With goals to improve public safety, reduce maintenance frequency, increase drainage system reliability, restore aquatic and riparian habitat, and improve water quality, the City of Issaquah sought to develop a surface and stormwater master plan (SSWMP). The plan develops a prioritized list of stormwater infrastructure and habitat restoration projects to be implemented within a 20-year timeframe, simultaneously helping the city many its grown while remaining compliant with NPDES permitting. The Otak team worked with the city to develop the plan and subsequent projects that would address the impacts of impaired surface water quality, aquatic and riparian habitat degradation, increased flooding, and aging infrastructure.
Expertise: Environmental
Cedar Crossing Restoration
In reconnecting Johnson Creek to its historic floodplain, the stream restoration plan provided flood storage for peak flow attenuation, and improved riverine and wetland habitats. Otak led the overall project management, geomorphic assessment, monitoring data interpretation, hydrologic analysis and hydraulic modeling, as well as the comprehensive restoration design.
Enhanced Habitats for Endangered Species
Working with the City of Portland Bureau of Environmental Services (BES), the Cedar Crossing Restoration enhanced native riparian vegetation and tree canopy, creating high-flow refugia for listed Endangered Species Act (ESA) species, particularly several ESA-listed salmonids. Overarching project elements involved demolition of structures, management of contaminated soil as well as a number of complex design, permitting and site preparation considerations. The stream restoration design included in-stream channel structures, bank stabilization, large woody debris structures and the removal of nearly 2,000 feet of a century-old WPA rock wall that lined both banks of the stream.
North Creek Channel Relocation
In restoring the natural morphology of North Creek, the channel was relocated to reconnected 3,800 feet of the stream to its associated 60 acres of wetlands. The restoration of native plant communities in the floodplain was also a focus in improving the habitat for both aquatic and terrestrial wildlife species.
Restoration to Natural State for An Improved Habitat and Open Space
As part of larger improvements to the campuses of University of Washington-Bothell and Cascade College (CC), the North Creek Channel relocation was designed alongside a regional trail and a boardwalk. The developed public open space features interpretive exhibits that extend out into the restored floodplain. In restoring this lowland portion of the site, the meander length, curve radius, bankfull depth, and width-to-depth ratio of North Creek were returned to its natural status. The Otak team based this design on historical information, regional references for similar streams, reference reaches both onsite and upstream.
Springwater Wetlands Restoration
The Springwater Wetlands Restoration project reconnects and restores the Johnson Creek floodplain while managing flooding by expanding on existing, historic wetlands. Similar to the adjacent Foster Floodplain Natural Area that preceded this work, the project also restores the wetland as a wildlife habitat and space for public access. Otak led the predesign research and worked iteratively with the City of Portland Bureau of Environmental Services (BES) and other stakeholders to meet a variety of goals throughout the permitting and design processes.
Enhancing a Wetland for Improved Habitat and Flood Mitigation
In addition to managing flooding through green stormwater infrastructure, the city aimed for broader benefits in restoring the Springwater Wetland, including the removal of contaminated fill and non-native species while enhancing habitat for native species such as the Red Legged Frog. An unsteady-state hydraulic model was used to evaluate flood storage capacity and develop hydraulic connections that convey flood flows to and from the wetlands. The modeling and hydrologic analysis was used to develop the Johnson Creek Restoration Plan that accounts for 10-year storm, 25-year flood, and 100-year flood events. Data gathered during modeling also informed the design of public access amenities, including the creative reuse of historic stonework as seating, new plaza areas, and architectural features throughout the site. Expanding the area’s use further, a dual-use trail system (that uses porous pavement to further reduce stormwater runoff) creates a safe, accessible connection to the Springwater Corridor Trail.
Spring Creek Pedestrian Bridge
Spring Creek Pedestrian Bridge is a 385-foot clear-span cable-stay structure over the Methow River. Its design incorporated historical context from the town with elements that reflect 19th Century steel construction. The main steel truss and towers were constructed of weathering steel and the deck planks were precast concrete while tower foundations were designed to accommodate significant scour potential. Otak was responsible for structural design, stream hydraulics, and construction staking in partnering with Mowat Construction for this $2.5 million design-build project.
Flood Reduction at South 180th Street
The flood reduction at South 180th aimed to address longstanding flooding issues in the area caused by a ditch with inadequate capacity. A critical areas assessment developed solutions focused on the goal of reducing flooding through flow control and additional conveyance capacity.
A Critical Area Assessment to Solve Persistent Flooding
Flooding at 180th would happen several times a year. An evaluation of flood risk to adjacent properties as well as capacity for the ditch and downstream drainage system delivered alternatives using both conventional or natural drainage systems. These alternatives considered factors such as construction cost, right-of-way availability, property acquisition, erosion in downstream ditches, and permitting requirements. Otak played a critical role in this project, providing project management, topographic survey, hydraulic and hydrologic modeling, and stakeholder engagement. The evaluation has the South 180th Flood Reduction project well-positioned to succeed in its mission to reduce flooding and protect the surrounding properties and community.
Smith and Bybee Natural Area Floodplain Management
An evaluation of options to manage hydrology in the Smith and Bybee Lakes complex led to the restoration of its connecting channel and subsequent mitigation of seepage found in an existing levee. The design adds capacity to the connection channel through excavation, allowing Metro to better manage water levels in the lakes. Otak led the hydraulic modeling of the system to compare the benefits derived from varying levels of channel excavation for the resulting restoration design as well as alternatives analysis of proposed remedies for the levee hazard mitigation design.
