Deployment Notes: Rostock Port, Baltic Interface, October 2023
I stepped off the tender and onto the quay just as a grey, biting wind began whipping across the Warnow. The air smelled of salt and diesel. Looking out over the water, you can't tell just by glancing that you're standing at a hydrodynamic war zone. To the untrained eye, it looks like a sleepy river mouth. To a hydrographer, it's a mess. We were there to get a real-world look at how the Baltic seawater is pushing back against the river discharge, and the timing couldn't have been worse—or better, depending on if you like a challenge.
Rostock Port is a peculiar beast. It's where the Warnow River meets the Baltic Sea, creating a volatile mix of riverine runoff and brackish tidal influences. The real headache is the salt wedge. Denser, saline water from the Baltic creeps inland along the bottom, while the fresh water from the Warnow slides over the top. This creates a sharp pycnocline—a density barrier that can bend acoustic signals and throw your data right out the window if you aren't paying attention. I've dealt with this in the Elbe, but Rostock's narrow channel geometry makes the vertical shear even more aggressive.
The bathymetry here is erratic, to say the least. You have these deep, dredged shipping channels flanked by shallow flats. It creates a "canyon effect." The current accelerates through the center of the channel while practically stagnating in the margins. While we call this a micro-tidal environment, that's a bit of a lie. Wind-driven surges from the Baltic can shove water levels up or down by over a meter in a few hours, completely reversing the flow of the Warnow. It's a constant tug-of-war between the river and the sea.
What We Found
The data came back with a shocker. We caught a period of extreme vertical velocity shear that would make a navigator sweat. At one point, the surface current was pushing 0.2 m/s toward the Baltic, but just a few meters down, the bottom layer was screaming inland at 0.1 m/s. If you're relying on a single-point measurement or a surface float, you're missing half the story. You're essentially flying blind. This kind of shear is a nightmare for deep-draft vessels trying to maintain steerage in a narrow channel.
We also saw some significant signal attenuation during a spike in river runoff. The Warnow carries a heavy sediment load, and when that silt hits the salt wedge, it creates a cloud of acoustic noise. We saw some "noisy data" in the lower bins, likely due to the high suspended solids increasing backscatter. It wasn't enough to kill the mission, but it was enough to make me double-check the sound velocity profiles. Without a proper sanity check against the CTD (Conductivity, Temperature, Depth) data, I wouldn't have trusted the discharge calculations for a second.
Equipment Performance
I pushed for the 1200kHz unit over the 600kHz for this run. Why? Because the water is shallow and the blanking distance on a 300kHz unit is simply too large. If you use a lower frequency here, you lose the top few meters of the water column—exactly where the wind-driven currents are strongest and most volatile. The 1200kHz gave us the resolution we needed, with a bin size of 0.25m that allowed us to pinpoint the pycnocline transition with surgical precision. We used a heavy-duty tripod with a customized leveling frame because the bottom of the Warnow is essentially soft silt. Any tilt at all—even two degrees—would have introduced unacceptable errors in the velocity vectors over a ten-meter column. The frame held, though we had to over-sample to compensate for the noise floor during the storm surge.
Recommendations for Future Deployments
If you're sending a team back into the Warnow estuary, don't wing it. The salinity gradients are too sharp and the bottom is too unstable for a standard drop-and-forget approach.
- Use High-Frequency Units: Stick to 1200kHz to minimize blanking distance and capture surface-driven flow.
- Rigid Bottom Mounting: Use a wide-footprint leveling frame to prevent the unit from sinking into the silt.
- Frequent Sound Velocity Profiles: Run CTD casts every 6 hours during storm events to correct for refractive index changes caused by the salt wedge.
- Tight Bin Resolution: Set bins to 0.25m or smaller to properly map the vertical shear at the pycnocline.
Field report by Capt. Marcus Thorne. Capt. Thorne is a maritime acoustics expert with 20 years of experience in port hydrography and deep-sea instrumentation deployment.
Field Deployment Report: Bottom-Mounted ADCP in the Warnow Estuary, Rostock Port