Understanding Risk for Channel Rehabilitation: Considerations for Installing Large Woody Material in Front Range Canyon Corridors

Jun 15, 2024

Contributed by Will deRosset, PE, Ayres Inc.

Channel Rehabilitation

Many of our channel rehabilitation projects are in remote locations, with little human built infrastructure and minor regulatory scrutiny. In these environments, we have a relatively free hand to envision uplift, reconnect a river system to its floodplain, and to seed beneficial change. We’re primarily limited by the resources we bring to bear and the coalition of willing stakeholders we can bring to the project. The water quality, habitat, and hydrologic ecosystem benefits are the primary goal.

But we don’t limit our work to those remote environments. We often undertake river rehabilitation in service to anthropogenic goods— the project is part of a larger water supply infrastructure, roadway protection, or flood mitigation, for example—or in a more-developed environment.

In this case, many of our clients look at river rehabilitation as a co-benefit rather than a primary goal.  And our developed rivers and floodplains are heavily regulated spaces, with regulatory approval processes that were developed for “traditional” hard infrastructure and land development.

Rehabilitation Diagram

Rehabilitation becomes an item with uncertain risk, particularly if the desired uplift reintroduces absent natural processes appropriate to a river corridor, such as large woody material (LWM) reintroduction, unconstrained lateral migration, and channel avulsion. This risk is shared between the designer, the builder, and the owner. A formal risk assessment that is performed internally and updated/expanded as the project progresses and new stakeholders get engaged can help both understand and mitigate project risk. The results may alter the goods we can provide with the rehabilitation!

For example, as part of CDOT’s US 34 permanent repair following the 2013 flood event, we (Muller and Ayres) worked with CDOT, USFS, Larimer County, CPW, CWCB, private landowners, and Ellen Wohl at CSU to verify appropriate wood dynamics for the Big Thompson River canyon corridor and to incorporate LWM in the river rehabilitation plans. The OTAK/Stillwater river rehabilitation construction observation team also advocated for appropriate LWM application for the permanent repair. Kiewit (the contractor) and Flywater (the river rehabilitation subcontractor) were all instrumental in its successful installation.

CDOT, the project owner, was wary but open to piloting wood installation on this project. There were three major sources of risk to LWM addition: downstream infrastructure, regulatory limitations, and material availability.

Downstream Infrastructure

The Big Thompson Canyon has many communities with privately-owned access from US 34. The Big Thompson River has twice severely damaged undersized bridge crossings in recent memory, and in 2013, many of these bridges were damaged or destroyed by bedload impact, debris, and overtopping/flanking. US 34 The river flanking local access bridges destroyed many segments of US 34.

CDOT Flood
Photo courtesy of CDOT

CDOT had no appetite to increase risk to these crossings and initially viewed LWM as a potential source of debris load. CDOT mitigated this major infrastructure risk as part of permanent repair. They collaborated with Larimer County to replace six critical access bridges with clear-span structures that would pass the predicted 100-year flow with adequate freeboard, limiting backwater and allowing free passage of debris. A clear-span structure crosses the channel with no piers in the water. Freeboard is the clearance between the design water surface elevation and the underside of the bridge. Consequently, in reaches upstream of these new structures, designers were able to incorporate wood, and would limit its in-stream application to limit future damage potential to infrastructure.

Big Thompson Metrics

Regulatory Limitations

The Big Thompson Canyon has an established regulatory floodway, and our work would necessarily alter the landscape in that floodway. Causing a water surface rise in the floodway was not permissible. We mitigated this regulatory risk in two ways.

First, given the extreme disturbance that the river experienced in 2013 and subsequent channel modifications performed as part of emergency US 34 repair 2013-2014, CDOT and CWCB committed to re-map the corridor. We also negotiated formal agreements with FEMA and CWCB to treat channel rehabilitation features (including LWM placements) as roughness elements rather than as fill or obstructions that must be individually modeled. We designed rehabilitation features that could be naturally mobilized and re-worked by the Big Thompson River’s fluvial processes in low to moderate-recurrence interval events.

Second, we could do no harm: we could not cause increases in flooding on remaining insurable structures. Consequently, the designs included inset floodplain development and other form-based restoration features to improve floodplain connectivity and still operate in a “no rise” context.

This required extensive excavation, fluvial hazard review, and thorough 1-d and 2-d hydraulic modeling to verify that we were not adversely impacting existing structures.

Material Availability

Finally, emergency repairs in the US 34 corridor included “debris removal.” Much of this debris was anthropogenic—remnants of cars, construction debris, propane tanks, and the like transported by the flood. Unfortunately, it also included most of the LWM recruited and deposited in the corridor by the 2013 flooding. These materials were removed and trucked off-site. That left us with very little wood to work with and limited our LWM options. Our mitigation: re-use all the suitable trees harvested as part of the permanent repair. This also reduced landfill and haul costs.

Tree Debris

Summary

Active risk assessment can help reduce risks for the owner, designer, and the contractor. It can also help optimize our designs, maximizing the lift we can achieve as part of river rehabilitation in built environments or as co-benefits to infrastructure projects. Also, this process improves project outcomes. In the case of US 34, its success allowed CDOT to entertain “softer” process interventions, including in-channel LWM placement and native stone embankment protection, in subsequent corridor repair projects, such as at CO 7 along lower St. Vrain Creek. Washington State DOT regularly incorporates LWM in their projects and includes a risk assessment template as part of their hydraulics manual.

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