Deployment Notes: Bacolod Coastline, October 2023
The humidity hit us the moment we stepped off the boat at the Bacolod waterfront. It was barely 5:30 AM, but the air already felt heavy, thick with the scent of salt and drying fish. We were there to deploy a bottom-mounted ADCP to get a real handle on the flow dynamics of the Guimaras Strait. This isn't your standard open-ocean deployment. The strait is a chaotic bottleneck. You have complex bathymetry, varying depths, and the constant push-pull of tidal currents that make the water move in ways that basic tide tables can't predict.
The water state was deceptive. On the surface, it looked like a mirror, but the subsurface was a different story. We noticed immediate signs of strong tidal flux as we navigated toward the drop-off. The challenge here is the sediment. Bacolod's coastal shelf is a mix of sandy patches and mangrove-rich silt. If you drop your gear in the wrong spot, you risk the transducer getting buried in a silt drift within forty-eight hours, which kills your signal. We spent an hour ground-truthing the seabed with a lead line to ensure we hit a stable, sandy bottom (though it was shallower than expected for October).
What We Found
The data came back with a shock. We saw velocity spikes that caught us off guard, particularly during the transition between the flood and ebb tides. In some of the deeper channels of the Guimaras Strait, the current didn't just shift direction; it sheared. We recorded high-velocity cores moving northeast, while just a few meters above the seabed, the water was practically stagnant or moving in a sluggish counter-current. This kind of vertical shear is classic for this region, but seeing it in the raw data is another thing entirely. It's a mess of hydrodynamic forces.
The monsoon influence was obvious. We were deploying during the transition toward the Amihan (Northeast Monsoon), and the surface bins showed a distinct wind-driven drift. This created a layered effect. The deeper water followed the tidal clock, but the upper water column was being pushed by the wind. When these two forces collided, we got noisy data in the middle bins. Honestly, it's a nightmare to post-process. You spend half your time filtering out the 'garbage' created by the turbulence. We also noticed a significant salinity dip near the river mouths, which likely skewed some of the acoustic backscatter. The freshwater inflow from local tributaries creates a density lens that can trick a less sensitive instrument into seeing a 'false bottom'.
Equipment Performance
We used a 600kHz ADCP for this run. I’ll be blunt: the 600kHz unit outperformed the 300kHz we considered because we needed the higher resolution in the lower water column to catch those shear layers. The instrument held its position well, and the internal compass remained stable despite the heavy current. However, we did run into some bin contamination. Because the water is so rich in organic matter—thanks to the surrounding mangrove forests—the acoustic signal bounced off suspended debris. I saw a lot of 'spikes' in the velocity plots that were clearly just schools of fish or clumps of seagrass drifting through the beam. We had to apply a strict median filter to get a clean signal.
Recommendations for Future Deployments
If you're heading back to the Guimaras Strait, don't wing it. The bathymetry changes too fast for general charts to be useful.
- Use a heavy-duty tripod base with oversized footpads to prevent the unit from sinking into the silt.
- Increase the ping rate during the spring tide cycle to capture the rapid acceleration of the current.
- Deploy a secondary CTD (Conductivity, Temperature, Depth) sensor alongside the ADCP to account for the salinity gradients caused by river runoff.
- Avoid deploying during the peak of the Habagat season unless you want to fight 3-meter swells just to recover your gear.
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: Profiling Current Shear in the Guimaras Strait off Bacolod