PNNL

Abstract

For this assessment, researchers at the Pacific Northwest National Laboratory (PNNL) developed a three-dimensional hydrodynamic model of the Skagit River delta region based on a prior version of the model developed at PNNL. The model is based on the Finite Volume Community Ocean Model (FVCOM), which solves the three-dimensional momentum, continuity, temperature, salinity, and density equations in an integral form by computing fluxes between non-overlapping, horizontal, and triangular control volumes. The new unstructured grid is the highest resolution yet produced by the PNNL modeling group for the Skagit River delta; it consists of 131,471 elements that vary in size from 400 meters (1,312 feet) to less than 10 meters (33 feet). Bathymetry was updated with recent Lidar and boat-based surveys available from sources including the U.S. Geological Survey (USGS) and the U.S. Army Corps of Engineers (USACE). Skagit River flow was determined by a USGS gauge near Mount Vernon and the flow distribution between North and South Forks of the river were calibrated with five short-term stage gauges maintained by WDFW. The model was forced with tides and resulting outputs were validated against the WDFW and SRSC monitoring stations. Simulations were conducted over a 7-month period from November 2014 through May 2015, which coincided with the WDFW and SRSC stream gauge deployment and encompassed several 2-year floods and a majority of the fish outmigration period. A total of 7 model simulations were planned to assess 22 of the 23 potential projects in the Skagit River delta. Projects were grouped so that the effects of each project would be isolated and quantifiable. This allowed small projects to be grouped, while some very large projects were simulated as stand-alone cases. Each simulation generated a set of deliverables including inundation area calculations, cumulative frequency plots for water surface elevation, distribution of water depths across the project site, stage-discharge curves, and GIS plots for depth of inundation, change in water surface elevation, change in bed shear stress, and change in salinity. Following this initial assessment, the SHDM Team identified a group of selected projects for a simulation to assess cumulative impacts. Cumulative effects are an important and often overlooked element in restoration planning, as restored area can alter the tidal prism or hydraulics in a way that changes the viability of other projects. Avon-Swinomish Bypass and NF Left Bank Levee Setback A were excluded because they had significantly high levels of impact when compared to other projects. Two more simulations were then conducted to assess the response of restoration projects to future climate change. The modeled future conditions included 0.57 m (1.87 ft) of sea level rise and a 2080 Skagit River hydrograph corresponding to the moderate emissions scenario (A1B-IPCC). This addresses questions about the longevity of restoration projects.