Deployment Notes: Tanjung Intan Port, South Sumatra, November 2023
The humidity hit us like a wall the moment we stepped off the boat at Tanjung Intan. It was 05:30, and the air was thick, smelling of brackish mud and diesel from the idling cargo vessels nearby. I watched the tide pushing in from the Java Sea, swirling violently around the pier supports. This isn't a standard open-ocean deployment. Tanjung Intan is a nightmare for acoustic instrumentation because of the extreme sediment load and the erratic salt wedge dynamics typical of South Sumatran estuaries.
The water was a milky, opaque brown. We were operating during the transition into the northwest monsoon, meaning the river discharge from the interior was pushing hard against the incoming tide. This creates a high-shear environment where the freshwater floats atop a denser saline layer. If you don't account for that salinity gradient, your sound speed corrections will be off, and your velocity data becomes useless. The wind was gusting, making the deployment of the tripod mount a clumsy, stressful affair.
What We Found
The data caught us off guard. We saw velocity spikes in the lower water column that completely contradicted the surface flow. While the surface current was sluggish, the bottom-hugging saline wedge was ripping through the channel at nearly 0.9 m/s during the flood tide. It's a classic estuarine trap. The sheer volume of agricultural runoff—palm oil residues and silt—creates a dense, turbid slurry that scatters acoustic signals. We saw significant 'noisy data' in the first few bins above the transducer, likely due to the high suspended sediment concentration (SSC) near the bed.
I noticed a strange oscillation in the current vectors every six hours. It wasn't just the tide. The interaction between the river's discharge and the coastal currents creates these weird, rotating eddies near the berths. For the local pilots navigating medium-sized cargo ships, these eddies are dangerous. A ship can be pushed off course in seconds if the pilot isn't expecting a sudden lateral shift in the current. Honestly, the local charts don't even hint at this level of instability. We spent three hours ground-truthing the ADCP readings against a handheld current meter, and the ADCP held its own, though the signal-to-noise ratio was tighter than I'd like.
Equipment Performance
We ran a 600kHz ADCP for this stint. I'm glad we didn't go with a higher frequency; a 1200kHz unit would have been blinded by the turbidity in Tanjung Intan within hours. The 600kHz unit penetrated the silt well enough to give us a clean signal in the mid-column. However, we dealt with some bin contamination near the bottom. The sediment is so thick here that the 'bottom track' occasionally locked onto a dense layer of suspended silt rather than the actual seabed. I had to manually scrub the data during post-processing to remove those artifacts. It worked, but it was tedious. The battery life held up, but the biofouling started almost immediately—Sumatran waters are aggressive.
Recommendations for Future Deployments
If you're heading back to Tanjung Intan or similar Indonesian ports, don't just drop and hope. You need a strategy for the silt.
- Use a 300kHz or 600kHz transducer. Anything higher will fail in this turbidity.
- Deploy a CTD (Conductivity, Temperature, Depth) sensor alongside the ADCP. Without real-time salinity data, your sound speed profile is just a guess.
- Increase the ping rate during spring tides to capture the rapid acceleration of the salt wedge.
- Apply a heavy-duty anti-fouling coating to the transducer face to avoid signal degradation from organic growth.
- Set the blanking distance slightly higher (approx 1.5 meters) to avoid the 'mud zone' contamination.
The port authority is mostly concerned with dredging schedules and vessel safety. I told them that the current shear we measured justifies more frequent dredging in the main channel. The silt isn't just settling; it's being actively pushed into the navigation lanes by these tidal wedges. It's a dynamic system, not a static hole in the ground.
We pulled the gear at 17:00 on the fourth day. The tripod was caked in a grey, clay-like slime. I checked the internal clock and the data logs—everything was intact. It was a messy deployment, but the velocity profiles we captured are the first of their kind for this specific reach of the port. Now we just have to see if the seasonal shift in the monsoon changes these patterns in January.
Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics with twenty years of experience mapping estuarine flow and salt wedge intrusions in tropical environments.
Field Deployment Report: Bottom-Mounted ADCP Profiling at Tanjung Intan Port