sarah is a senior oceanographer specializing in tidal asymmetry and the deployment of acoustic instrumentation in complex estuarine environments.
Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling in the Hooghly Estuary at Kolkata Port