Levee Seepage Mitigation and a Restored Channel for Improved Hydrology Management
From the simulation of spring freshet floodplain functions to control of invasive species and habitat for migratory waterfowl, Metro manages water levels across the seasons in Smith and Bybee Lakes for a variety of objectives. A control structure was constructed in 2003 to aid in this management. However, the water levels in Smith Lake were not responding to control structure manipulations due to sedimentation and partial obstruction in the channel connecting Smith Lake to Bybee Lake. After comparing options, a design concept to excavate the channel and restore capacity was developed along with the construction documents. After construction, an inspection revealed seepage in the levee near the control structure and also identified burrowing animals which further increased a risk of levee failure. An alternatives analysis to resolve the issue led to a 200-foot-long sheet pile cutoff wall with segments on either side of the water control structure. With the piles driven 20-feet below ground surface, the design increases the sub-surface flow path to reduce seepage to minimal levels while providing a physical barrier that prevents animals from burrowing.
Miller Creek Restoration
By daylighting a stream that previously flowed almost entirely through a failing pipe network, endangering private property and downstream wetlands, the restoration of Miller Creek created a stable natural feature while supporting an adjacent trail along its embankments. This work followed an extensive planning effort in the area for redevelopment. Otak provided preliminary concepts, surveying, and permitting documentation, while also coordinating between multiple agencies and municipalities to see the project through construction.
Daylighting a Stream for Improved Stormwater and Habitat in a Redeveloped Subarea
With a focus on stormwater and habitat, the Miller Creek restoration utilizes a 162-acre subarea adjacent to the SeaTac Airport. The area’s proximity to Port of Seattle, as well as the Cities of SeaTac and Burien, required each municipality’s involvement, and their collaboration made this work possible. In addition to relocating 900 linear feet of stream to ground level, the design also added another 300 linear feet as part of its newly formed floodplain. Two fish passable culverts under Des Moines Memorial Drive and Miller Creek Trail further enhance the natural habitat for wildlife like salmon that spawn in the creek and contribute towards the broader goals of restoring the Puget Sound Watershed. Along with native vegetation along the streambank, the project also connects with multiple green stormwater facilities to treat stormwater runoff from approximately 50 acres of impervious surfaces, complementing continued redevelopment in the area.
Hess Creek Preserve
Creating more than 50 acres of natural habitat and public space, the restoration of Hess Creek Preserve protects the community from failing infrastructure while balancing human recreation with ecological health. Removal of two dams and invasive species combined with the addition of two bridges, unique earth art features, and a looped trail system are part of this Otak designed and managed project that celebrates the restored stream corridor.
Restoring a Natural Area that Blends Art, Ecology, and Education
Hess Creek is a tributary of the Willamette River that drains runoff from the slopes of the Chehalem Mountains. For the community of Newberg, Oregon, two small dams on the waterway presented an ongoing liability for property owners with the potential for failure. Plans to remove the dams and develop a publicly accessible park and natural habitat in their place took shape. The channels would also be re-graded to activate the natural floodplain and improve resilience during storm events. The design includes a looped trail system, bridges, and earth-themed art and educational installations. Restoration work began with the removal of invasive species and revegetation across 36 acres of riparian forest and reestablishing 10 acres of oak savannah, a once common native planting in the Willamette Valley that is now rare due to urbanization. These improved areas will support habitat for a variety of local birds, mammals, and aquatic organisms. This rare combination of private land, public access, and multi-agency support results in a community asset that blends ecology, education, and recreation.
Foster Floodplain Natural Area
In restoring 5,600 feet of stream bank and retrofitting seven stormwater outfalls, a flood-prone neighborhood was transformed into the Foster Floodplain Natural Area, benefiting both regional wildlife and the surrounding community. The large undertaking was completed in close collaboration with the City of Portland and also spurred the Springwater Wetlands restoration. Otak provided both the restoration and transportation design, including hydraulic modeling and topographic surveying while supporting public involvement, permitting, and construction.
Transforming a Flood-Prone Neighborhood into Habitat and a Public Asset
Centered on a reach of Johnson Creek, the very wide and complex nature of the Foster Floodplain posed unique challenges to managing flooding in for the area. The restoration design required a thorough understanding of the geomorphology and flooding dynamics in the area, developing an unsteady flow hydraulic model that properly evaluated the acre-feet of flood storage capacity available within the project footprint. Based on this modeling, design alternatives accounted for 10-year storm, 25-year flood, and 100-year flood events. That data also supported floodplain permitting as well as both Conditional Letter of Map Revision (CLOMR) and Letter of Map Revision (LOMR) applications. The excavation and grading combined with a hydraulic control structure maximizes flood storage in the area while the removal of existing bank armoring and addition of vegetated stormwater facilities contribute to improved water quality. Three of the stream’s outfalls would also be redirected as part of mitigation efforts and log jams were engineered to protect the stream banks from lateral migration toward major infrastructure while still allowing the channel to naturally adjust in other locations. From various fish to the Red-Legged Frog and Bald Eagle, the area provides improved habitat for a wide range of the region’s native species.