Field Deployment Report: Bottom-Mounted ADCP Velocity Profiling at Munalaiu Port

Deployment Notes: Munalaiu Port, October 2023

We hit the quay at Munalaiu just as the morning mist was lifting, though the humidity was already stifling. The air smelled of diesel and brine. My first look at the water showed a choppy surface, a classic sign of the complex interaction between the outgoing tide and the riverine discharge feeding into this hub. This isn't your standard open-ocean deployment. Munalaiu is a nightmare for acoustics because of the extreme turbidity and the way the salt wedge behaves here.

The site is a mess of competing forces. You have heavy commercial traffic—bulk carriers and container ships—creating massive wake turbulence that can easily mask the actual current signals. The water column is stratified, with a dense saline layer hugging the bottom and a fresher, sediment-heavy layer on top. This creates a sharp pycnocline that often bends acoustic signals, leading to potential errors if you aren't careful with your sound speed corrections.

What We Found

The data came back with a shocker: we recorded peak ebb velocities of 1.1 m/s in the mid-channel, far higher than the port's historical charts suggested. It was a wake-up call. The current wasn't uniform across the channel. We saw an intense shear zone near the berths where the flow suddenly decelerated, creating dangerous eddies. For a captain trying to dock a medium-sized bulk carrier, these cross-currents are a liability. If you don't know exactly where that shear layer sits, you're essentially guessing your approach angle.

The most interesting part was the salt wedge dynamics. We caught the flood tide pushing the saline front upstream, effectively "lifting" the fresher surface water. This caused a massive spike in suspended sediment concentration. In the raw data, this looked like a wall of noise. I spent three hours scrubbing the signal to separate the actual water velocity from the movement of the sediment plumes. It's a classic case of bin contamination where the high particle load in the upper 2 meters skewed the vertical profile. Honestly, the data from the bottom 5 meters was crystal clear, but the surface bins were a fight to clean up.

Equipment Performance

We deployed a bottom-mounted ADCP, and for the most part, it held its own. I opted for a higher-frequency unit to get better vertical resolution, which was the right call. However, the biofouling started almost immediately. Within ten days, we saw a thin film of organic growth on the transducers. It didn't kill the signal, but it did increase the noise floor. I suspect the nutrient-rich runoff from the nearby agricultural lands is fueling this growth. I’ve seen cleaner signals in the open Atlantic than I did in this harbor. That said, the Doppler shift calculations remained stable, and the internal tilt sensor confirmed the unit didn't budge an inch despite the heavy vessel traffic overhead.

Recommendations for Future Deployments

If we go back to Munalaiu, we can't just drop and pray. We need a more aggressive strategy to handle the turbidity and the traffic.

• Shift to 600kHz or 1200kHz transducers to improve spatial resolution in the shear zones.
• Deploy a concurrent CTD (Conductivity, Temperature, Depth) sensor to get real-time sound speed profiles (essential for correcting the salt wedge refraction).
• Use a heavier tripod base with an integrated leveling plate to prevent scouring in the high-velocity mid-channel areas.
• Implement a shorter sampling interval during spring tides to capture the rapid acceleration of the ebb flow.
• Apply a more aggressive blanking distance to avoid the surface noise caused by ship wakes.

The port management thinks they have a handle on the currents, but the data says otherwise. The discrepancy between the modeled flow and the actual measured velocity is significant enough to warrant a rewrite of the pilotage guidelines for the main channel. We need more ground-truthing across different seasons, specifically during the monsoon transitions when the river discharge peaks. Until then, the captains are flying blind on the approach.

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 coastal environments.