The influence of a dredge hole on tidal asymmetry and salinity structure in the Mullica River estuary

Estuarine salinity and velocity patterns are controlled by freshwater input, tides, and bottom topography. Artificially deepening estuary channels through dredging modifies water column stratification and tidal currents. We examined these processes in a 14-meter deep dredged hole near Chestnut Neck, within the Mullica River estuary.  Data were collected during two vessel-based Acoustic Doppler Current Profiler (ADCP) surveys with intermittent Conductivity-Temperature Depth (CTD) casts in and adjacent to the Chestnut Neck dredge hole. A stationary ADCP was placed within the hole during the vessel surveys. Analysis of stationary ADCP data yielded a semidiurnal tidal current amplitude of 0.45 m/s near the surface that diminished to 0.16 m/s near the bottom of the dredge hole. In contrast, data previously collected downstream of the dredge hole showed uniform amplitudes of ~0.4 m/s for both near surface and near bottom currents. The near-bottom velocities were highly asymmetrical (0.4 m/s at maximum flood and 0.1 m/s at maximum ebb tide), potentially trapping higher salinity water in the dredge hole during ebb tide. ADCP transects confirm the ebb velocity asymmetry and its restriction to the dredge hole.  CTD salinity profiles revealed that the vertical salinity gradient varied with tide stage between 0.07 to 0.24 psu/m.  During ebb tide, the dredge hole developed a higher salinity layer coinciding with the weak ebb velocities. Trapping of high salinity waters within the dredge hole appears to alter the water column salinity structure and tidal flow, potentially impacting the local benthic environment.