Deployment Notes: Al Hudaydah Coastal Shelf, October 2023
The heat hit us the moment we stepped off the vessel at the Al Hudaydah docks. It was barely 6:00 AM, but the humidity was already thick, smelling of salt and diesel from the idling tankers. We were here to get a real-world look at the current dynamics in this specific stretch of the Red Sea, and the conditions were immediately hostile. The water was a murky green, churned up by heavy port traffic and the constant shifting of seabed sands.
This isn't your typical open-ocean survey. Al Hudaydah presents a nightmare for acoustics because of the extreme salinity gradients and the sheer volume of suspended sediment. The Red Sea is already some of the saltiest water on Earth, and in this shallow coastal zone, the density layers shift rapidly. We spent the first hour just fighting the current to position the deployment frame, realizing quickly that the local ebb and flow are far more erratic than the regional tide tables suggest.
What We Found
The data shocked us. We saw velocity spikes during the ebb tide that nearly doubled our pre-deployment estimates. I noticed a weird shear layer about three meters above the seabed where the current direction actually flipped. It's a classic case of bottom friction fighting the surface push, but the intensity was unusual. We recorded peaks of 0.9 m/s in the upper bins, while the bottom bins were practically stagnant. This kind of vertical shear is a headache for anyone trying to model sediment transport in the port basin.
The wind played a massive role here. We had a strong northeasterly breeze blowing throughout the deployment, which pushed the surface water hard toward the shore. This created a setup that amplified the tidal surge. When we compared the ADCP profiles to the surface floats, the discrepancy was glaring. The surface water was racing, but the deep water was sluggish. Honestly, if you only rely on surface measurements in Al Hudaydah, you're missing half the story. You get a completely skewed picture of how the water actually moves through the harbor.
Equipment Performance
We used a 600kHz ADCP for this run, and it was the right call. I had a colleague suggest a 300kHz unit for deeper penetration, but in these shallow coastal waters, you need the higher resolution to avoid bin contamination from the seabed. The signal-to-noise ratio was shaky at first—likely due to the high turbidity and bubble interference from the nearby shipping lanes—but once the unit settled into the sand, the signal cleaned up. We did lose a few pings during the peak flood tide when the sediment load spiked, but the overall data set remained robust. The bottom-mount frame held steady, though we had to over-engineer the ballast to ensure the unit didn't tip in the shifting sands.
Recommendations for Future Deployments
If you're heading back to the Yemen coast for current profiling, don't wing it. The environment is too volatile.
- Use 600kHz or 1200kHz transducers to maintain a usable blanking distance in shallow depths.
- Over-weight your deployment frames by at least 30% to prevent tipping during high-velocity ebb tides.
- Sync your ADCP clock with a high-precision GPS to ensure tide-phase correlation is accurate to the second.
- Increase the ping rate to capture short-duration turbulence events common near the port infrastructure.
- Perform a manual sanity check with a handheld current meter during the initial drop to verify the ADCP's zero-velocity calibration.
The Red Sea is a temperamental beast. In Al Hudaydah, the intersection of high salinity, wind-driven currents, and heavy maritime traffic creates a complex acoustic environment. You can't just drop a sensor and hope for the best. You have to account for the 'noise' of the port and the physical instability of the seabed. For us, the goal was ground-truthing the theoretical models, and the results prove that the local coastal currents are far more aggressive than the charts indicate. It's a reminder that the ocean doesn't always follow the manual.
Field report by Dr. Kenji Sato. Dr. Sato is a specialist in underwater acoustics and oceanographic instrumentation with 20 years of experience in global river and coastal discharge monitoring.
Field Deployment Report: Bottom-Mounted ADCP Profiling in the Al Hudaydah Port Basin