Deployment Notes: Charleston Harbor, South Carolina - October 2023
We hit the water just before 0500, fighting a stiff breeze that smelled of pluff mud and salt. The visibility was poor, but the tide was the real story. We were timing the flood to get the unit seated exactly where the Ashley and Cooper rivers collide with the Atlantic. It is a chaotic stretch of water. You can practically feel the energy of the tidal prism pushing against the hull of the boat, a reminder that this isn't a lazy estuary—it's a high-velocity mixing zone.
The surface conditions were choppy, with short, aggressive periods that made the deployment tricky. The water was that classic Lowcountry tea-color, thick with organic tannins and suspended sediment. I checked the salinity at the surface; it was fresh enough to make you think you were in a lake, but we knew the salt wedge was lurking just a few meters down, waiting to flip the velocity profile on its head.
What We Found
The data coming off the first 48 hours was wild. We caught a massive 'velocity flip' that would have completely fooled a standard flow meter. At the surface, the freshwater head from the Cooper River was screaming seaward. But just five meters down, the dense, saline Atlantic water was creeping landward, sliding under the fresh layer like a conveyor belt. The shear was intense. We saw a complete reversal in flow direction within a narrow vertical band. If you're only measuring the top meter, you're missing half the story. In fact, you're getting the wrong story entirely.
The tidal range was hitting a peak of nearly 6 feet, which is aggressive for this time of year. This creates a brutal cycle of flushing. We watched the sediment plumes move in rhythmic pulses, dragging nutrients out of the marshes and shoving them back in with surprising force. The most surprising part? The bottom-boundary layer dynamics. The flow near the bed was significantly slower than the mid-column, but the turbulence was off the charts. It's a high-friction environment. The organic-rich mudflats flanking the deep shipping channels create a strange bathymetric contrast that steers the current in ways that don't always align with the dredged channel's axis.
Equipment Performance
I stuck with the 600kHz ADCP for this run, and it was the right call. I've used 300kHz in deeper waters, but here it doesn't provide enough bins to actually resolve the salt wedge interface. Conversely, 1200kHz would have choked on the turbidity. The 'brown water' of Charleston is notorious for attenuating high-frequency signals. We did hit some noisy data during a heavy rain event mid-deployment, but the signal remained stable enough for a reliable sanity check. We used a heavy tripod mount to keep the unit vertical. I've seen people try side-mounting on piers in this harbor—don't do it. The turbulence from the pilings creates massive bin contamination that ruins the dataset. Moving the unit 50 meters offshore was the only way to get a clean signal. We also installed a signal fence to stop the beams from bouncing off the hulls of Neo-Panamax container ships passing overhead.
Recommendations for Future Deployments
If you're heading into the Lowcountry, don't wing it. The stratification is too volatile for guesswork. For anyone planning a run in the harbor or the surrounding inlets, I suggest the following:
- Stick to 600kHz: It's the sweet spot for balancing bin resolution and signal penetration in turbid, shallow-shelf waters.
- Bottom-Mount Only: Avoid pier-mounting to prevent structural turbulence from contaminating your velocity bins.
- Heavy Ballast: Use an oversized tripod. The tidal prism in Charleston can shift massive volumes of water fast enough to tilt a light mount.
- Coordinate with Tide Tables: Deploy during a neap tide if you want to establish a baseline, but hit the spring tides if you actually want to see the salt wedge in action.
- Check your Blanking Distance: Set it carefully to capture as much of the bottom-boundary layer as possible without getting signal noise from the seabed.
Honestly, the biggest mistake I see in these environments is relying on average flow. In a place like Charleston, an 'average' is a lie. You need the full profile or you're just guessing. Ground-truthing with a CTD probe is a must to see where that salinity interface actually sits during the cycle. Without that context, the ADCP data is just a bunch of vectors without a home.
Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience profiling continental shelf currents and tidal asymmetry.
Field Deployment Report: Bottom-Mounted ADCP Profiling in Charleston Harbor