Deployment Notes: Merak Port, Banten, October 2023
The humidity hit us the moment we stepped off the boat at Merak Port. It was 05:30, and the air felt like a warm, wet blanket. I watched the ferry traffic pulsing between Java and Sumatra—a relentless stream of vessels navigating the narrow bottleneck of the Sunda Strait. This isn't your typical quiet harbor. The water here is a chaotic mix of tidal surges and heavy ship wakes, making it a nightmare for stable instrumentation.
The current state of the water was troubling. High turbidity from recent runoff and the constant churning of propellers meant we were dealing with a lot of suspended sediment. The tide was ripping through the channel, pulling hard toward the west. We had to time our deployment perfectly to avoid the ADCP being dragged across the seabed before it could settle. The wind was light, but the surface chop was aggressive, likely caused by the clash of deep-ocean swells entering the strait and the shallow coastal shelf.
What We Found
The data came back with a shock. We saw velocity spikes that peaked far higher than the historical averages for this sector of the Sunda Strait. In some bins, the current was screaming past 1.2 m/s during the spring tide peak. This is a high-energy environment. The vertical profile showed a sharp shear layer just a few meters above the seabed, which explains why smaller vessels in the port struggle with steerage during certain tidal windows. Honestly, the sheer volume of water moving through this corridor is staggering when you see it on a plot rather than just reading a chart.
We also spotted significant 'noisy data' in the lower water column. I suspect this was caused by the massive propeller wash from the Ro-Ro ferries. When a large vessel passes overhead, the ADCP captures these artificial turbulence events. It creates these jagged spikes in the velocity profile that look like errors but are actually real, localized disruptions. It's a mess to clean up during post-processing, but it gives us a realistic look at how ship traffic alters the local hydrodynamic regime. The salinity gradients were also erratic (likely due to freshwater plumes from nearby coastal drainage), which can mess with the speed of sound calculations if you aren't careful.
Equipment Performance
I used a 600kHz ADCP for this run, and it was the right call. A 300kHz unit would have given us better depth, but we didn't need it, and the 600kHz provided the spatial resolution required to see that shear layer. The bottom-mount tripod held steady, though the sediment is soft—almost silty—which makes me nervous about long-term tilt. The signal-to-noise ratio stayed acceptable for the most part, but we lost some pings during the peak turbidity events. I've seen better signals in the open ocean, but for a working port like Merak, this is as clean as it gets. The internal compass performed well, though I had to run a sanity check against the known coordinates because the proximity to large steel hulls can sometimes induce magnetic interference.
Recommendations for Future Deployments
If you're heading back to Merak or any high-traffic strait, don't wing it. You need a rigorous deployment strategy to avoid losing your gear to a dragging anchor or a stray trawler.
- Use heavy-duty galvanised tripods with wider footprints to prevent sinking into the silt.
- Increase the ping rate to capture the rapid turbulence changes caused by vessel wakes.
- Deploy a separate CTD probe for 24 hours to get real-time salinity and temperature data for accurate sound speed correction.
- Avoid placing the sensor directly in the primary ferry lanes to reduce bin contamination from propeller wash.
- Schedule recovery windows strictly around neap tides to simplify the retrieval process.
The real challenge at Merak isn't the depth; it's the activity. You are fighting a battle against sediment, ships, and a very powerful tide. But once you get the sensor on the bottom and the data flowing, the Sunda Strait reveals exactly why it's one of the most complex navigational waters in Indonesia.
Field report by Dr. Kenji Sato. Dr. Sato is a specialist in underwater acoustics and oceanographic instrumentation with 20 years of experience in hydrodynamic monitoring.
Field Deployment Report: Bottom-Mounted ADCP at Merak Port, Sunda Strait