Field Deployment Report: Bottom-Mounted ADCP at the Yalu River Mouth, Dandong Port

Deployment Notes: Dandong Port, August 2023

The air was thick with humidity and the smell of brackish silt when we hit the docks at Dandong before 05:00. I remember looking out over the Yalu River mouth and seeing the water—it wasn't blue or even green. It was a heavy, opaque ochre. This is the brutal reality of the Yalu estuary. You aren't just measuring water; you're measuring a slurry of terrestrial sediment fighting a losing battle against the Yellow Sea's tidal push.

The conditions were volatile. We were operating right in the teeth of the monsoon season, which means the Yalu was dumping massive volumes of freshwater into the port. The wind was kicking up, and the current was ripping. I've spent years in the Yangtze delta, but Dandong feels more constrained. The geography here forces the freshwater and saltwater into a violent, narrow mixing zone that makes acoustic propagation a gamble.

What We Found

The data came back with a profile that would make a textbook author sweat. The most jarring discovery was the sheer intensity of the salt wedge. We saw a massive velocity shear that was almost surgical in its precision. The surface water was screaming seaward, driven by the river's discharge, while the bottom layer—pushed by the incoming flood tide from the Yellow Sea—was actually moving landward. It's a conveyor belt of sediment. One layer goes out, the other comes in, and the boundary between them is a chaotic mess of turbulence.

We caught several spikes in the backscatter profiles that confirmed my suspicions about the silt load. During the peak of the flood tide, the suspended sediment concentration spiked so hard that we saw significant signal attenuation in the lower bins. I noticed a recurring pattern of 'noisy data' just above the seabed. This is classic bin contamination. The fine silts from the Yalu don't just float; they create a dense, acoustic-absorbing blanket that can trick a sensor if you aren't paying attention. I spent three hours ground-truthing the velocity readings against our secondary sensors just to be sure we weren't seeing ghosts in the machine.

Equipment Performance

We deployed a 600kHz ADCP on a heavy concrete ballast frame, and honestly, it was the only sane choice. I had a colleague suggest a 300kHz unit for better range, but in these shallow waters, the blanking distance would have eaten our most critical data. We would have been blind to the bottom 3 meters of the water column. The 600kHz unit hit the 'sweet spot.' It gave us the resolution to track the shear zone without getting completely swallowed by the turbidity. The bottom-tracking held up well, though we did see some drop-outs during the highest sediment pulses. If we had used a vessel-mounted unit without a rock-solid motion sensor, the data would have been useless—we'd be measuring the ship's drift in the current rather than the actual water movement. The frame stayed put, which is a win given the erratic bathymetry of the port channel.

Recommendations for Future Deployments

If you're heading into the Yalu estuary, don't wing it. This environment eats poorly configured gear for breakfast. Based on this run, here is my checklist:

• Frequency Choice: Stick with 600kHz. 1200kHz attenuates too fast in this silt, and 300kHz has too large a blanking zone for the shallow depths at Dandong.
• Mounting: Forget side-mounts. Between the heavy commercial traffic in the port and the sediment scour, a weighted bottom-mount is the only way to ensure a clean signal.
• Positioning: Set the transducer exactly 1.5 meters off the seabed. Any lower and you're fighting the seabed shadow zone; any higher and you miss the salt wedge interaction.
• Calibration: You must calibrate for local backscatter. The Yalu's silt profile is unique and will skew your readings if you use factory defaults.
• Timing: Schedule your deployments around the monsoon peaks if you want 'stable' data, though the real science happens during the discharge spikes.

The interaction between the river and the sea here is a constant tug-of-war. We managed to capture the transition, but it requires a level of precision that 'plug and play' setups just can't provide. You have to respect the sediment, or the sediment will ruin your dataset.

Field report by Dr. Kenji Sato. Dr. Sato is a specialist in underwater acoustics with over 20 years of experience designing instrumentation for high-turbidity estuarine environments.