Determination of Critical Shear Stresses for Erosion and Deposition Based on In Situ Measurements of Currents and Waves over an Intertidal Mudflat

Accurate determination of the critical shear stress associated with the erosion and deposition of sediments is an important component of numerical models used to predict and quantify sediment behavior and transport across intertidal flats. In this study, water depth, wave parameters, near-bed turbulent velocity, suspended sediment concentration (SSC), and intratidal bed-level changes were measured to determine the erosion (tau(ce)) and deposition (tau(cd)) thresholds of sediments on an intertidal mudflat at Jiangsu, China. Based on integrated field measurements of bed-level changes and hydrodynamics, the bed shear stresses of currents (tau(c)), waves (tau(w)), and combined current-wave action (tau(cw)) were calculated, and the critical shear stress required for erosion (tau(ce) = 0.14 N/m(2)) and deposition (tau(cd) = 0.08 N/m(2)) of these sediments was determined. Both values are in agreement with an estimate of tau(ce) (0.13 N/m(2)) that was based on water content and tau(cd) values calculated by previous works, indicating that the value of tau(ce) estimated for these sediments is controlled primarily by the water content of the sediments. During field measurements, deposition occurred (tau(cw) < tau(cd)) when current action exceeded wave energy (tau(c) > tau(w)) during calm weather, whereas erosion occurred (tau(cw) > tau(ce)) when wave action increased dramatically during rough weather. Our field data showed that high current velocities lead to low tau(c), possibly because high SSC reduced the drag coefficient, which is variable during a tide, and further caused low tau(c) under high current velocities. Additionally, bedforms characteristic of intertidal mudflat (e.g., gullies, small creeks, ripples, or saltmarsh) has a significant influence on the drag coefficient of the bed. These observations suggest that the bed level responds strongly to changing hydrodynamic conditions, and also that waves are of great importance to erosion. This study demonstrates that the in situ determination of the parameters that control erosion and deposition is a useful approach to obtaining values of tau(ce) and tau(cd), which provide the basis for a mechanistic understanding of the morphological evolution and development of predictive sediment transport and erodibility models.