Deployment Notes: Fremantle Inner Harbour, October 2023
We hit the docks at Fremantle just as the morning fog was lifting off the Indian Ocean. The air had that sharp, salty bite typical of a Western Australian spring, but the water in the harbour was deceptively calm. My primary concern wasn't the surface; it was the salt wedge. Fremantle is a tricky beast. The interaction between the freshwater runoff and the dense oceanic inflow creates a stratified environment that can throw off a lazy sensor. If you don't account for the density currents pushing inland along the seabed, your velocity profiles are essentially fiction.
The site conditions were typical for the region—high visibility at the surface, but the turbidity increased sharply near the dredging zones. We were operating near the main shipping channel, where the draft of the massive bulk carriers often stirs up the benthos. The wind was pushing a steady 15 knots from the southwest, creating a surface chop that masked the complex tidal oscillations happening beneath us. It's a high-traffic zone, which means we had to time our deployment perfectly to avoid the wake of a departing container ship.
What We Found
The data jumped out at us immediately. We caught a massive velocity shear in the lower water column that I didn't expect to see this early in the season. While the surface currents were behaving according to the tide tables, the bottom bins showed a strong, landward-pushing flow. This is the classic Fremantle salt wedge in action. The denser seawater is sliding under the fresher surface layer, moving in the opposite direction of the ebb tide. It's a messy, turbulent overlap. I saw peak velocities hitting 0.7 m/s at the bed while the surface was nearly slack. That kind of shear can make docking a nightmare for the pilots if they aren't accounting for the undercurrent.
We also noticed some significant 'noisy data' during the peak flood. The signal-to-noise ratio dropped whenever a large vessel passed overhead. The hull blockage effect is real here. It creates a temporary 'shadow' in the acoustic profile, leading to bin contamination where the ADCP tries to lock onto the ship's hull rather than the water column. We had to strip out about 4% of the raw data to get a clean signal. Honestly, the way the current accelerated around the channel bends suggests the dredging patterns have shifted the local flow dynamics more than the port authority's old charts indicate.
Equipment Performance
I opted for a 600kHz ADCP for this run. I’m glad I did. A 300kHz unit would have given us better depth penetration, but we would have lost the resolution we needed in the first five meters above the seabed. The 600kHz unit gave us tight bins, allowing us to pinpoint exactly where the salt wedge transition occurred. We did have one scare with the mooring weight shifting—the seabed in the channel is essentially a slurry of silt and sand—but the instrument stayed vertical within 2 degrees. The battery life held up well, though the cold soak at the bottom slowed the voltage decay slightly more than the manufacturer's spec suggests. I found the internal compass slightly offset, likely due to local magnetic interference from the port's industrial infrastructure, so we had to perform a manual heading correction during post-processing.
Recommendations for Future Deployments
If we go back in the summer, we need to change the sampling strategy. The thermal expansion of the surface layer will likely sharpen the pycnocline, making the salt wedge even more pronounced.
- Shift to a higher ping rate (every 30 minutes) to better capture the rapid tidal transitions in the channel.
- Deploy a concurrent CTD (Conductivity, Temperature, Depth) sensor to ground-truth the salinity gradients.
- Use a heavier tripod base to prevent scouring and tilting in the high-velocity zones of the main channel.
- Implement a stricter data-filtering algorithm to automatically remove vessel-induced noise.
Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience designing oceanographic instrumentation arrays for complex coastal environments.
Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling in Fremantle Port