Measurement of critical shear stress for mud mixtures in the San Jacinto estuary under different wave and current combinations

In situ measurements of the critical shear stress for the onset of erosion were made at three sites in the shallow, microtidal San Jacinto estuary under different hydrodynamic conditions. Sediment at the three sites was composed of a mixture of sand, silt, and clay with the total silt and clay fraction ranging from 45% to 95% of the total mixtures. The measurements were made by deploying an acoustic Doppler velocimeter over a tide cycle to measure mean and turbulent velocity as well as suspended sediment concentration 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/m2 and 0.06 N/m2 respectively (which correspond to a Shieldʹs stress value of 0.1), but values were not found in the lowest energy environment. The hydraulics of the region are such that flood currents produce the highest shear stresses and therefore the largest degree of erosion. Deposition largely occurs over the lengthy high-water condition. All six methods for estimating the bed shear stress showed general trend correlation, however, significant variation in the estimated magnitude of the bed shear stress was observed among the methods. Based on theoretical considerations and analysis of the data, it was found that the logarithmic profile method performed best in current dominated regions and that the modified turbulent kinetic energy method performed best in the wave dominated zone.