Deployment Notes: Richard's Bay Main Channel, November 2023
We hit the water at 04:00, just as the first grey light touched the horizon. The air was thick with that oppressive KwaZulu-Natal humidity, and the smell of industrial sulfur from the nearby berths hung heavy over the harbor. I watched the surface of the bay; it looked deceptively calm, but the tide was pushing in hard. In Richard's Bay, the surface is a liar. Below that glassy veneer, the Agulhas Current is shoving high-salinity oceanic water into the basin, creating a subterranean river that moves independently of the wind or the surface tide.
The conditions were typical for November. The water was turbid, tea-colored from the recent summer rain runoff flushing out of the estuarine system. We were operating right in the throat of the harbor, where the bathymetry shifts violently from deep-water channels to shallow flats. It is a chaotic environment. The interaction between the freshwater runoff and the Indian Ocean brine creates a persistent salt wedge that makes acoustic profiling a nightmare if you don't know exactly where your pycnocline is sitting.
What We Found
The data came back with a shock. We clocked a bottom current ripping at 0.6 m/s while the surface was practically dead. That is a massive vertical shear. The salt wedge was sitting at roughly 8 meters, acting like a physical barrier between two different worlds of water. I've seen this in the Port of Houston, but the gradients here are far more aggressive. The dense, saline water from the Agulhas influence simply slides under the fresher runoff, creating a subterranean conveyor belt of sediment and salt.
We saw significant tidal asymmetry during the spring cycle. The flood tide pushes oceanic water deep into the basin with surprising force. Then the ebb tide comes, but it struggles to clear the silt. This creates these localized eddies near the industrial berths that are a total headache for navigators. If you're piloting a capesize vessel, these hidden bottom flows can push your stern in ways the surface current doesn't suggest. It's a dangerous game of guesswork if you rely on simple flow meters. We found several zones where the flow was squeezed by the channel morphology, spiking the velocity and increasing the shear stress on the seabed to levels that would strip away poor-quality anchoring.
Equipment Performance
I'll be honest: the 600kHz units struggled in the main channel. We hit a wall of 'noisy data' almost immediately. The suspended solids from the summer rains were so thick that the signal attenuated before it could reach the bed. We had to swap in the 300kHz workhorse for the deep-channel profiling. It gave us the penetration we needed, though we still fought some bin contamination near the seabed. The real killer was the pycnocline. At the density interface, the acoustic signal occasionally bounced, creating a 'shadow zone' where the data just vanished for a few meters. We spent three hours ground-truthing the data against a physical CTD cast just to make sure the ADCP wasn't hallucinating the velocity spikes. Once we calibrated for the salinity gradient, the 300kHz unit performed reliably, but the 600kHz was only useful in the shallower berths where the water column was more homogenous.
Recommendations for Future Deployments
Stop relying on vessel-mounted sensors for transport volume calculations in this bay. You will miss the salt wedge every single time. For any serious monitoring at 28.7° S, you need bottom-mounted arrays with a high sampling rate to catch the tidal asymmetry.
- Use 300kHz ADCPs for all main channel work to bypass attenuation from suspended solids.
- Schedule deployments to coincide with spring tides if you want to map the maximum extent of the salt wedge.
- Always run a concurrent CTD profile to identify the pycnocline depth; otherwise, your velocity profiles are just guesses.
- Increase the blanking distance to avoid surface noise from the heavy shipping traffic in the industrial zone.
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 instrumentation.
Field Report: Mapping the Salt Wedge and Bottom Currents at Richard's Bay