Deployment Notes: Calapan Port, Oriental Mindoro, October 2023
The humidity hit us the moment we stepped off the ferry at Calapan Port. I remember looking at the water—a murky, olive-green mix—and knowing immediately that we were dealing with a complex estuary environment. The tide was pushing in hard from the Tablas Strait, swirling around the berthing areas where massive RORO ferries were docking. It was chaotic. The air smelled of diesel and salt, and the current was already tugging at our small deployment boat before we even dropped the first weight.
Calapan isn't your typical deep-water port. It is a critical bottleneck for goods moving between Luzon and Mindoro. The bathymetry here is tricky. We saw significant sediment transport and fluctuating salinity gradients, likely driven by the interaction between the incoming tide and the freshwater runoff from the Mindoro hinterlands. The water was surprisingly turbid. This kind of suspended load usually creates a nightmare for acoustic signals, often leading to signal attenuation or excessive noise in the lower water column.
What We Found
The data caught us off guard. We saw peak current velocities that spiked far beyond the average tidal predictions for this sector of Oriental Mindoro. In some bins, the flow was practically erratic, shifting direction rapidly as the current hit the port's structural piers and the narrow channel boundaries. We found strong shear layers. The surface water was ripping along at nearly 1.1 m/s, while the water just a few meters above the seabed was almost stagnant. This kind of vertical shear is dangerous for pilots maneuvering heavy cargo ships into the berths.
I noticed a recurring pattern of 'noisy data' during the peak ebb tide. The sediment concentration was so high that the ADCP struggled to maintain a clean signal in the bottom-most bins. We had to discard about 15% of the near-bottom data because of bin contamination from the seabed. Still, the overall profile showed a clear, dominant flow pattern that explains why berthing is such a headache for the local port authorities during certain lunar phases. The current doesn't just move in and out; it swirls in these localized eddies that can push a vessel off course in seconds.
Equipment Performance
We deployed a bottom-mounted ADCP, and honestly, the 600kHz unit was the only logical choice here. A higher frequency would have been swallowed by the turbidity, and a lower one wouldn't have given us the vertical resolution needed for such a shallow channel. The unit held its position well despite the heavy bottom currents, but the deployment frame was a bit light. We had to add extra ballast to ensure the transducer head didn't tilt. If the head tilts even a few degrees, your entire velocity vector is skewed. Once we stabilized it, the signal-to-noise ratio improved. The battery life held up, though the extreme heat on the surface during the recovery phase made me worry about the electronics for a moment.
Recommendations for Future Deployments
If we go back to Calapan, we can't just rely on a single point measurement. The spatial variability is too high.
- Deploy a multi-point array to map the eddies near the main ferry berths.
- Increase the ballast weight by 20% to prevent tilting during peak spring tides.
- Shorten the sampling interval to 10 minutes to better capture the rapid tidal reversals.
- Run a concurrent CTD profile to correlate the velocity spikes with salinity changes.
Measuring currents in a hub like Calapan isn't just an academic exercise. It is a safety requirement. When you have agricultural products and construction materials moving in bulk on massive ships, knowing exactly how the water is moving in that channel is the difference between a smooth docking and a costly collision. We need more ground-truthing with handheld current meters to verify the ADCP's bottom-track accuracy in these silty conditions.
Field report by Dr. Kenji Sato. Dr. Sato is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience in river discharge and maritime monitoring.
Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling at Calapan Port