Deployment Notes: Figueira da Foz, October 2023
We hit the docks in Figueira da Foz just as the Atlantic was starting to churn. The wind was whipping off the water, smelling of salt and diesel, making the walk to the deployment vessel a fight against the gusts. I spent the first hour staring at the charts of the Mondego River mouth, knowing that the transition from riverine discharge to open ocean creates a chaotic mixing zone that can chew through poorly anchored equipment in a matter of hours.
The water state was erratic. We were dealing with a nasty combination of a strong ebbing tide and significant freshwater runoff from the Mondego catchment. This isn't your typical steady coastal current. The salinity gradients here are sharp, and the suspended sediment load—typical for this part of the Portuguese coast—means we were heading into high-attenuation waters. Visibility was less than a meter. It was a messy, turbid environment that makes acoustic pings struggle.
What We Found
The data came back with a spike that caught us off guard. We saw localized velocity surges near the seabed that completely contradicted the surface wind-driven flow. While the surface water was being pushed east by the prevailing westerlies, the bottom currents were behaving like a different animal entirely. We recorded peak ebb velocities that were far more aggressive than the local tidal tables predicted. It turns out the bathymetry near the mouth of the Mondego acts like a nozzle, accelerating the outflow and creating these intense, short-lived jets of water.
I suspect the tidal asymmetry here is more pronounced than previous papers suggest. The flood tide doesn't just reverse the flow; it hits the river plume like a wall, creating a shear zone that generates significant turbulence. We saw 'noisy data' in the lower bins during these transitions, which I attribute to the massive amount of organic debris and silt being kicked up from the bed. It's a classic case of bin contamination where the signal-to-noise ratio drops because the water is simply too thick with sediment for the 600kHz signal to return cleanly.
Equipment Performance
We deployed a bottom-mounted ADCP (Acoustic Doppler Current Profiler) to get a full water column profile. Honestly, the unit held its own, but the high-frequency pings struggled in the first two meters above the seabed. I’ve used these units in clearer waters, but the Mondego's silt load creates a 'blanking distance' that is frustratingly wide. We had to adjust the bin size manually to get a usable signal. The mooring held, though the tilt sensor showed the instrument was leaning about 5 degrees due to the sheer force of the river outflow. If we had used a lighter tripod, it probably would have tipped. The Doppler shift calculations remained accurate for the mid-column, but I wouldn't trust the near-bottom data without a serious sanity check against a secondary current meter.
Recommendations for Future Deployments
If you're heading back to Figueira da Foz, don't rely on standard mooring weights. The sandy bottom here is deceptive and shifts during storm surges.
- Switch to a 300kHz transducer if you need deeper penetration through the sediment plumes.
- Increase the sampling rate to 15-minute intervals to better capture the rapid tidal reversals at the river mouth.
- Use heavy-duty galvanized steel cabling to prevent abrasion against the coarse Atlantic sands.
- Deploy a conductivity-temperature-depth (CTD) sensor alongside the ADCP to ground-truth the salinity gradients.
The interaction between the North Atlantic circulation and the river's discharge makes this spot a nightmare for timing. You have to time your recovery perfectly between the spring tides, or you'll spend four hours fishing for a lost instrument in the surf. Despite the headaches, the data proves that the coastal currents here are far more volatile than a simple tidal model can predict. The wind is a major driver, but the river's pulse is what really dictates the energy in the system.
Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience mapping continental shelf currents.
Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling at the Mondego River Mouth