Deployment Notes: Gijón Coast, Asturias, October 2023
We hit the docks in Gijón just as a grey, Atlantic drizzle began to soak through our gear. The Bay of Biscay doesn't do 'calm'—even in October, the swell was pushing hard against the harbor walls. My main concern wasn't the weather, though. It was the complex bathymetry of the Asturian coast. The way the shoreline bends here creates erratic eddies and unpredictable shear zones that make standard current modeling a nightmare. If you aren't careful with your placement, you end up with data that looks like white noise.
The water was cold and surprisingly turbid. We were operating in a zone where the deep Atlantic waters push up against the coastal shelf, creating a volatile mix of salinity and temperature gradients. The current was ripping—probably hitting 0.7 m/s during the peak flood—which meant we had to move fast to get the instrument on the seabed before the window closed. The seabed here is a messy mix of sand and rock, which always makes me nervous about the ADCP tilting during the drop.
What We Found
The data came back with a shock: the tidal asymmetry is far more aggressive than the regional charts suggest. We saw a massive disparity between the flood and ebb velocities. The flood tide rushes into the bay with a violent intensity, but the ebb is sluggish, dragging out slowly over several hours. This creates a net landward transport of sediment that explains why the local beaches, like Playa de San Lorenzo, behave the way they do. I noticed a significant 'slug' of water pushing inland during the high tide, which likely traps nutrients and pollutants near the shoreline longer than we expected.
We also caught some weird wind-driven residuals. A strong northwesterly wind kicked up during day three, and the ADCP picked up a surface current that completely contradicted the tidal flow. It was a classic battle between the moon and the wind. The surface bins showed a strong eastward drift, while the bottom bins remained locked into the tidal cycle. Honestly, seeing that sheer in real-time is the only way to truly understand how the Gijón coast breathes. If we had relied on a single-point sensor, we would have missed the vertical structure entirely.
Equipment Performance
I opted for a 600kHz ADCP for this run. I'm glad I did. The 300kHz units often struggle with 'bin contamination' in shallower coastal waters where the bottom boundary layer is messy, but the 600kHz gave us a clean signal across the water column. We did hit a snag with the acoustic backscatter; the turbidity was so high during a storm surge that the signal-to-noise ratio dipped. I had to tweak the correlation thresholds in post-processing to get rid of the junk data. It worked, but it was a tedious process. The mounting tripod held firm, though I suspect the sandy substrate shifted slightly—the tilt sensor showed a 2-degree deviation, but not enough to ruin the velocity vectors.
Recommendations for Future Deployments
If you're heading back to the Asturian coast, don't trust the general tide tables for your deployment window. You need local, real-time telemetry to time your drop perfectly. I'd also suggest a heavier ballast than the standard kit; the Bay of Biscay will move your gear if it gets a chance.
- Use 600kHz or 1200kHz transducers to avoid bottom-bounce interference in shallow shelf waters.
- Increase the sampling rate to 15-minute intervals to capture the rapid onset of wind-driven surges.
- Apply a strict quality control filter for 'noisy data' during high-turbidity events.
- Conduct a sanity check with a handheld current meter during the initial deployment to ground-truth the ADCP's first few bins.
Field report by Sarah Jenkins. Sarah is a senior oceanographic engineer specializing in high-resolution velocity profiling and tidal dynamics in complex coastal environments.
Field Deployment Report: Bottom-Mounted ADCP Profiling in the Bay of Biscay off Gijón