Deployment Notes: Lahad Datu, Sabah - November 2023
We hit the water just before 0500 hours, the air thick with that oppressive Sabah humidity that clings to your skin. The light was barely breaking over the horizon, casting a grey haze across the Celebes Sea. My team and I were maneuvering the launch boat through the fringes of the mangrove forests, where the water turns a murky, tea-color from the tannins. It is a claustrophobic environment for a navigator. One wrong move and you are snagged on a submerged root or grounded on a shifting sandbar that wasn't there on the chart from six months ago.
The conditions were volatile. We were operating right on the edge of the Northeast Monsoon transition. The wind was gusting intermittently, whipping the surface into a choppy mess that made deploying the gear a nightmare. More importantly, the water column here is a chaotic mix. You have massive freshwater plumes from the inland rivers dumping into the coast, creating sharp salinity gradients. This stratification makes acoustic profiling tricky because the sound speed varies wildly between the fresh surface layer and the salty depths. If you don't account for that, your data is junk.
What We Found
The velocity data surprised us. We saw surface currents screaming at nearly 0.9 m/s, but the moment we looked at the bottom bins, the water was almost stagnant. That kind of shear is aggressive. It tells us that the wind-driven surface flow is completely decoupled from the deeper water masses. We spotted several areas of intense upwelling near the offshore reefs. The current hits those underwater ridges and gets forced upward, bringing nutrient-rich cold water to the surface. It explains why the local fisheries are so productive here, but it makes the hydrography a headache to map accurately.
I noticed some weird spikes in the data around the midnight tide change. We saw brief, high-velocity pulses that didn't align with the tidal clock. My gut says it's localized turbulence caused by the complex geometry of the seabed. The bathymetry here is a mess of ridges and valleys. When the tide pulls back, the water gets squeezed through these gaps, creating miniature jets. It's not a steady flow; it's a series of pulses. Honestly, anyone relying on a simple tidal table for this region is guessing.
Equipment Performance
We deployed a bottom-mounted ADCP (Acoustic Doppler Current Profiler) and a series of surface drifting buoys for ground-truthing. The 600kHz unit performed well, but we struggled with 'noisy data' in the first few bins. The water is simply too turbid. Suspended sediment from the river runoff creates too much backscatter, leading to bin contamination. I had to manually filter out the top 2 meters of data to get a clean signal. The drifting buoys were a mixed bag. Satellite tracking is great until the buoy gets trapped in a mangrove creek or snagged on a floating log. We lost two buoys to the vegetation within 48 hours. It's a reminder that in coastal Sabah, the environment always wins.
Recommendations for Future Deployments
If you're heading back to Lahad Datu, don't just drop a sensor and hope for the best. You need a strategy for the sediment.
- Use a higher-frequency ADCP to better resolve the shear layers, but expect shorter ranges.
- Increase the sampling interval during the monsoon transitions to catch those erratic velocity spikes.
- Deploy CTD (Conductivity, Temperature, Depth) sensors alongside the ADCP. You cannot trust the default sound speed settings in these brackish waters.
- Avoid drifting buoys in the inner coast; stick to fixed-point moorings with heavy anchors to prevent gear drift into the mangroves.
- Run a sanity check on all depth readings. The seabed here shifts with the silt, and your 'bottom track' might actually be a layer of dense suspended mud.
The interaction between the Celebes Sea and the riverine discharge creates a dynamic that is far more complex than a standard coastal model suggests. You have to respect the monsoon. If you ignore the wind-driven currents, you're only seeing half the picture. We spent three days on site and still felt like we were scratching the surface of how this water actually moves.
Field report by Capt. Marcus Thorne. Captain Thorne is a senior consultant in underwater acoustics with 20 years of experience in maritime instrumentation and port hydrography.
Field Deployment Report: Current Profiling in the Lahad Datu Coastal Zone