Field Deployment Report: Bottom-Mounted ADCP Profiling in the Gulf of Aqaba, Sharm El-Sheikh

Deployment Notes: Sharm El-Sheikh, October 2023

We hit the water at 0400 hours, the air still thick with humidity but the wind unusually dead. The Red Sea looks like a sheet of glass from the deck, but anyone who has worked the Sinai coast knows that surface stillness is a lie. Beneath the skin of the water, the Gulf of Aqaba is a complex machine. We were targeting a specific set of coordinates off the coast of Sharm El-Sheikh to get a handle on the subsurface velocity profiles.

The site is a nightmare for instrumentation if you aren't prepared. You've got massive vertical walls—underwater cliffs that drop off into the abyss almost instantly. The salinity is higher than your average coastal site, which shifts the speed of sound and can throw off your distance calculations if you don't calibrate for the local salt load. The water was crystal clear, which is great for diving but terrible for acoustics; without enough suspended particles (backscatter), some sensors struggle to find a signal.

What We Found

The data came back with a spike that caught us off guard. We saw a strong, narrow jet of water moving south-southeast, far faster than the regional averages. It wasn't a tide—the Red Sea has negligible tidal ranges—but rather a wind-driven surface current that was punching deep into the water column. We recorded velocities hitting 0.6 m/s in the upper bins, which then sheared off violently as we hit the thermocline. It's a classic example of how the local topography of the Sinai coastline channels flow, turning a general current into a localized accelerator.

I suspect the interaction between the northerly winds and the steep bathymetry of the reef edge is creating these eddies. We saw significant 'noisy data' in the bottom three bins, likely due to the ADCP being too close to the seabed (bin contamination). However, the mid-water column data was clean. The salinity gradients here are aggressive. We noticed the current direction shifted by nearly 30 degrees over a vertical distance of just 20 meters. That's a lot of shear for a relatively small area.

Equipment Performance

I ran a 300kHz ADCP for this stint. Honestly, the unit performed well, but the low turbidity of the Red Sea pushed it to its limits. In some of the deeper bins, the signal-to-noise ratio dropped, making the velocity readings twitchy. I’ve used 600kHz units in these waters before, and while they have a shorter range, they often provide a cleaner signal in the upper 50 meters. The bottom mount held firm despite the current, though the sandy patches near the rocky coves make leveling a pain. If the tripod isn't perfectly plumb, your vertical bins are useless. We spent an extra hour just ensuring the tilt was within 2 degrees to avoid skewed data.

Recommendations for Future Deployments

If you're heading back to Sharm El-Sheikh for current profiling, don't just trust the factory settings. You need a rigorous ground-truthing plan.

• Adjust the speed of sound profile manually. The high salinity in the Gulf of Aqaba will lie to you if you use standard seawater constants.
• Increase the blanking distance. The seabed here is rugged; you'll get too much bottom-bounce interference if you keep the blanking distance tight.
• Use a heavy-duty mooring. The wind-driven surface currents can put unexpected drag on your gear.
• Schedule deployments during the transition between seasons to capture the shift in wind-driven flow patterns.

We did a quick sanity check against a handheld flow meter during the recovery phase. The numbers aligned within 5%, which gave me confidence in the ADCP's integrated readings. The Red Sea is a fickle environment. One day it's a lake; the next, the subsurface currents are ripping through the coral canyons with enough force to shift a poorly anchored sensor.

The operational window in Sharm is tight. Between the tourist boat traffic and the protected marine park zones, you have to be precise with your drop points. We managed to avoid the diving corridors, but the congestion in the bay makes the deployment phase the most stressful part of the job. Once the gear is on the bottom, it's just a waiting game for the data to accumulate.

Field report by Capt. Marcus Thorne. Capt. Thorne is a senior consultant in maritime acoustics with 20 years of experience deploying hydrographic instrumentation in extreme oceanic environments.