Deployment Notes: Port of Tacoma, Puget Sound, October 2023
The mist was thick enough to chew when we pushed off from the dock at 05:30. We were racing against the clock to hit our deployment coordinates before the peak flood tide pushed too much debris through the channel. The Port of Tacoma isn't your typical open-water site; it's a chaotic intersection of deep-water berths and narrow navigation channels where the tide doesn't just flow—it surges. Navigating the Puget Sound's complex bathymetry means dealing with erratic current shears that can push a vessel off course in seconds if you aren't paying attention to the helm.
The water state was typical for a Washington autumn: cold, brackish, and surprisingly turbid. We noticed a significant salinity gradient near the surface, likely a result of recent rainfall runoff mixing with the saltwater wedge. This stratification makes acoustic profiling tricky. If you aren't careful with your bin settings, the pycnocline can create acoustic reflections that look like velocity shifts but are actually just density boundaries. The wind was gusting from the northwest, chopping up the surface and making the launch of the instrument frame a bit of a gamble.
What We Found
The data came back with a shock: we saw localized velocity spikes in the lower water column that completely contradicted the surface drift. While the surface current seemed manageable, the bottom-hugging flows were ripping through the channel at speeds that would make any harbor pilot sweat. We caught several instances of tidal asymmetry where the ebb flow was significantly more concentrated and violent than the flood. It's a classic Puget Sound phenomenon, but seeing it mapped in real-time across the vertical profile is another thing entirely. These subsurface jets are likely driven by the unique geometry of the Tacoma Tideflats.
I noticed some noisy data in the bottom few bins, which I suspect is bin contamination from the sediment-heavy bottom. The Port handles massive amounts of cargo, and the constant dredging and vessel movement keep the silt in suspension. We saw a clear correlation between the passing of a deep-draft container ship and a sudden surge in suspended particulate matter. This 'wake-induced turbidity' momentarily blinded the ADCP's signal-to-noise ratio, but the instrument recovered quickly. Honestly, the 600kHz unit handled the particulate load better than the higher-frequency alternatives we've tested in similar harbors.
Equipment Performance
The ADCP performed solidly, though the battery life took a hit due to the high sampling rate we set to capture the tidal transitions. We opted for a bottom-mounted configuration with a heavy tripod to prevent tilting. It stayed upright, which is a win given the current speeds. I did find the internal compass slightly off—about 2 degrees—which required a manual correction during post-processing. It's a common quirk, but you have to catch it during the sanity check or your vectors will be skewed. The acoustic pings remained clean for the most part, and we managed to get a high-resolution look at the shear layers without too much signal dropout.
Recommendations for Future Deployments
If we head back to the Port of Tacoma for a winter run, we need to adjust our strategy to account for higher runoff and increased turbidity.
- Increase the blanking distance to avoid surface noise from heavy vessel traffic.
- Swap the 600kHz transducer for a lower frequency if we need deeper penetration through the silt layers.
- Use a heavier concrete anchor block to ensure zero tilt during the spring tide surges.
- Implement a staggered sampling interval to save battery without losing the peak flow data.
The key is ground-truthing these acoustic measurements against physical tide gauges. Without that, you're just guessing at the absolute velocity. In a high-stakes environment like a commercial port, guessing isn't an option.
Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and oceanographic instrumentation with a focus on tidal asymmetry and continental shelf currents.
Field Deployment Report: Bottom-Mounted ADCP Profiling in the Port of Tacoma