The CalWood Fire began on October 17, 2020, and burned approximately 10,112 acres of private, county, state, and federal land within Boulder County, Colorado. The fire left approximately 46% of the burn area with “moderate to high soil burn severity,” leading to post-fire flooding, debris flow, and erosion concerns. 

Trihydro Corporation and Na Ali’i Consulting & Sales, LLC supported Boulder County Parks and Open Space (BCPOS) in designing and constructing mitigation measures to capture potential debris and mudflows in areas impacted by the CalWood Fire. The project was funded primarily through the Natural Resources Conservation Service (NRCS) Emergency Watershed Protection (EWP) Program, with BCPOS entering into a cooperative agreement with the NRCS to provide emergency stabilization and infrastructure protection to areas most at-risk due to conditions created by the wildfire.

The initial phase of the work included reviewing existing pre- and post-fire datasets to identify data required to support analysis and modeling efforts. Identified data included remote sensing and satellite-based imagery, land cover and vegetation mapping, digital elevation models, historical streamflow and precipitation, and soils information. 

Prior to beginning the design process, the project team conducted a series of site assessments to evaluate mitigation measure types and locations. One objective included qualitatively validating results of the initial hazard assessment data review. The project team also performed an assessment to identify locations where detailed modeling was required to inform mitigation measure designs.

The team performed modeling analyses to yield the required design inputs at the appropriate spatial and temporal scales. The team used debris flow modeling results to evaluate mitigation alternatives and size the final designs. Passive and active mitigation measures were considered, which included open debris flow control structures (e.g., rock check dams, lateral walls, deflection walls, berms and barriers), closed control structures (e.g., debris racks, storage basins), and other sediment control structures (e.g., beaver dam analogs). NRCS reviewed and approved the final designs. In addition to the final plan set and specifications, the team developed an operations and maintenance plan, a temporary construction stormwater pollution control plan, and a final design report. The team also supported BCPOS with local and federal permitting processes. 

The team worked collaboratively with BCPOS and other stakeholders throughout the project. Based on client and stakeholder feedback, the team adapted initial designs to achieve multiple objectives (e.g., debris and sediment control, ecological and habitat needs, access and maintenance requirements). In addition, the team made design adjustments during construction to better match the natural terrain and maximize the use of on-site, native materials. In the lower Geer Creek watershed, the team designed and constructed a floodplain restoration area to reduce peak discharges and protect downstream infrastructure along Left Hand Canyon. Returning the area to its natural setting, including native wetlands vegetation, provides long-term hydrologic and wildlife benefits.  

Outcomes

• Adaptive design that targeted small, strategically located debris/sediment control structures that blended into the natural environment
• Eliminated the need for large, obtrusive, and more environmentally degrading structures downstream
• New flash flood and debris flow control structures mitigate peak flows and help prevent the movement of harmful debris and sediment
• 2022 American Public Works Association Colorado Award