Deployment Notes: Sonsonate Coastline, October 2023
The humidity hit us like a wall the moment we stepped off the boat. I remember looking toward the Izalco Volcano, the 'Lighthouse of the Pacific,' and realizing just how much geographic chaos we were stepping into. We hit the water at 0400 hours to time the flood tide perfectly, but the Pacific doesn't play by the rules here. The water wasn't blue; it was a thick, opaque brown, churned up by the Lempa River's massive sediment discharge. You could practically taste the silt in the air.
Sonsonate is a hydrodynamic nightmare for any acoustics expert. You have the semi-diurnal tides of the Pacific slamming directly into the freshwater plumes of the Lempa system. This creates a volatile saltwater wedge that pushes inland during the flood and then rips back out during the ebb. The result is extreme vertical shear. One minute you're measuring a slow-moving salt layer at the bottom, and the next, a high-velocity freshwater surge screams overhead. It's a constant tug-of-war between buoyancy-driven flow and tidal forcing.
What We Found
The data came back messier than I expected. The most jarring point was the sheer velocity of the ebb tide near the estuaries. We saw spikes that nearly doubled our projections, likely due to the coastal bathymetry funneling the outflow through narrow rocky outcrops. These features create localized eddies that act like centrifuges, spinning the water in ways that make standard linear models useless. I've seen this kind of turbulence in the Mekong Delta, and it's just as unpredictable here.
We also hit a wall with the thermocline. Because of the unique coastal geometry, cold, nutrient-dense water upwells from the deep Pacific and crashes into the warm, river-fed surface layers. This created a sharp temperature gradient that caused significant ray bending. In several of the lower bins, we saw 'ghost' velocities—noisy data caused by the acoustic signal refracting as it hit the cold wedge. If you aren't accounting for this refraction, your depth-averaged velocity is a lie. We spent three days ground-truthing the data against surface floats just to make sure we weren't chasing phantoms.
Equipment Performance
I opted for a 300kHz ADCP for this run, and it was the only right call. A 600kHz unit would have been blinded by the 'thick soup' of suspended solids coming off the Lempa, and a 1200kHz unit would have been useless after ten meters. Even at 300kHz, we struggled with signal attenuation during the peak of the river discharge. The silt simply absorbs the pings. We had to widen the sampling intervals to maintain a clean signal, but that meant sacrificing some temporal resolution. I'll be honest: the mooring was a headache. The sandy shelves shift rapidly here, and we nearly lost the unit to a sudden scour event that undermined the tripod base (shallower than expected for October). We got the data, but it was a fight every step of the way.
Recommendations for Future Deployments
If you're heading into the Sonsonate zone, don't trust the charts blindly. The bathymetry is too chaotic for a 'set and forget' approach.
- Stick to 300kHz transducers to penetrate the sediment plumes without losing the entire return signal.
- Over-engineer your mooring weights. The Lempa's outflow creates enough bottom-current energy to shift a standard tripod.
- Increase the sampling interval during the rainy season (May to October) to avoid bin contamination from excessive backscatter.
- Perform a manual salinity profile at the time of deployment to calibrate for the saltwater wedge interaction.
Field report by Capt. Marcus Thorne. A specialist in underwater acoustics and maritime instrumentation with twenty years of experience in high-turbidity port hydrography.
Field Deployment Report: Bottom-Mounted ADCP Profiling near the Lempa River Mouth, Sonsonate