Deployment Notes: Alicante Coast, September 2023
The humidity hit us the moment we stepped off the boat near the Port of Alicante. It was barely 06:00, but the Mediterranean sun was already baking the deck. We spent the morning fighting a stubborn surface chop that made deploying the tripod a nightmare. The water here is deceptive; it looks like a swimming pool from the beach, but the bathymetry drops off with a steepness that catches you off guard if you aren't checking your charts every five minutes.
We were targeting a specific shelf break where the coastal currents interact with the deeper Mediterranean circulation. The sea state was moderate, but the salinity gradients were erratic. We noticed a distinct lack of freshwater runoff—typical for a dry Spanish September—which meant we were dealing with high-density brine that shifted the acoustic velocity of sound. I had to manually adjust the sound speed profile in the software to avoid significant distance errors in our binning.
What We Found
The data came back with a spike that stopped us cold: a localized surface jet moving south-southeast at nearly 0.6 m/s, completely decoupled from the bottom currents. It was a classic example of wind-driven transport. The prevailing easterlies were pushing water right against the coast, creating a vertical shear that would make any hydrodynamic modeler sweat. While the bottom layers remained sluggish (barely moving at 0.05 m/s), the top 10 meters were in a frenzy.
I suspect the submerged ridges near the shoreline are acting as baffles, tripping the current and creating small-scale eddies. We saw some noisy data in the lower bins, likely due to the sandy substrate reflecting the signal. It wasn't a deal-breaker, but it required some aggressive filtering to get a clean signal. Honestly, the interaction between the anti-clockwise Mediterranean gyre and the local topography creates a chaotic mixing zone here that most textbooks simplify too much.
Equipment Performance
We ran a 300kHz ADCP for this leg. It handled the depth well, but we hit a snag with 'bin contamination' near the seabed. The proximity to the bottom caused some side-lobe interference, which skewed the first two bins of data. I’ve seen better performance from the 600kHz units in shallower berths, but for this specific depth, the 300kHz was the only way to get a full profile. The battery life held up, though the biofouling on the transducer face was surprisingly aggressive for a short deployment. We found a layer of Mediterranean slime that almost dampened the pings by day ten.
Recommendations for Future Deployments
If you're heading back to the Alicante littoral, don't rely on standard sound speed presets. The salinity shifts are too volatile.
- Use a CTD cast every 48 hours to ground-truth the sound speed; otherwise, your depth bins will be off by several meters.
- Deploy a heavier mooring weight. The bottom currents are weak, but the surface drag from wind-driven surges can tilt a light tripod, ruining your tilt-correction math.
- Swap to a 600kHz head if you are working within the 20-meter contour to minimize the blanking distance.
- Schedule deployments outside the peak tourist season to avoid the nightmare of navigating around pleasure craft in the port channels.
The most critical takeaway? Watch the winds. In Alicante, the wind is the engine. If the easterlies are howling, expect your surface velocities to decouple from the benthos entirely. It's a volatile environment that demands a sanity check on every single data string before you commit it to the final report.
Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience deploying oceanographic instrumentation in complex coastal environments.
Field Deployment Report: Bottom-Mounted ADCP Profiling off the Costa Blanca, Alicante