Deployment Notes: Bayonne Estuary, October 2023
We hit the banks of the Adour river just as the morning mist was lifting over the Basque rooftops. The air was damp, smelling of salt and river silt. My team and I were there to set up a bottom-mounted ADCP to track the complex flow where the Adour and Nive rivers collide before pushing out into the Atlantic. It is a chaotic stretch of water. The confluence creates a mixing zone that makes standard current modeling nearly impossible without real-time, in-situ data.
The water state was aggressive. We were fighting a strong ebb tide that turned the river chocolate-brown with suspended sediment. This isn't a clear-water environment. The turbidity here is a nightmare for acoustic sensors because the particles scatter the signal. We spent an hour just scouting the bed for a stable footing, avoiding the shifting sandbanks that characterize the Bayonne coastal transition. The wind was whipping in from the Atlantic, pushing surface water back against the river's outflow, creating a shear layer that I suspected would give us some very noisy data in the upper bins.
What We Found
The data hit us hard. We saw velocity spikes that caught us off guard, with peak ebb currents ripping through the channel at speeds far higher than the historical averages for October. It turns out the tidal prism in this specific reach of the Adour is much more volatile than the regional charts suggest. We saw a massive salinity wedge pushing inland during the flood tide, which created a density interface that essentially acted as a mirror for some of our acoustic pings. It was a mess of data for the first six hours, but once the tide stabilized, the patterns became clear.
The most interesting part was the sediment transport. We noticed a direct correlation between the peak velocity bursts and a sudden drop in signal strength in the lower bins. This is classic bin contamination. The water was so thick with silt that the ADCP was essentially 'seeing' a wall of mud moving at 0.9 m/s. Honestly, I was worried we'd lose the signal entirely, but the unit held on. We captured the exact moment the Atlantic surge collided with the river discharge, creating a turbulent eddy that probably stays stationary for hours, trapping nutrients and debris in a localized swirl.
Equipment Performance
I used a 600kHz ADCP for this run, and it was the right call. A higher frequency would have been attenuated by the sediment too quickly, and a lower frequency wouldn't have given me the vertical resolution I needed for the shallow estuary floor. The unit performed well, though we had some 'ringing' in the first two bins due to the proximity of the riverbed. I had to manually strip those bins during post-processing to get a clean signal. The mounting tripod shifted about 15 centimeters during a particularly violent tidal swing (shallower than expected for October), but the internal tilt sensor caught it, so we could correct the vectors in the lab. If we had used a lighter mount, the whole rig would have rolled.
Recommendations for Future Deployments
If you're heading back to the Bayonne coast, don't trust the tide tables blindly. The interaction between the Nive and Adour creates local anomalies that can shift your window by an hour or more. Also, ground-truthing is non-negotiable here; you need a handheld flow meter to sanity check the ADCP's bottom track.
- Use heavy-duty galvanized steel tripods to prevent scouring and rolling in high-velocity ebb tides.
- Set the blanking distance to at least 0.5m to avoid bottom-bounce interference in the turbid Adour silt.
- Deploy during a neap tide if you want a baseline, but go for spring tides if you're hunting for maximum sediment transport rates.
- Double-check the salinity gradients; the halocline here is sharp and can mess with your sound speed corrections.
The sheer energy of the Bayonne waterfront is a reminder that estuaries are living things. They breathe, they shift, and they fight back. Getting a clean velocity profile in a place this dynamic requires more than just expensive gear—it requires a bit of intuition and a lot of patience with the mud.
Field report by Elena Rodriguez. Elena is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience mapping coastal sediment transport.
Field Deployment Report: Bottom-Mounted ADCP at the Adour-Nive Confluence, Bayonne