Deployment Notes: Port of Londonderry, November 2023
We hit the quay at the Port of Londonderry just as the grey November mist was lifting off the River Foyle. The air was damp, smelling of salt and diesel, and the current was already pulling hard against the pier. I spent the first hour watching the surface chop; the Foyle is a tricky beast. It is not just a river, but a tidal estuary where the Atlantic pushes deep into Northern Ireland, creating a complex dance of salt wedges and freshwater runoff that makes stable instrumentation a nightmare.
The site conditions were volatile. We were dealing with high turbidity—typical for this stretch of the Foyle—which usually messes with acoustic backscatter. The water was a murky olive green, thick with suspended sediments that threaten to choke a low-frequency signal. Wind was gusting from the northwest, pushing surface waters toward the bank and creating a shear layer that I knew would make our vertical velocity profiles look chaotic.
What We Found
The data shocked us. We saw velocity spikes during the ebb tide that far exceeded the historical averages for this specific reach of the channel. In some bins, the flow was ripping through at nearly 1.2 m/s, while just a few meters above the bed, the water was almost stagnant. This kind of extreme vertical shear is a red flag for sediment transport. It means the river is actively reshaping its own bed, moving massive amounts of silt and sand toward the mouth of the port. If the port authority isn't tracking these shifts, their dredging schedules are basically guesswork.
I noticed some weird 'noisy data' in the lower water column during the flood tide. After a quick sanity check against the local tide gauges, I realized we were seeing the salt wedge pushing inland. The density interface creates a refractive boundary that can trick a cheap sensor, but our ADCP held the lock. We caught the exact moment the saline front hit the sensor, marked by a sharp jump in the speed of sound. It's a classic Foyle phenomenon, but seeing it in real-time on the profile is always a bit jarring.
Equipment Performance
We used a 600kHz ADCP for this run, and honestly, it was the only right choice. A 300kHz unit would have had too much bin contamination from the riverbed, and a 1200kHz unit would have been blinded by the turbidity. The instrument stayed pinned to the bottom despite the heavy flow, though the tripod legs sank about ten centimeters into the soft silt (shallower than expected for November). The signal-to-noise ratio stayed clean enough for a reliable profile, but I did have to toss out the bottom two bins to avoid 'ringing' from the sediment interface. The battery life held up, though the cold water drained it faster than the lab specs suggested.
Recommendations for Future Deployments
If you're heading back to the Foyle, don't trust the charts for bed composition. The silt is deceptive. To get a clean signal without losing data to the boundary layer, try these:
- Use heavy-duty spiked tripods to prevent the unit from tilting in the ebb current.
- Increase the ping rate to 2Hz to better capture the rapid tidal reversals.
- Set the blanking distance to at least 0.5 meters to avoid bed-interference noise.
- Perform a manual sound-velocity profile every 6 hours to account for the salt wedge movement.
The Port of Londonderry is a vital hub for containers and bulk cargo, but the River Foyle is a living, moving thing. You can't just drop a sensor and walk away. You have to fight the turbidity and the tides to get a signal that actually means something. For this project, we got the ground-truthing we needed, but the Foyle reminded us that the river always wins if you get lazy with your calibration.
Field report by Elena Rodriguez. Elena is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience in coastal sediment transport.
Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling in the River Foyle