Deployment Notes: Sambas Regency, West Kalimantan, November 2023
The humidity hit us the moment we stepped off the boat in Sambas. It was barely 5:30 AM, but the air felt thick, smelling of salt and decaying mangrove peat. We arrived just as the flood tide began pushing seawater deep into the estuaries, turning the brownish river water into a swirling, brackish mess. This isn't a standard open-ocean deployment. In Sambas, you aren't just fighting the current; you're fighting the sediment load of the Borneo rainforest washing out to the South China Sea.
The water state was chaotic. We saw surface ripples from the wind, but beneath that, the tidal prism was moving massive volumes of water through narrow mangrove channels. The visibility was abysmal—maybe two meters on a good day. We spent the first hour just trying to find a stable bottom for the tripod because the silt layer was deeper than the charts suggested. It felt like deploying into a bowl of wet cement.
What We Found
The data caught us off guard. We saw velocity spikes during the ebb tide that far exceeded the local averages, likely compressed by the narrow geometry of the mangrove fringes. The semi-diurnal cycle here is aggressive. We recorded two high tides and two low tides daily, but the asymmetry was striking. The flood tide pushed nutrients and salt wedges far inland, while the ebb tide tore back out, scouring the seabed. Honestly, the sheer volume of suspended solids in the water column created a lot of acoustic backscatter that would have fooled a less experienced eye.
The monsoon influence was the real story. We were deploying during the transition toward the Northeast Monsoon (Amihan). The wind-driven currents were fighting the tidal flow, creating these weird, sheared velocity profiles where the top two meters moved in one direction while the bottom five meters moved in another. It's a nightmare for anyone trying to model sediment transport without high-resolution data. We saw evidence of localized upwelling near the coast, which explains why the local fishermen are seeing such high shrimp and crab yields in these specific pockets. The water is alive, turbulent, and completely unpredictable.
Equipment Performance
We used a bottom-mounted ADCP, and for the most part, it held its own. However, we dealt with significant bin contamination in the lowest 0.5 meters due to the fluffy nature of the seabed sediment. I suspect the acoustic signal was bouncing off the silt clouds rather than the actual water movement. The 600kHz unit provided a clean signal in the mid-water column, but the turbidity levels in the Sambas estuaries are punishing. We had to manually filter out a lot of noisy data during the post-processing phase to get a reliable mean velocity. Still, the instrument survived the debris—which included everything from floating driftwood to plastic waste—without any sensor fouling.
Recommendations for Future Deployments
If you're heading back to West Kalimantan, don't trust the bathymetry maps. They are outdated. You need to over-engineer your mooring weights or you'll lose your gear to the silt shift.
- Use a higher frequency ADCP to better resolve the shallow-water shear layers common in these estuaries.
- Increase the sampling interval during the transition between Amihan and Habagat seasons to catch the rapid current reversals.
- Deploy an auxiliary CTD (Conductivity, Temperature, Depth) sensor to ground-truth the salinity gradients; the freshwater lens here is massive.
- Avoid deploying during peak spring tides if you aren't using a heavy-duty tripod; the ebb currents in the narrow channels can literally walk your equipment downstream.
Field report by Elena Rodriguez. Elena is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience mapping coastal sediment transport in Southeast Asia.
Field Deployment Report: ADCP Velocity Profiling in the Sambas Estuaries