Deployment Notes: Porbandar, Gujarat, October 2023
The salt air hit us the moment we stepped off the boat. It was early morning, and the Arabian Sea was churning with a restless, grey energy that only the post-monsoon transition brings. I remember looking at the shoreline of Porbandar and realizing how deceptive this coast is. To a tourist, it is just a beach. To an acoustician, it is a chaotic intersection of tidal surges and remnants of the southwest monsoon winds.
The water was surprisingly turbid. We saw a lot of suspended sediment, likely stirred up by the strong currents pushing against the coastal shelf. The wind was whipping at about 15 knots, creating a choppy surface that made deploying the instrument frame a nightmare. We had to fight the swell just to keep the tripod stable during the descent. It was a messy start, but the site was exactly why we came: high-energy water movement that fluctuates wildly between the ebb and flow of the tides.
What We Found
The data came back with a shock. We saw velocity spikes that completely contradicted the surface drift buoy readings we had analyzed previously. While the surface buoys suggested a steady drift, the ADCP revealed a complex, sheared profile. In some bins, the water was screaming eastward at 0.7 m/s, while just a few meters above the seabed, the current had almost stalled or even reversed. This kind of vertical shear is common in the Arabian Sea near Gujarat, but seeing it in the raw data is a different story. It proves that relying on surface data alone is a gamble.
The influence of the monsoon was still visible in the deeper layers. Even though we were in October, the residual momentum from the summer winds was pushing water masses in ways that didn't align with the local tidal clock. I noticed some noisy data in the bottom few bins—likely 'bin contamination' from the acoustic signal bouncing off the sandy bottom. I had to trim the data manually to get a clean signal. Once I did, the picture became clear: the seabed topography here acts like a series of speed bumps, tripping up the current and creating localized eddies that could easily toss a small vessel off course.
Equipment Performance
I used a 600kHz ADCP for this run, and honestly, it was the only right choice. A higher frequency would have lacked the penetration needed for the depth, and a lower one wouldn't have given me the resolution to see that shear layer. The unit held its position well despite the current, though the tripod shifted about two degrees during a particularly strong ebb tide. The battery life held up, but the biofouling started almost immediately. Within a week, a thin film of organic matter began to cloud the transducers. It didn't kill the signal, but it added a layer of noise that required some aggressive filtering during post-processing. If we go back, I'm bringing a more robust anti-fouling coating.
Recommendations for Future Deployments
If you are planning to monitor the Gujarat coast, don't trust the seasonal averages. The variability is too high. I suggest the following for any team heading to Porbandar:
- Use bottom-mounted frames with heavy concrete anchors to prevent 'walking' during tidal reversals.
- Set the blanking distance higher than usual to avoid seabed interference (the sand here is highly reflective).
- Deploy for at least 28 days to capture a full spring-neap tidal cycle; anything less is just a snapshot.
- Pair the ADCP with a CTD sensor to track salinity gradients, as the freshwater runoff from inland can create density layers that warp the sound speed.
We spent a few hours ground-truthing the results against local fishing charts. The fishermen told us the currents 'pull hard' during certain moon phases. The data confirmed it. It's one thing to see a vector on a screen; it's another to realize the physical force of the Arabian Sea pushing against the Porbandar coast. We left the site exhausted and covered in salt, but the data set was gold.
Field report by Dr. Kenji Sato. Dr. Sato is a specialist in underwater acoustics with 20 years of experience designing oceanographic instrumentation for high-energy environments.
Field Deployment Report: Bottom-Mounted ADCP at Porbandar Coastline