Projects & Programs
Nisqually National Wildlife Refuge Wetlands Habitat Management and Restoration Project, Washington
The Nisqually River estuary is located in south Puget Sound near Olympia, Washington. Approximately 3,000 acres of this delta between Interstate-5 and Puget Sound are protected by the United States Fish and Wildlife Service (USFWS) as a National Wildlife Refuge. About 1,000 acres of this refuge are enclosed within a dike system and are currently managed as a mosaic of freshwater wetlands and grasslands. During the past 20 years, management of this area has become increasingly difficult and habitat quality has declined. Weeds and reed canary grass, which limit access for birds and wildlife, have overrun the habitat inside the dike.
The general opinion of the scientists, engineers and wetland biologists who evaluated the status of the refuge was that the best alternative for restoring habitat quality was to remove or breach the dike system, restoring tidal and marine habitat to the refuge. USFWS, in collaboration with Ducks Unlimited, Inc., is planning and implementing the habitat restoration efforts.
PNNL staff conducted hydrodynamic and sediment transport modeling to support USFWS efforts using the model RMA-10 for hydrodynamics and RMA-11 for sediment transport. The models were applied simulating a mean tide, mean river flow conditionand the 1996 Nisqually River flood. Numerical modeling was used to evaluate the hydrodynamic characteristics of several alternative configurations where breaches in existing dikes and dike removal are proposed to allow tidal flushing.
The study showed that full tidal and marine environment restoration was feasible with breaches and dike removal options. Alternatives with dike removal along the western side of the refuge were successful in moving floodwaters off the refuge, thereby reducing flood effects. The highest levels of sedimentation occurred during the river flood event, primarily within dike enclosures in all alternatives. Average tidal and river flow conditions did not show excessive erosion at breach/removal locations. Salinity was shown to encroach into newly breached regions, returning the habitat to marine tidal conditions. A frequency analysis for water depth and salinity along a transect was performed to examine ecological succession when the wetlands are returned to estuarine tidal conditions.
Project Highlights:
- RMA-10 and RMA-11 models were used to conduct simulation of flow through a marsh with wetting and drying schemes
- Models showed clearly that proposed action would reduce flood impacts and restore marine and tidal environment
- Model results can be used to evaluate habitat changes when returning the site to estuarine tidal wetlands
- Models allowed design and selection of restoration actions.