Confluence of Stephens Creek and the Willamette River: Vigil-Agrimis, Inc. (VAI) provided City of Portland Bureau of Environmental Services (BES) with design services for an aquatic and riparian habitat enhancement project at the confluence of Lower Stephens Creek and the Willamette River.  The proposed enhancement elements include the removal of an abandoned Combined Sewer Overflow (CSO) pipe that runs parallel to Stephens Creek and the addition of complex Large Wood (LW) structures along Stephens Creek and the surrounding banks of the Willamette River.  The LW structures were designed to be secure by embedment only, without the use of cables or anchors.  The LW structures were designed to withstand prolonged inundation and to provide habitat value at a wide range of water levels.

The VAI team’s scope of work included site grading, engineering analysis, restoration design, hydraulic analysis, floodplain and habitat enhancement, bank stabilization, and preparation of detailed plans and specifications for bidding purposes.

Site Grading:  The VAI design team modified the site contour design to enhance an historic side channel to improve its habitat value and increase frequency of inundation.  The design team lessened the steep slopes on some stream banks impacted by urbanization to reduce erosion and scour.  The design team utilized AutoCAD software to prepare grading design/construction drawings.

Habitat/Wood Analysis:  The VAI team added large wood to enhance habitat in many areas.  They completed detailed buoyancy and stabilization calculations to ensure that wood elements would remain in place.  They completed detailed analysis of forces to ensure that the gravity and friction forces that hold the wood in place would counter the buoyancy and momentum forces that would move the wood.

Floodplain Analysis:  This project was constructed in the floodplain of the Willamette River.  The design team reviewed the Willamette River HEC-RAS model developed by the City of Portland to determine water surface elevations for design purposes.  The intent of the project was to create additional floodplain and off-channel fish habitat at the confluence of Stephens Creek through limited excavation, and removal of old pipe infrastructure.

An exceptionally large rain event occurred during construction, creating unexpected challenges to which VAI responded immediately in collaboration with City staff to provide additional design services—resulting in a very successful project.  Construction of the Lower Stephens Creek project was completed in October 2008.

The Upper Stephens Creek Ecosystem Restoration and Habitat Enhancement Project repaired a leaking trunk sewer and restored habitat throughout the project reach (approximately 4,300 feet).  VAI developed design alternatives and a fluvial geomorphic assessment that identified critical areas where an existing 36-inch sanitary sewer main had become exposed, and also identified areas for potential stream and habitat enhancement.  VAI completed the final design for this project which included floodplain restoration, bio-engineered bank stabilization, streambed gravel replacement and armored bank stabilization to protect the exposed sewer pipe.  VAI assisted Bureau of BES staff during the construction phase of this project which was completed during the summer of 2008.