A three-dimensional hydrodynamic and sediment transport model of the mouth of the Columbia River (MCR) was constructed using the Delft3D4 (D3D) modeling suite (Deltares, 2021) to simulate water levels, flow, waves, and sediment transport for the time period of September 22, 2020 to March 10, 2021. The long time period and high spatial resolution required for this study necessitated a nested modeling scheme to reduce computational expense. The nested detail model domain consists of a structured, orthogonal, curvilinear, grid that covers an area of 37 km along shore and 33 km cross shore centered on the MCR. The grid dimensions are 140 by 246 cells with resolution varying between 26 m and 1.2 km. Ten equally spaced vertical sigma layers were used to simulate 3D effects within the model domain. The grid was aligned with coastal engineering structures, including the three primary stone jetties, as well as several training dikes along the north side of the navigation channel. Flow through the structures was limited in the model by using thin dams (no transmission) or dry points. The open boundaries in the detailed model were prescribed as a time-series of Riemann invariants and salinity values for each vertical layer were derived from an overall model domain that extended roughly 150 and 100 km to the north and south of the MCR, respectively. The bathymetry for the overall, detailed, and wave grids was derived from recent data sets collected by the USGS, NOAA, and USACE between 2004 and 2020 as described in Stevens and others (2020). The source bathymetric data were converted to a common horizontal datum (NAD83) and to the land-based North American Vertical Datum of 1988 (NAVD88), then projected into the Cartesian UTM Zone 10 coordinate system (meters). Deep areas associated with the Astoria submarine canyon were removed from the model bathymetry to improve stability along the oceanic boundaries. Oceanic boundaries of the overall model were forced using astronomic tidal constituents derived from the TPXO 7.2 global tide model (Egbert and Erofeeva, 2002). A vertical offset of 1.15 m (positive values are up) derived from NOAA VDatum (version 3.2; https://vdatum.noaa.gov/
) was applied at the oceanic boundary to account for the difference between local mean sea level and NAVD88. Oceanic subtidal variations were imposed at the oceanic open boundary as a time-varying correction to the astronomical tides. The subtidal time-series was derived from observations of water levels at NOAA stations 9440910 (Toke Point, WA), 9437540 (Garibaldi, OR), and 9440581 (Cape Disappointment, WA). Water-level time-series from the three stations were low-pass-filtered using a 66-hr cutoff to remove fluctuations at tidal frequencies. The low-pass-filtered values from the stations were highly correlated, and an average was applied to the oceanic model boundaries. The landward boundary of the overall model was forced with a time-series of river discharge measured at 30-minute intervals at USGS gauge 14246900 (http://waterdata.usgs.gov/usa/nwis/uv?site_no=14246900)
. The overall model’s oceanic and fluvial boundaries were prescribed constant salinity values of 33 and 0 practical salinity units (psu), respectively. The effects of temperature variations on circulation were neglected in the present model application.