Projections of compound floodwater depths for the lower Nooksack River and delta, western Washington State

Generated a numerical unstructured grid consisting of a set of one-dimensional (1D) nodes as cross-channel transects along the mainstem river and distributary channels of the lower Nooksack River and two-dimensional horizontal (2DH) nodes across the coastal embayments and floodplains using Delft3D Flexible Mesh (Delft3D FM). Numerical grid node spacing ranges between 6 and 50 m with highest resolution in areas of greatest concern to flood model outputs, namely along the floodplain in the vicinity of greatest flood exposure and identified mitigation alternatives for assessment. The grid nodes and transects were attributed with elevations derived from a seamless topographic and bathymetric digital elevation model (Tyler and others, 2020). Modifications of model grid elevations for identified flood mitigation/habitat restoration alternatives were made by prescribing channels across the floodplain following specifications in Whatcom County Public Works (2017) and by raising mainstem channel stream bed elevations to represent recent observed decadal scale bed aggradation patterns (Anderson and Grossman, 2017; Anderson and others, 2019).

Generated boundary condition forcing for the marine and freshwater boundaries of the model. The marine boundary was prescribed tidal harmonics and nontidal residual values (anomalies) derived from the 27-year observational record at the National Oceanic and Atmospheric Administration (NOAA) Cherry Point tide gage (station 9449424). The freshwater boundary was forced with measured stream discharge at the USGS stream gage Nooksack River at Ferndale (Station 12213100) for historical flows. Modifications for future conditions included elevating sea level position following Miller and others (2018) and increasing stream discharge following the future hydrologic projections of Hamlet and others (2013) for mean and high change scenarios described in Grossman and others (2023).

Ran model simulations of existing conditions for validation and calibration period of 2017 for comparison to measured water levels in Anderson and others (2019) and for two historical floods of Jan 8, 2009, and Feb 2, 2020, to test model performance for extreme events (Grossman and others, 2023). Ran model simulations of all individual and combined effects of existing and future 2040 and 2080s changes in sea level and stream flooding, flood mitigation/habitat restoration alternatives, and bed sediment aggradation scenarios.

Generated floodwater level outputs, and in turn, calculated water depth by subtracting CoNED land elevations (Tyler and others, 2020) for all individual and combined effects of sea level-tide-storm surge and streamflows, flood mitigation/habitat restoration alternatives, and bed sediment aggradation scenarios in the form of geoTIFF. Final geoTIFFs were transformed from unstructured grids to uniform grids with 5 meter cell size and organized by the two historical floods (2009 and 2020) for existing conditions, 2040s and 2080s.