Salish Sea water level hindcast simulations: 1985-2015

A two-dimensional hydrodynamic model of the Salish Sea constructed using the Delft3D Flexible Mesh (DFM) modeling suite (Deltares, 2020) was used to simulate water levels between October 1, 1984, and September 30, 2015. The model domain extends across the entire Salish Sea and Vancouver Island to ~180 km south of the entrance to the Strait of Juan de Fuca. Elevations and bathymetry for US waters were prescribed from Tyler and others (2020) and for Canadian water utilizing Carignan and others (2013). Seasonal salinity representing average autumn and winter condition were prescribed as an initial condition based on average October through March depth-profile temperature, salinity, and density measurements spanning 1999-2017 by the State of Washington Department of Ecology Marine Waters Monitoring Program (Washington Department of Ecology, 2022). Stream discharge was prescribed at the mouth of the 24 largest streams in Washington State (USGS, 2022) and the Fraser River in British Columbia (Environment and Natural Resources Canada, 2020). Oceanic boundaries were forced using astronomic tidal constituents derived from the satellite-derived FES 2012 global tide model (Lyard and others, 2006). Sea-level anomaly was prescribed on the marine boundary using HYbrid Coordinate Ocean Model (HYCOM) sea surface height data (Chassignet and others, 2007) for the period 1994-2015 and a sea-level anomaly predictor described in Grossman and others (XXXX) for the period 1985-1994 when HYCOM was unavailable. Hourly wind and atmospheric pressure fields from the 6-km dynamically downscaled Weather Research and Forecasting (WRF) of the North American Regional Reanalysis spanning the period 1985�2015 (Chen and others, 2018) were prescribed on the open boundary and to compute the SLA prescribed at the ocean boundary. Roughness values were calibrated to optimize tidal propagation.

Simulated water levels were compared against time-series measurements of observed water levels from NOAA tide gages throughout the study area. All water level observations were made or converted to a common vertical datum (NAVD88) and water level data were interpolated in time from the native measurement interval (between 6 and 15 minutes depending on site) to the 10-minute output interval of the model simulations. Bulk error metrics describing the comparisons between model and observations over the entire simulation time frame (October 1, 1984 to September 30, 2015) were calculated following Jolliff and others (2009). Total root-mean-square error (RMSE) was less than 15 cm for water levels at all of the measurement sites and described in detail in Grossman and others (XXXX).

Annual Maxima/Generalized Extreme Value (AM/GEV) and peak-over-threshold/Generalized Pareto Distribution (POT/GPD) analyses following Coles and others (2001) were conducted on the simulated 1985-2015 water level results to estimate water level extreme recurrence.