Taiwan-U.S. Source-to-Sink Research Workshop – NSYSU, Kaohsiung, Taiwan. Feb 2008. High Resolution Numerical Modeling of Cohesive Sediment Transport in Estuary and Continental Shelf Tom T.-J. Hsu,Assistant Professor Civil & Coastal Engineering University of Florida
Wright & Nittrouer , Estuaries. Fate of Terrestrial Sediment in the Coastal Ocean • Initial Deposition 2. Negatively buoyant plume: Occurrence of hyperpycnal flow in Taiwan’s rivers: • 1. Positively buoyant plume: • Stable density stratification. • Flocculation process. • Estuarine Turbidity Maximum (ETM); • frontal trapping. • Resuspension Dadson et al. 2005, JGR-Earth Surf.
Need detailed field measurements and theoretical/numerical modeling on boundary layer flow and sediment transport dynamics near the bed. ~10m ~20cm Motivation Large-scale coastal models, e.g.Delft3D, Mike21, NearCoM, NOPP-CSTM, … and many others, require accurate parameterization on wave boundary layer processes. NOPP-CSTM (Community Sediment Transport Model): NOPP-CTSM, figure provided by Dr. John Warner (USGS) NearCoM, figure provided by Dr. Fengyan Shi (U Delaware).
Eel Shelf slope ≈ 1/200 Wave-supported gravity-driven mudflows Overview: a recent review paper Wright & Friedrichs (2006), Cont. Shelf Res. e.g., STRATAFORM at Eel river: Traykovski et al. , Cont. Shelf Res.
z dilute turbulent suspension < o lutocline mobile fluid mud o < < gel Consolidating bed > gel ~10g/l ~100g/l Fluid Mud Modeling Develop a 1DV numerical modeling framework for fluid mud transport based on Fast Equilibrium Eulerian Approximation (Ferry & Balachandar 1994) to the Eulerian Two-phase formulation – Mixture Approach • Existence of fluid mud: Hsu et al. (2007).turbulence-sediment interaction, downslope gravity-driven mudflow. • Floc properties and floc dynamics: Winterwerp (1998), Khelifa & Hill (2006) • Erodibility (Type I erosion): critical bottom stress depends on cumulative eroded mass, which is due to consolidation. • Rheological stress: e.g., Bingham-plastic. Wave-mud interaction. • Directly resolve bed consolidation and fluidization (e.g., Gibson et al. 1967). Evolution of floc aggregate structure. Winterwerp & van Kesteren (2004)
z y, along-shelf α=0.002 x, cross-shelf Sherwood et al.  Wave-supported Gravity-driven Mudflows Fluid mud at Po Prodelta EUROSTRATAFORM: Traykovski, Wiberg and Geyer , Cont. Shelf Res. Case 1: Moderate Concentration Event Hydrodynamic Condition: r.m.s. wave velocity=0.52 m/s, weak currents Floc Properties: D=24 mµ,a = 1440 kg/m3, nf =2.26 Erodibility: Type I erosion with Stevens et al. (2007): Rheology: OFF Floc dynamics: OFF EUROSTRATAFORM data obtained in collaboration with Peter Traykovski.
Case 2: High Concentration Event Hydrodynamic Condition: r.m.s. wave velocity=0.51 m/s, weak currents Floc Properties: D=21 mµ,a = 1440 kg/m3, nf =2.2 Erodibility: Type I erosion with Stevens et al. (2007): Rheology: ON Bingham rheology with Floc dynamics: OFF Large erodibility and rheological stress are required to model the observed wave-supported gravity-driven mudflows
Summary - Wave-supported Gravity-driven Mudflows • Based on our numerical model study, we find: • critical mechanisms controlling wave-supported gravity-driven mud flows are: • Total amount of available unconsolidated mud • Turbulent-sediment interaction (density stratification) • Floc properties (fractal dimension) • Rheology • detailed intra-wave time-dependent erodibility and floc dynamics are of less importance • need concurrent measurements on floc properties and rheology