Relating accretion and erosion at an exposed tidal wetland to the bottom shear stress of combined current-wave action

Sediment dynamics have an important influence on the morphological evolution of tidal wetlands, which consist of mudflats and salt marshes. To understand the nature of sediment behavior under combined current-wave action at an exposed tidal wetland, we measured the waves, currents, water depths, bed-level changes, and sediment properties at a mudflat-salt marsh transition on the Yangtze Delta, China, during five consecutive tides under onshore winds of ~8m/s, and calculated the bed shear stresses due to currents (τ_c), waves (τ_w), combined current-wave action (τ_cw), and the critical shear stress for erosion of the bottom sediment (τ_ce). The bed shear stresses under combined current-wave action (τ_cw) were approximately five times higher on the mudflat (up to 1.11N/m²; average 0.27N/m²) than on the salt marsh (up to 0.14N/m²; average, 0.06N/m²). On the mudflat, τ_cw was larger than the critical erosion shear stress (τ_ce=0.103N/m²) for 70% of the period of submergence, whereas τ_cw was always lower than τ_ce at the salt marsh site (τ_ce=0.116N/m²). This result indicates that the sediment dynamics on the mudflat were dominated by erosion, whereas at the salt marsh they were governed by deposition, which is in agreement with the observed bed-level change during the study period (-3.3mm/tide on the mudflat and 3.0mm/tide on the salt marsh). A comparison of τ_cw values calculated using the van Rijn (1993) and Soulsby (1995) models for bed shear stresses under combined current-wave action indicates that both models are applicable to the present case and effectively predict the bottom shear stress under combined current-wave action. Overall, we conclude that τ_cw in combination with τ_ce is useful in assessing the hydrodynamic mechanisms that underlie the morphological evolution of exposed tidal wetlands.