Hydrology Monitoring for Floodplain Enhancements in the Tualatin Basin

The PCC Rock Creek Floodplain Enhancement Project in the upper Tualatin River Basin of northwest Oregon aims to improve eco-hydrological functions of a floodplain-wetland complex while mitigating for hydromodification effects from rapid urban growth upstream. Its overall goal has been to benefit flora and fauna by increasing soil inundation through prolonging residence time of surface-water and groundwater storage. 

On behalf of Clean Water Services, Stillwater developed and implemented a multi-year, pre- and post-implementation surface-water and groundwater monitoring program that helped inform floodplain enhancement designs and gage project effectiveness. Monitoring included data collected via an array of groundwater piezometers and streamflow gages, mapping of floodplain inundation, and inventorying of beaver activity. For inundation mapping, Stillwater utilizes our unmanned aircraft system (UAS) equipment to aid in the characterization of inundation perimeters, runoff pathways, beaver dams, and as-built conditions of restoration features.

Pre-implementation monitoring findings significantly refined the restoration design process by simplifying engineering challenges and, ultimately, reduced construction costs through identifying existing natural processes while dealing with the confounding influences of beaver activity and hydromodification. Post-implementation monitoring results indicate improved floodplain connectivity, a higher water-table in summer, value of integrating beaver into the design, and resultant changes in vegetation-community structure, all of which confirm the project is achieving its goal of providing eco-hydrological uplift.

 

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Restoration and construction activities occurred in 2017 when native plants, logjams, and snags were installed at the Project site.

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