Deployment Notes: Puerto Madryn, Chubut Province, November 2023
The wind hit us the moment we stepped off the quay. It is a relentless, biting gale typical of the Chubut coast, whipping spray across the concrete berths where cargo vessels and fishing boats sat idling. We arrived at the deployment site just as the tide began to shift, the Atlantic pushing a heavy, sediment-laden surge into the harbor basin. The water looked opaque, a milky turquoise that signaled high suspended particulate matter—exactly the kind of environment that makes acoustic backscatter unpredictable.
Puerto Madryn isn't your standard deep-water port. It's a complex transition zone where the open Atlantic meets the sheltered Gulf. The bathymetry here is erratic. We dealt with sudden depth changes and a seabed that behaves more like a shifting sandbank than a solid floor. The water state was choppy, with short-period swells reflecting off the harbor walls, creating a chaotic surface layer that I knew would complicate our initial velocity readings.
The real challenge here is the salt wedge dynamics. Because of the specific geometry of the gulf and the seasonal runoff patterns, we often see sharp salinity gradients. These pycnoclines can bend acoustic beams, leading to 'ghost' velocities if you aren't careful. We spent the first hour just checking the sound speed profiles to ensure we weren't chasing artifacts. If the sound speed is off by even 2 m/s, your depth bins shift, and your data becomes a guessing game.
What We Found
The data came back noisier than I expected, but the results were fascinating. We caught a massive surge in bottom-layer current velocities that completely contradicted the surface drift. While the surface water was sluggish, the lower bins showed a concentrated jet of high-velocity water moving southeast. It was a classic case of internal tide modulation. We clocked peak velocities at 0.72 m/s near the bed—surprisingly high for this time of year (certainly higher than the October baseline data).
I noticed some significant bin contamination in the bottom three cells. This usually happens when the ADCP is too close to the seabed or when the bottom is too reflective. We had to manually strip the bottom cells during post-processing to get a clean signal. Once we cleaned the noise, the profile showed a distinct shear layer. The water column was essentially split in two; the top half followed the wind-driven surface currents, while the bottom half obeyed the deeper, denser tidal oscillations of the Atlantic. It's a violent tug-of-war happening right under the hulls of those cruise ships.
Equipment Performance
I opted for a 600kHz unit for this run, and honestly, it was the right call. A 300kHz unit would have given us more range, but we would have lost the resolution we needed in the lower water column. The instrument held its position well despite the scouring currents, though the tilt sensor showed a 3-degree lean by the time we recovered it. The battery life held up, but the acoustic signal suffered during the peak of the flood tide due to the sheer volume of suspended sediment. I’ve seen better signals in a muddy river, but for a high-energy port environment, the ADCP performed reliably enough for a sanity check against the tide tables.
Recommendations for Future Deployments
If you're heading back to Puerto Madryn for another run, don't rely on standard sound speed assumptions. The stratification is too volatile.
- Deploy a dedicated CTD (Conductivity, Temperature, Depth) sensor alongside the ADCP to correct for sound speed variations in real-time.
- Increase the blanking distance to 1.5 meters to avoid the worst of the bottom-bounce noise.
- Use a heavier tripod mount. The seabed shifts, and a light frame will end up on its side within a lunar cycle.
- Sample at a higher frequency during the spring tides to capture the rapid acceleration of the salt wedge.
Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience deploying instrumentation in high-energy coastal zones.
Field Deployment Report: Bottom-Mounted ADCP Profiling in Puerto Madryn Port