Deployment Notes: Mukah Coastline, Sarawak, November 2023
We hit the docks in Mukah just as the morning mist was lifting off the South China Sea. The air was thick, humid, and smelled strongly of salt and drying fish from the local markets. My primary concern wasn't the equipment—it was the water. The Mukah coastline is a nightmare for acoustics if you aren't prepared. You have this volatile mix of freshwater runoff from the interior and heavy saline intrusion from the sea, creating a salt wedge that shifts with every tide. If you place your transducer in the wrong layer, the salinity gradient will bend your acoustic signal before it even hits the first bin.
The conditions were typical for the onset of the Northeast Monsoon. We saw choppy surface waters and a turbidity level that would make a lesser sensor choke. The seabed here is a messy blend of silt and mud, which means any bottom-mounted gear risks sinking into the muck if the tripod isn't weighted perfectly. We spent the first three hours just scouting for a spot where the bathymetry wouldn't cause immediate signal shadowing.
What We Found
The data came back with a spike that caught us off guard. We recorded peak tidal velocities that surged far beyond the historical averages for this specific stretch of the Sarawak coast. It seems the local headlands are funneling the flood tide into narrow corridors, accelerating the flow in ways the coarse regional models completely miss. We saw currents hitting 1.2 m/s in the lower water column, while the surface remained sluggish. This vertical shear is classic salt wedge behavior, but the intensity here is aggressive.
I noticed a significant amount of noisy data during the transition between ebb and flow. In my experience, this is usually due to suspended sediment plumes being kicked up by the current. The ADCP was fighting through a wall of organic debris and silt. When the tide turned, the backscatter intensity jumped, suggesting a massive movement of nutrients and sediment being pushed inland. It's a chaotic system. The water isn't just moving; it's churning.
Equipment Performance
We deployed a 600kHz ADCP, and honestly, it was the only right choice. I’ve seen colleagues try to use higher-frequency units here, but they lose the signal too quickly in the turbid Mukah waters. The 600kHz unit gave us a clean signal through the majority of the water column, though we did suffer from some bin contamination near the seabed. The mud is so soft that the instrument settled a few centimeters deeper than planned during the first 24 hours. This shifted our blanking distance slightly, but it didn't ruin the dataset. The battery life held up well, though the biofouling on the transducer face started appearing within a week—these tropical waters are aggressive.
Recommendations for Future Deployments
If you're heading back to Mukah, don't trust the general charts. The seabed topography is too erratic for blind deployments. You need a sanity check with a handheld sonar before you drop your gear.
- Use heavy-duty mud mats for the tripod to prevent the unit from sinking into the silt.
- Stick to 600kHz or lower; anything higher will get swallowed by the turbidity.
- Increase the sampling rate during the spring tide cycles to capture the rapid velocity shifts.
- Apply a rigorous filter to the bottom-most bins to account for sediment-induced noise.
The real challenge here is ground-truthing. Without a secondary current meter for validation, you're just guessing if those high-velocity spikes are real or just acoustic artifacts from a passing school of shrimp. Next time, I'll bring a Lagrangian drifter to see if the surface flow matches the ADCP's top bins. It probably won't, given how the monsoon winds push the surface layer independently of the tide.
Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience deploying instrumentation in challenging coastal environments.
Field Deployment Report: Bottom-Mounted ADCP in the Mukah Estuarine Wedge