Deployment Notes: Semayang Port, North Kalimantan, October 2023
We hit the docks at Semayang Port just as the morning humidity began to settle over the harbor. The air was thick, smelling of diesel and salt. I watched a medium-sized cargo ship maneuver slowly toward the berth, fighting a current that looked deceptively sluggish from the pier but was clearly pushing the bow off course. This is the reality of working in North Kalimantan; the water looks calm until you're in it, and then you realize the tidal flux here is a beast of its own.
The site conditions were challenging. We were operating in a high-traffic zone where palm oil exports and construction materials move in a constant stream. The water turbidity was high—typical for this region—with suspended sediments likely stirred up by recent dredging in the main channels. The tide was swinging hard, and the salinity gradients were shifting as freshwater runoff from the hinterland hit the saltwater wedge of the port basin.
What We Found
The data came back with a spike that caught us off guard. We recorded peak flow velocities that exceeded our initial estimates by nearly 20% during the spring tide cycle. It wasn't just the speed; it was the shear. We saw massive velocity differences between the seabed and the surface, creating a twisting effect in the water column that would make any pilot sweat during a docking maneuver. I've seen this in other Indonesian ports, but Semayang's specific geometry seems to funnel the current in a way that creates localized accelerators.
Most of the noise in the initial data came from the heavy vessel traffic. Every time a deep-draft cargo ship passed over the sensor, we got a blast of acoustic interference. We had to do some aggressive filtering to separate the actual current flow from the wake turbulence of the ships. Once we cleaned the signal, the pattern became clear: the port's currents aren't just tidal; they're influenced by the complex bathymetry of the coastal shelf. The 'dead zones' we expected near the berths aren't actually dead—they're just swirling eddies that can push a ship's stern unexpectedly.
Equipment Performance
I ran a bottom-mounted ADCP for this stint. Honestly, the 600kHz unit was the only way to go here. The lower frequencies would have been too coarse for the depth, and the higher frequencies would have been choked out by the sediment load. We had some initial bin contamination near the seabed—basically, the 'blanking distance' was eating into our lowest data points—but it was manageable. The unit held its position on the seabed despite the strong currents, though I suspect the tripod legs shifted a few centimeters during the peak flood. It's a rugged piece of kit, but in these turbid waters, you have to keep a close eye on the signal-to-noise ratio or you'll end up with a dataset full of ghosts.
Recommendations for Future Deployments
If you're sending a team back to Semayang, don't trust the historical charts. The dredging has changed the flow dynamics. I suggest the following:
- Increase the sampling rate during the first 48 hours to capture the full tidal oscillation without aliasing.
- Use a heavier ballast for the ADCP frame to prevent 'walking' on the sandy bottom.
- Coordinate with port authority to time deployments during low-traffic windows to minimize wake interference.
- Deploy a secondary CTD sensor to ground-truth the salinity changes, as these are likely driving the density currents we observed.
We spent a few hours doing a sanity check against manual flow meters, and the ADCP held up. It's the only way to get a real vertical profile of what's happening under the hulls of those ships. Without this data, the port is basically guessing when it comes to vessel safety and sediment transport management.
Field report by Capt. Marcus Thorne. Capt. Thorne is a senior specialist in underwater acoustics with twenty years of experience managing hydrographic surveys in complex maritime environments.
Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling at Semayang Port