Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling in Stavanger Port

Deployment Notes: Stavanger Port, Rogaland Coast, October 2023

We hit the docks at Stavanger just before 0500, the air thick with that biting North Sea dampness that gets right into your bones. The harbor was a chaotic mix of grey water and orange sodium lights, with a few OSVs already churning the basin into a froth. My first look at the water state told me exactly what to expect: a volatile mix of North Sea surges pushing in and heavy freshwater runoff bleeding out from the Rogaland hinterlands. It's a messy junction.

The bathymetry here is a nightmare for anyone who likes a flat seabed. You have these jagged, deep channels that suddenly pinch into shallow basins, creating localized eddies that act like underwater centrifuges. I've spent plenty of time off Aberdeen, and while the North Sea is always temperamental, Stavanger has a specific kind of unpredictability. The water is stratified, and in October, that pycnocline is aggressive. You can practically feel the density shift as the cold saltwater fights the freshwater plumes sliding over the top.

What We Found

The data came back with a vertical shear that was honestly staggering. We saw velocities in the upper five meters that were completely decoupled from the bottom flow. In some bins, the surface current was ripping eastward while the bottom-driven tidal flow was barely moving or drifting in a different direction entirely. This isn't just a minor variance; it's a stratified column that would make a low-frequency instrument go blind. If you aren't configured for high spatial resolution, you're just guessing at the average, which is useless for actual port operations.

The most frustrating part was the noise. Every time a 10,000-ton supply vessel pushed through the main transit lane, the signal spiked. We saw these massive velocity anomalies that looked like a storm surge on paper but were actually just the wake of a tanker passing five meters from the sensor. It's classic bin contamination. Without a strict signal fence, the data is essentially junk. We had to strip out several hours of 'noisy' data just to get a baseline of the semi-diurnal tidal regime. Once we cleaned the signal, the actual tidal velocities varied wildly depending on the proximity to the inner harbor walls, proving that the internal basin dynamics are governed by a very localized, very erratic set of rules.

Equipment Performance

I went with the 600kHz frequency for this run, and it was the right call. A 300kHz unit would have given us more range, but the bin size would've been too coarse to catch that sharp vertical shear in the top 10 meters. We used a bottom-mount setup with a heavy-duty tripod to keep the unit from tilting during the surge. The hardware held up, but the bottom-tracking struggled during the peak freshwater runoff. The salinity gradient caused some ray bending—refraction that messed with our positioning. I had to do a manual sanity check against known seabed markers to ensure the ADCP hadn't 'walked' during the deployment. Despite the refraction, the 600kHz unit outperformed every expectation in terms of capturing the fine-scale turbulence of the fjord-like channels.

Recommendations for Future Deployments

If you're dropping gear in the Rogaland coast, don't wing it. The interaction between the North Sea and the local runoff is too volatile for a 'plug-and-play' approach.

• Stick to 600kHz: High spatial resolution is non-negotiable for capturing the stratified shear in shallow port basins.
• Tighten the Averaging Interval: Set your averaging to exactly 5 minutes. Anything shorter captures wave orbital motion (noise); anything longer smooths out the peak tidal flows you actually need to see.
• Aggressive Signal Fencing: Use a strict acoustic filter to kill the interference from OSV propellers and hull displacement.
• Heavy Tripods Only: The bottom currents in these channels can be deceptively strong during surges. A light mount will tilt, and a tilted ADCP is just a fancy piece of scrap metal.
• Cross-Reference Salinity: Always run a CTD cast alongside the deployment to ground-truth the pycnocline depth.

Field report by Capt. Marcus Thorne. Thorne is a specialist in underwater acoustics with twenty years of experience managing hydrographic surveys in high-energy maritime environments.