Deployment Notes: Dahaban Coastal Sector, October 2023
We hit the shoreline at Dahaban just as the first light broke over the horizon. The air was thick with salt and the smell of drying nets from the local fishing boats. My team and I spent the morning fighting the surf to position our tripod mounts. The water here is deceptive. One moment it looks like a mirror; the next, a sudden swell pushes you three meters back toward the dunes.
The site conditions were volatile. We dealt with a choppy surface and a surprising amount of suspended organic matter. The local geography—specifically those deep inlets and jagged headlands—creates a chaotic mixing zone. It's a nightmare for standard flow meters because the current doesn't just move in one direction; it swirls. This makes ground-truthing the velocity data incredibly difficult when you're dealing with rapid tidal shifts and localized wind-driven surges.
What We Found
The data came back with a shock. We saw peak current velocities that spiked far beyond the regional averages for this time of year. In the narrower channels near the headlands, the flow accelerated violently during the ebb tide. I noticed a sharp shear layer just a few meters above the seabed. It was a clean signal, but the magnitude was unexpected. The water wasn't just moving; it was pulsing. (Probably a result of the specific bathymetry funneling the tide into a tighter space than the charts suggested).
We also caught some noisy data during the peak wind events. When the prevailing winds kicked up, the surface currents shifted almost instantly, creating a vertical velocity profile that looked like a twisted ribbon. The top bins showed strong shoreward movement, while the bottom bins were still pushing seaward. This kind of stratification is common in coastal zones, but the intensity at Dahaban is extreme. It explains why the local fishermen struggle with their gear during certain moon phases—the underwater currents are fighting the surface wind.
Equipment Performance
I deployed a 600kHz ADCP for this run, and honestly, it was the right call. I’ve used 1200kHz units in similar coastal environments, but they often struggle with range in these shallower, high-energy zones. The 600kHz unit gave us the vertical resolution we needed without sacrificing too much of the water column. We did run into some bin contamination near the seabed—typical for sandy bottoms—but the signal-to-noise ratio remained acceptable. The battery life held up well, though the biofouling started to creep in around day ten. If we leave a unit here for a month, we'll need a better anti-fouling coating or we're looking at garbage data by week four.
Recommendations for Future Deployments
Next time, we can't rely on the general bathymetry maps. They are too coarse for Dahaban's erratic seabed. We need a more granular approach to placement to avoid the 'dead zones' created by the underwater ridges.
- Shift deployment coordinates 50 meters further offshore to avoid the high-turbulence surf zone.
- Increase the ping rate during spring tides to capture the rapid velocity swings.
- Use a heavier tripod base. The sandy substrate here shifts during storms, and I don't trust the current mounts to stay level.
- Pair the ADCP with a standalone conductivity-temperature-depth (CTD) sensor to see if salinity gradients are driving these strange shears.
The sheer variability of the Dahaban coast makes it a fascinating, if frustrating, place to work. You can't just drop a sensor and walk away. You have to understand the rhythm of the coast, or you'll end up with a data set that makes no sense.
Field report by Dr. Kenji Sato. Dr. Sato is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience in high-energy aquatic environments.
Field Deployment Report: Bottom-Mounted ADCPs along the Dahaban Coastline