Deployment Notes: El Nido Port, Palawan; November 2023
We hit the docks at El Nido Port just as the morning light broke over the Bacuit Archipelago. The humidity was already oppressive, and the air smelled of salt and diesel from the idling outrigger boats. I spent the first hour watching the current rip through the main channel; it's a chaotic mix of tidal flux and runoff from the surrounding limestone karst topography. This isn't your standard open-water deployment. The geography here creates a nightmare for acoustic profiling because the narrow channels act like nozzles, accelerating water speeds in ways that defy simple tidal tables.
The water was deceptively clear, though the turbidity spikes during the incoming tide suggested significant sediment transport from the nearby mangroves. We were operating in a window between the southwest monsoon and the onset of the northeast monsoon (Amihan). The surface was choppy, and the vessel traffic—mostly tourist boats ferrying crowds to the Big Lagoon—made precise positioning of the ADCP frame a logistical headache. I had to fight with the boat captain twice just to keep us stationary long enough to ensure the transducer head was perfectly vertical.
What We Found
The data surprised me immediately. We caught a peak velocity spike of 1.1 m/s during the spring tide ebb, which is significantly higher than the historical averages for this section of the port. The velocity shear was aggressive. In the bottom three meters, the flow was sluggish, but just above that, the current accelerated violently toward the surface. It's a classic salt wedge scenario, though the freshwater input from the terrestrial runoff is more episodic than constant. I suspect the complex bathymetry of the surrounding reefs is funneling the water, creating localized jets that could easily push a drifting tourist boat off course if the captain isn't paying attention.
We also saw some weirdness in the mid-water bins. I noticed significant signal attenuation during the peak flood. Honestly, it looked like biological interference—likely schools of fish or dense plankton blooms moving through the channel. I ran a sanity check against the tide gauge data, and the timing aligned perfectly with the flood peak. The current wasn't just moving water; it was moving biomass. This creates 'noisy data' that a novice might mistake for turbulence, but for those of us who've spent time in Palawan's waters, it's a standard feature of the ecosystem.
Equipment Performance
I opted for a 600kHz ADCP for this run. I'm glad I did. A 300kHz unit would have had too large a blanking distance, and we would have lost the most critical data in the lower water column. The unit held its position on the seabed thanks to a heavy galvanized steel tripod, though I noticed some slight tilting in the post-deployment tilt-correction logs (about 2 degrees). The signal-to-noise ratio remained strong for the first ten days, but we started seeing some bin contamination near the surface as the weather turned. The bubbles from the choppy surface water created an acoustic ceiling that chopped off our top two bins. It's a nuisance, but manageable. The battery life held up well, though the extreme tropical heat on the deck during the initial soak period always makes me nervous about the internal electronics.
Recommendations for Future Deployments
If you're heading back to El Nido for a follow-up, don't trust the surface readings during a storm surge. You need a more robust anchoring system to prevent any lateral shift in the tripod.
- Swap the standard tripod for a weighted concrete base to eliminate that 2-degree tilt.
- Increase the sampling rate to 15-minute intervals during the full moon cycle to better capture the peak velocity spikes.
- Use a higher-frequency transducer to minimize the blanking distance in these shallow port channels.
- Coordinate with the port authority to mark the deployment site with a permanent buoy to prevent accidental snagging by fishing nets.
The real challenge here isn't the tech; it's the environment. The interplay between the limestone cliffs and the tidal currents creates a hydrodynamic environment that is constantly shifting. Ground-truthing these results with a handheld current meter would be the next logical step, provided we can find a window where the tourist boats aren't blocking every square inch of the channel.
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 tropical environments.
Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling at El Nido Port, Palawan