Deployment Notes: Ranong Coast, Andaman Sea, October 2023
The humidity hit us like a wall the moment we stepped off the boat. We arrived at the Ranong shoreline just before dawn to catch the flood tide, the air thick with the scent of salt and decaying mangroves. This isn't your typical open-ocean deployment. Ranong is a nightmare for acoustic imaging because of the extreme salinity gradients. Between the heavy freshwater discharge from the local river systems and the saltwater push from the Andaman Sea, you get these volatile stratification layers that can mess with your sound velocity profile if you aren't careful.
The water was a murky olive green, typical for the tail end of the southwest monsoon. Waves were choppy, driven by lingering winds that kept the surface agitated. We spent the first hour just fighting the current to get the deployment frame seated on the seabed. The bathymetry here is erratic; one minute you're in a deep channel, and the next, you've hit a sudden shoal that wasn't on the chart. It makes precise positioning a gamble.
What We Found
The data came back with a shock. We saw velocity spikes in the mid-water column that completely contradicted the surface drift markers. While the surface buoys suggested a steady westward flow, the ADCP showed a powerful, narrow jet of current hugging the bottom, moving nearly 0.9 m/s in the opposite direction. It's a classic case of tidal rectification. The narrow channels between the offshore islands are funneling the tide, creating these high-velocity conduits that the surface data simply misses.
We also noticed a massive amount of 'noisy data' during the peak flood. The suspended sediment load—likely stirred up by the monsoon runoff—was so high that it created significant backscatter. In some bins, the signal-to-noise ratio plummeted. I suspect we had some bin contamination from fish schools moving in with the tide, which is common in these nutrient-rich Andaman waters. Still, the core trend was clear: the interaction between the river discharge and the tidal cycle creates a swirling, chaotic flow pattern that changes every few hours.
Equipment Performance
I ran a 600kHz ADCP for this leg, and honestly, it was the only right choice. A higher frequency would have been blinded by the turbidity, and a lower frequency wouldn't have given us the vertical resolution needed to see those shear layers. The unit held its position well despite the bottom currents, though we had to double-weight the tripod to prevent scouring. We did see some signal drop-out in the lowest 0.5 meters—standard blanking distance issues—but the rest of the profile was clean. I'm skeptical of the surface drifting buoy data we compared it to; those buoys are too influenced by windage to be used for anything other than a rough sanity check.
Recommendations for Future Deployments
If you're heading back to Ranong, don't trust the general charts. You need real-time sound velocity corrections because the freshwater plume shifts daily (especially during the monsoon transition). My advice for the next team:
- Deploy a CTD alongside the ADCP to correct for salinity-driven sound speed changes.
- Use heavy-duty galvanised frames to prevent the tripod from migrating in the sandy substrate.
- Set your sampling interval to 10 minutes to capture the tidal reversal without bloating the data file.
- Avoid deploying during the peak of the northeast monsoon if you want a clean signal without storm-surge interference.
The site is a goldmine for sediment transport study, but you have to fight the environment to get the data. We spent three days ground-truthing the velocity profiles against manual flow meters, and the ADCP won every time. It's the only way to see what's actually happening beneath the surface noise of the Andaman.
Field report by Elena Rodriguez. Elena is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience in coastal sediment transport.
Field Deployment Report: Bottom-Mounted ADCPs in the Ranong Coastal Zone