Deployment Notes: San Diego Harbor, October 2023
We hit the water at 0400 hours, the air still thick with that damp, salt-heavy fog that clings to the San Diego coastline. My primary concern wasn't the weather, but the tide. We had a narrow window to drop the gear before the flood tide pushed us off-station. Looking out toward Point Loma, you can see how the landmass acts like a massive concrete fist, punching into the Pacific and forcing the currents to wrap violently around the peninsula. It's a hydrodynamic bottleneck. If you've never worked the San Diego Bight, you don't realize how quickly the 100-fathom curve swings shoreward, squeezing the California Current into a localized surge that makes harbor navigation a nightmare for heavy-load transits.
The surface was deceptively calm, but the water state beneath us was chaotic. We were operating near the coordinates 32.7° N, right where the sheltered berths transition into the exposed mouth of the bay. The water was a murky green, typical for this time of year, and the temperature felt slightly elevated (shallower than expected for October). I could tell just by the way the boat was drifting that the tidal asymmetry was peaking. In this harbor, the water doesn't just rise and fall; it surges horizontally through those narrow channels, creating velocity spikes that would make a standard current meter scream.
What We Found
The data came back messier than I anticipated. We caught a massive velocity shear at the 10-meter mark that completely contradicted the surface observations. While the top layer was pushing south, driven by the wind, the deeper water was locked in a heavy tidal ebb. It was a classic fight between atmospheric forcing and bathymetric constraint. I've seen this in the Mediterranean, but San Diego's stratification during these seasonal shifts is surprisingly aggressive. We saw distinct layering that likely caused significant acoustic refraction. If you aren't ground-truthing your sound speed profile in these conditions, you're basically guessing.
The real shocker was the data near the North Island Naval Air Station pinch points. We recorded localized current accelerations that far exceeded the mean tidal flow. This isn't just 'noise'—it's the result of the harbor's geometry forcing a huge volume of water through a tiny gap. For a ship pilot, these are the zones where the vessel starts to crab unexpectedly. We also spotted significant bin contamination. Because the dredged channels have such steep slopes, the ADCP beams were hitting the bottom at angles that sent the signal bouncing back unpredictably. It created these 'ghost' currents in the lower bins that I had to scrub manually during post-processing.
Equipment Performance
I insisted on the 600kHz frequency for this run, and thank god I did. A 300kHz unit would have been useless here. With the shipping channels often sitting under 20 meters, a lower frequency unit has a blanking distance too large to capture the upper-water column. We would have been blind to the exact area where the wind-driven currents were fighting the ebb. The bottom-mounted tripod held firm, thanks to a heavy concrete anchor that resisted the scouring effect of the sandy bottom. However, the placement was tricky. We had to keep the unit centered in the channel; any closer to the quay walls and the concrete would have caused side-lobe interference, ruining the signal. Honestly, the 600kHz unit outperformed every expectation, provided you have the patience to filter out the slope-induced noise.
Recommendations for Future Deployments
If you're sending gear into the San Diego Bight, don't rely on generic tidal tables. You need site-specific calibration or you'll end up with data that looks right on paper but fails the sanity check in the field.
- Use 600kHz sensors to minimize blanking distance in shallow channel sections.
- Perform a manual sound speed profile every 24 hours to account for salinity gradients and refraction.
- Avoid mounting within 15 meters of concrete quay walls to prevent signal bounce.
- Position sensors specifically at the North Island pinch points to capture peak velocity spikes.
- Deploy heavy-duty concrete anchors to prevent scouring during spring tide cycles.
Field report by Capt. Marcus Thorne. Capt. Thorne is a senior specialist in maritime acoustics with 20 years of experience in port hydrography and underwater instrumentation.
Field Deployment Report: Velocity Profiling at the San Diego Bight and North Island Channels