Acoustic Signal Attenuation and Velocity Shear in the Samut Sakhon Estuarine Mixing Zone

The Convergence of Freshwater Plumes and Semi-Diurnal Tidal Forcing in Samut Sakhon

Field observations from November 2023 revealed peak current velocities in the Samut Sakhon coastal zone that significantly exceed historical regional averages. We observed localized current spikes reaching magnitudes that suggest a high-energy environment, driven by the constriction of water as it moves through the intricate network of canals and river mouths characteristic of this coastline. This is not a standard open-water scenario. The interaction between the river discharge and the Gulf of Thailand creates a volatile saltwater wedge. This wedge shifts position rapidly with the tide, dragging a massive load of suspended solids along with it.

The transition into the Northeast monsoon adds another layer of complexity. The wind-driven surface currents act in opposition to the flood tide, creating a violent shear zone within the water column. During our sanity check of the initial data, we saw a distinct flip in direction between the upper and lower bins of the ADCP. This vertical velocity gradient is a signature of the mixing zone. The water doesn't move as a single block; it swirls in a three-dimensional chaos of opposing forces. This makes single-point measurements useless for any serious sediment transport model.

Working in this region requires an understanding of the 'nozzle effect.' As the tidal prism is forced through narrow channels, the velocity increases. This acceleration scours the seabed and keeps organic matter in suspension. The resulting 'soup' of sediment creates a challenging medium for acoustic propagation. If you don't account for the high attenuation caused by these suspended solids, your data will be noisy at best and completely misleading at worst.

The Tha Chin River Mouth and Bight Bathymetry

The operational area centers around the discharge point of the Tha Chin River into the Gulf of Thailand, roughly between 13.8°N and 13.9°N. The bathymetry here is treacherous for instrumentation. We dealt with shallow contours that fluctuate wildly due to rapid siltation and dredging activities. Depth profiles show a steep gradient moving away from the shore, but the seabed is far from stable. The river mouth acts as a conduit for massive volumes of freshwater and silt, which settles in the bight, creating a fluctuating bottom boundary layer that can bury a bottom-mounted sensor in a matter of days if the site selection is poor.

This geographic bottleneck concentrates the semi-diurnal tidal flow. We tracked the movement of the saltwater wedge as it pushed inland during the flood tide, fighting the outward pressure of the river's discharge. The resulting turbulence is extreme. The interaction between the coastal geometry and the tidal cycle creates localized eddies that can trap pollutants or nutrients, making the precise measurement of flow vectors critical for any environmental impact study in the Samut Sakhon province.

Acoustic Propagation Challenges in This Environment

The water in Samut Sakhon is a nightmare for high-frequency acoustics. The combination of high turbidity, fluctuating salinity, and warm tropical temperatures creates a medium that absorbs sound waves aggressively. We observed significant signal attenuation. The suspended sediment—mostly fine silts and organic debris—acts as a series of acoustic dampers. In cleaner waters, a sound pulse travels further with less loss. Here, the signal fights through a thick curtain of mud. This attenuation limits the effective range of the instrument and can lead to 'blanking' in the lower bins where the sediment concentration is highest.

Salinity gradients also play a role. The freshwater plume from the Tha Chin River creates a pycnocline—a layer of rapid density change. Sound speed varies with salinity and temperature. When a sound wave hits this density interface, it can refract or scatter. This creates 'noisy data' that requires rigorous post-processing to clean. We found that the speed of sound was not constant across the water column, which, if left uncorrected, introduces a systematic error in velocity calculations. You cannot simply use a standard 1500 m/s constant in these waters; you'll get the wrong answer.

Low-Frequency ADCP Deployment and Signal Penetration

I opted for a lower frequency ADCP unit for this deployment. In these turbid conditions, a 600kHz unit would have struggled. It would have lacked the 'punch' needed to penetrate the suspended solids, likely resulting in a total loss of signal in the bottom 20% of the water column. By using a lower frequency, we ensured the acoustic energy could travel through the silt and return to the transducer with enough strength to be processed. It was the only way to get a clean signal across the majority of the water column.

The trade-off for lower frequency is a loss in vertical resolution. You get fewer bins and a larger blanking distance. However, in the Gulf of Thailand's coastal zones, penetration is more important than pinpoint precision in the top meter. We accepted the coarser resolution to avoid the risk of bin contamination. The high sediment load near the seabed often creates a 'false bottom' or introduces noise that masks the actual flow. The lower frequency unit allowed us to see the bottom boundary layer without the data turning into a mess of random spikes.

Data Interpretation and Field Findings

The resulting velocity profiles were eye-opening. We recorded current speeds that peaked during the spring tide, showing a clear correlation between the lunar cycle and the intensity of the nozzle effect at the river mouth. The data showed that the surface currents were often dominated by the NE monsoon winds, while the bottom currents remained locked to the tidal cycle. This created a shear zone that shifted vertically throughout the day. It's a complex, shifting system. The surface water was moving shoreward while the bottom water was still draining out to sea (a classic salt-wedge behavior).

We also noticed that the 'scatterers'—the plankton and silt—were surprisingly consistent in their distribution. This actually helped the ADCP maintain a lock on the water mass. Without these particles, the water would be too clear for the Doppler shift to be measured. It's a paradox: the mud that makes the water murky is the same thing that allows us to measure its movement. The challenge is simply finding the frequency that balances scattering with absorption. Our data confirmed that the energy levels in the Samut Sakhon bight are far higher than previously modeled, likely due to the changing coastal morphology.

Operational Implications for Port Hydrography

These findings have immediate implications for dredging and maritime navigation in Samut Sakhon. The aggressive tidal jets we measured move sediment with surprising force. This means that dredged channels can refill faster than expected. If a port authority relies on outdated historical averages, they will be surprised by how quickly silt accumulates in critical navigation lanes. The 'nozzle effect' creates hotspots of erosion and deposition that are highly localized. You can't generalize the flow across the harbor; you need site-specific data.

Furthermore, for any aquaculture operation in the region, understanding this shear zone is vital. The flip in current direction between the surface and the bottom affects how nutrients and oxygen are distributed. If you're placing cages or monitors, you need to know that the surface flow is not representative of the entire water column. Relying on a single-point float measurement is just guessing. Only a full-profile acoustic measurement provides the ground-truthing required to manage these waters effectively.

About the author: Capt. Marcus Thorne. A veteran oceanographer and acoustics expert with twenty years of experience in maritime instrumentation and port hydrography. He specializes in deploying acoustic sensors in high-attenuation estuarine environments.