Deployment Notes: Raas Cabaad, October 2023
We hit the shoreline at Raas Cabaad just as the first light broke over the horizon, fighting a stiff onshore breeze that threatened to push our skiff off course. The smell of salt and decaying marine vegetation was thick in the air. I remember looking at the water—a deceptive, glassy turquoise—knowing full well that the subsurface shear in this specific bay is a nightmare for any instrumentation engineer. This isn't your standard open-ocean deployment. Raas Cabaad is a chaotic intersection of tidal flux and complex seabed topography that makes velocity profiling a gamble.
The site is a mess of sandy patches and jagged rocky outcrops. These features create localized eddies that can throw off a sensor in seconds. We spent the first hour just scouting the bottom. The water was surprisingly warm for October, but the salinity gradients were shifting rapidly. We're dealing with a delicate balance here; the ebb and flow of the tides aren't just moving water, they're shifting the entire chemical signature of the bay. It's a high-energy environment that demands gear that can actually hold its ground.
What We Found
The data came back noisier than I expected, but the results were eye-opening. We caught a massive surge in current velocity during the peak flood tide that nearly doubled our initial estimates. It was a shock. The water wasn't just flowing; it was pulsing. We saw velocity spikes that suggest the local rocky formations are funneling the current into high-speed jets. This explains why the coastal erosion patterns here are so erratic. Most of the 'steady' flow we've seen in historical reports is a myth. The reality is a series of violent, short-lived bursts of kinetic energy.
I noticed a weird trend in the lower bins of the ADCP data. We had significant bin contamination near the seabed, likely caused by the suspended sediment being kicked up by the current. It looked like a wall of noise. But once we filtered the signal, we saw that the bottom-most layer of water was actually moving in the opposite direction of the surface flow. This kind of vertical shear is classic for this region, though the magnitude was far higher than I've seen in similar bays. It's a volatile system. One minute you have a calm surface, and ten meters down, the water is screaming toward the coast.
Equipment Performance
We deployed a 600kHz ADCP, and honestly, it was the only right choice. I've used 300kHz units in similar depths, but they lack the resolution needed to catch these tight shear layers. The unit stayed vertical (mostly), though we had a scare when a rogue swell shifted the tripod by a few degrees. The battery life held up, but the acoustic backscatter was all over the place. I suspect the high concentration of organic matter in the water column created too many 'false targets' for the sonar. We spent three days just cleaning the data to get a signal we could actually trust. If we had used a lower-frequency unit, we would have missed the fine-scale turbulence entirely. It would have been a wasted trip.
Recommendations for Future Deployments
Next time, we can't rely on a standard tripod. The seabed at Raas Cabaad is too unstable. We need something with a heavier footprint to prevent tilt during peak tidal surges. I'd also suggest a multi-sensor array to ground-truth the ADCP data.
- Switch to heavy-duty gravity anchors to avoid tilt-induced error.
- Deploy a CTD string alongside the ADCP to correlate velocity spikes with salinity shifts.
- Increase the ping rate to 2Hz to better capture the transient nature of the tidal jets.
- Use a protective cage for the transducer to prevent damage from floating debris during storm surges.
Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience in salt wedge modeling and oceanographic sensor deployment.
Field Deployment Report: Bottom-Mounted ADCP Profiling at Raas Cabaad