Mud transport in the microtidal San Jacinto Estuary

This study investigates the sediment transport processes in the microtidal San Jacinto Estuary under variable hydrologic and hydrodynamic conditions. This was done through a combination of laboratory, field, and numerical modeling experiments. The laboratory tests were run to formulate the relationship between an acoustic Doppler velocimeter (ADV) backscatter and suspended sediment concentration (SSC). For the field study, knowledge gained from the laboratory was used to collect needed data to determine the critical shear stress parameters with the ADV. This information was then used as part of the required input to numerically model the general sediment transport processes within the estuary under variable hydrologic conditions.

In the laboratory work, a 6 MHz Nortek Vector velocimeter was tested in a water tank with suspensions of four different synthetic and natural muds. For concentrations less than 1500 mg/l, a region of linearity between the logarithm of concentration and time-average SNR was found and a calibration equation could be developed. However, the exact calibration coefficients were strongly dependent on the sediment grain size distribution.

The field measurements of the critical erosion shear stress were made at three sites containing mixtures of sand, silt, and, clay under different hydrodynamic conditions. This was done by deploying the ADV over a tide cycle to measure mean and turbulent velocity as well as SSC and significant changes in boundary elevation. Six different techniques were used to convert velocity time series to bed shear stress time series. Two of the sites yielded estimates of the critical shear stress condition for erosion of 0.14 N/m^2 and 0.06 N/m^2 respectively.

Two synthetic freshwater inflows, simulating dry and wet conditions, were used in the numerical modeling experiments. Simulations showed that the change in freshwater inflow has significant impact on the distribution of salinity within the system but does not greatly change sedimentation patterns. This indicates that ecosystems sensitive to salinity level change in the estuary are vulnerable to extreme droughts and floods, but that the overall sediment transport patterns are relatively less affected by seasons of drought. The work does not however account for hurricanes which could strongly alter sediment transport patterns.