Author: Julie Ash
The Colorado Stream Restoration Network’s (CSRN) workshop on Designing for Resiliency and Aquatic Organism Passage (AOP) at Road-Stream Crossings, led by US Forest Service instructors Dan Cenderelli (USFS National Stream and Aquatic Ecology Center) and Mark Weinhold (USFS Hydrologist) was CSRN’s 9th workshop since its inception in 2014, following the September 2013 flood.
This workshop was targeted to culvert designers and proponents of projects with culvert design. The workshop overviewed the USFS Stream Simulation approach to culvert design, which has been developed over the past 10 years to reduce problems with typical crossing design by applying geomorphic, engineering, and biologic considerations. The goal of the Stream Simulation approach is to improve crossing design to increase resiliency, reduce risk, and improve stream health. The path to meeting this goal is to understand the transportation of water, sediments, wood, and organisms over a broad range of flows and to apply this understanding to design crossings that reduce damage to transportation infrastructure and improve stream health. USFS experts overviewed details of this design approach and shared information on recent performance of crossings designed under this method. This workshop was a short overview of the method and CSRN is working with the USFS on the potential to offer the week-long full workshop on Designing for Resiliency in 2017.
The target of the Stream Simulation method is to make crossings as unnoticeable as possible by establishing natural dimensions of the channel to the maximum extent practicable for the project and by selecting optimal culvert types/configurations and avoiding rigid culvert bottoms. For projects in degraded reaches, where the reference reach approach is inappropriate for determining dimensions, the target is only to match existing upstream and downstream conditions. While this may not restore natural channel dimensions, the crossing will perform as best possible in its setting because it as unnoticeable as possible to the water, sediment, wood, and organisms passing through. Restoring natural channel dimensions to a degraded reach is a larger-scale effort that typically cannot be undertaken as part of a small road-stream crossing project. The complexity and interrelated nature of dynamic stream systems preclude “band-aid” fixes at a localized point, such as a crossing, and using a reference reach in this situation has high potential for failure. Workshop discussions considered the use of the term “representative reach,” rather than reference reach, to describe the application of conditions in upstream and downstream reaches for purposes of the stream simulation approach without causing confusion with the reference reach approach.
Problems with traditional culvert design are commonly linked to gradients that are too steep at low flow with very rapid velocities during high flows. The first parameter for selecting appropriate representative reaches is gradient (a driver of geomorphic process), and vicinity to the crossing is also important. Secondary considerations include confinement (clues to geomorphic processes) and planform (e.g., riffle vs bend, etc.), among other factors. The challenge that cannot be met by traditional culvert design is to provide velocity-depth combinations for a wide range of aquatic species over a broad range of flows, which is necessary because aquatic organisms move at different times, combining to effectively constant movement. No hydraulic solution can accommodate the full range; the best resilient solution is to match the natural system.
USFS experts stressed the importance of higher-level evaluations before digging into design analyses. USFS begins each crossing project with an initial assessment that includes whether the road at this location can go away, such that a crossing is not needed (maximum risk reduction). If not, can the road be relocated or realigned for improved interaction with the stream? Relocating a crossing site can greatly reduce costs for construction and long-term repair and maintenance. Resource values and risk-based planning drive the pre-design evaluations. Consideration of watershed- and site-scale risk factors at the beginning and throughout the design process maximizes long-term culvert performance. Two examples are the importance of understanding floodplain flows and headcut potential. Simple solutions, if these risks are considered, include inclusion of floodplain flow conveyance in crossing design and consideration of inflection point location relative to the culvert. Knowledge of source versus transport versus response reaches is additionally key prior to starting design analyses because successful channel design differs for these reaches in order to accommodate different fluvial processes. The USFS instructors presented a helpful graph of the Montgomery and Buffington classification system with Rosgen classifications overlain along the source-transport-response continuum.
Presentation of recent performance of crossings designed using the Stream Simulation method shows substantially reduced damage during large events, reduced maintenance costs, and increased culvert service life, often close to the longer service life of (substantially more expensive) bridges. In many cases, the higher construction cost of upsizing and better culvert design directed by the Stream Simulation method is offset by cost savings (e.g., reduced counter weight depths due to lowered exit velocities).
Applying the Stream Simulation method solves problems that typical crossing design cannot by applying geomorphic, engineering, and biologic principles. The method simulates the natural channel and minimizes the hydraulic effect of the culvert using a combination of reference reach (or representative reach) and geomorphic design. Understanding and reading the stream, then incorporating that understanding into design increases resiliency, reduces risk, lowers damage, repair, and maintenance costs, and improves stream health.
Streams have something to tell us…we only need to listen. When we do, we see reduced damage to infrastructure during large flood events and we benefit from healthier streams every day of the year.