Khangaonkar Middle Skagit Floodplain

Hydrodynamic Modeling for Assessment of Floodplain Restoration Potential on the Middle Skagit River

SRFB Grant through Skagit Watershed Council: $55,000, 2010

Background and motivation: Due to the construction of dikes and flood-protection levees, natural floodplain functions in the Skagit River have been lost. The Skagit Watershed Council led this effort to identify and prioritize opportunities to restore and protect floodplain functions for salmonids in the Middle Skagit River. Hydromodification structures along the banks of the river and in the floodplain were assessed. To better understand the correlation between removal of selected hydromodifications and the benefit of restoring natural river conditions and habitat, a modeling-based approach was proposed. This would help move the goal of identifying habitat availability in the middle reach forward in a timely way.

Model Grid of Middle Skagit River Floodplain

Objective: The overall objective of this project was to develop a model of the Middle Skagit River to examine restoration potential in the Skagit River floodplain. This modeling effort would allow mapping and quantifying by reach, the habitat available at various flows significant to juvenile Chinook. The work involved extending the existing model from Sedro Woolley, WA, upstream past its current boundary at Concrete, WA, to a new location past the confluence with the Sauk River. The model was validated using the additional velocity and cross-section data provided by Seattle City Light and the revised channel cross-section data provided by US Army Corps of Engineers.

Project Team: The project team consisted of Skagit Watershed Council, Skagit River System Cooperative, Pacific Northwest National Laboratory, and Seattle City Light.

Simulated inundation during a flood event.

Project Highlights: When the model was applied to the Middle Skagit River floodplain system, it demonstrated that an unstructured-grid, coastal ocean model with a wetting and drying capability can be used to simulate the 3-D hydrodynamics in the nearshore and floodplain regimes. Such a model would also provide crucial habitat parameters, such as inundation frequency and duration, flooding areas, velocity, water depth, and salinity distributions. Specific highlights were as follows:

  • The Middle Skagit floodplain model was extended to the confluence of the Skagit and Sauk rivers and validated against measured data.
  • The model was applied to three flood flows for existing and alternative (without hydro-mod) conditions.
  • The results, which consist of velocity and inundation information, were used in the development of habitat maps under existing and projected alternative conditions for different flood flow scenarios.