Abstract

Numerical models can complement observations in investigations of marine sediment transport and depositional processes. A coupled hydrodynamic and sediment transport model was implemented for the Waipaoa River continental shelf offshore of the North Island of New Zealand, to complement a 13-month field campaign that collected seabed and hydrodynamic measurements. This paper described the formulations used within the model, and analyzed the sensitivity of sediment flux estimates to model nesting and seabed erodibility. Calculations were based on the Regional Ocean Modeling System—Community Sediment Transport Modeling System (ROMS-CSTMS), a primitive equation model using a finite difference solution to the equations for momentum and water mass conservation, and transport of salinity, temperature, and multiple classes of suspended sediment. The three-dimensional model resolved the complex bathymetry, bottom boundary layer, and river plume that impact sediment dispersal on this shelf, and accounted for processes including fluvial input, winds, waves, tides, and sediment resuspension. Nesting within a larger-scale, lower resolution hydrodynamic model stabilized model behavior during river floods and allowed large-scale shelf currents to impact sediment dispersal. To better represent observations showing that sediment erodibility decreased away from the river mouth, the seabed erosion rate parameter was reduced with water depth. This allowed the model to account for the observed spatial pattern of erodibility, though the model held the critical shear stress for erosion constant. Although the model neglected consolidation and swelling processes, use of a spatially-varying erodibility parameter significantly increased export of fluvial sediment from Poverty Bay to deeper areas of the shelf.

Highlights

  • This paper describes the implementation of the Regional Ocean Modeling System (ROMS)-Community SedimentTransport Modeling System (CSTMS) numerical model for the Waipaoa continental shelf and examines the sensitivity of sediment flux estimates to model nesting and seabed erodibility parameterizations

  • For panels (e)–(i), black lines indicate model estimates made for the grid cell nearest the tripod site, and grey lines indicate acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) observations provided by Hale, R. and Ogston, A. (University of Washington; [30]) from the tripod at 40 m water depth

  • The model underestimated peak water speeds at the three tripod locations, it replicated the spatial patterns of the tripod-observed time- and depth-averaged current speeds, which increased from the depocenter to the shallow site to the deep tripod (Figure 8)

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Summary

Sediment Transport Models

Field experiments carry a high cost and are hampered by difficulties of observing water column sediment fluxes during energetic conditions such as floods and storms, except at discrete points served by deployed instruments. Numerical models of continental shelf sediment transport have specified conditions along open boundaries based on estimates of coastal currents, temperature, and salinity from larger-scale, lower resolution models [14,15]. Like these examples, we build on previous efforts by nesting a finer-scale grid within a larger-scale hydrodynamic model, thereby accounting for larger-scale circulation patterns. We developed a simpler parameterization that modified the erosion rate parameter to account for spatial variations in erodibility, based on seabed microcosm erodibility experiments (see section 3.6)

Study Site
Objective
Hydrodynamic Model and Numerical Schemes
Background salinity psu
Surface Boundary Formulation
Bottom Boundary Layer Formulation
Seabed Model
Open Boundary Conditions
Model Grid Construction and Bathymetry
Atmospheric Forcing and Waves
River Discharge
Sediment Characteristics
Sensitivity Tests
Model Evaluation
Model Nesting
Sediment Erodibility
Computational Concerns
Summary

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