Maximizing the Benefit of Smaller Engineered Log Jams
Project ID: 5796
Principal Investigator: Sean Kimbrel
Research Topic: Sediment Management and River Restoration
Funded Fiscal Years: 2014
Keywords: large woody debris, engineered log jams, stability, liability, river restoration
Currently there is a technical gap for the guidance of maximizing the habitat benefits with smaller-sized ELJ as opposed to large ELJs. Typically it is assumed that larger wood material is better and little attempt is made to optimize the size of design materials. The following research questions address the issue of decreasing or optimizing the size of wood used in ELJs and the potential to buffer smaller potential habitat benefits by using more structures.
1. What body of knowledge (literature) is available documenting the efficacy of designing and implementing smaller (less number of members) ELJs as opposed to large, racked ELJs on smaller fluvial systems in terms of habitat benefit and reducing risk?
2. Of the documented cases of implementing smaller ELJs, what were the associated factors of stability/mobility, failure modes, and what was the fate and damages associated with smaller mobile wood? How do these damages compare to damages sustained with larger ELJs?
3. Can smaller ELJs be designed to maintain the same stability (e.g. through orientation or ballasting) as larger ELJs or to passively break up during mobilization to minimize downstream damages?
The goal of this research is to provide documentation/guidance on the design of smaller, less risky ELJs that also maximize habitat benefits. This information is beneficial for future planning, design, installation, and monitoring of ELJs for habitat restoration, which is applicable to several Reclamation habitat enhancement programs across the Western United States.
Need and Benefit
Using large wood structures for the purposes of reducing stream bank erosion, while at the same time adding in-stream habitat, is a fairly recent concept, dating back to 1995. The concept began with the observation that natural logjams not only introduce physical complexity to rivers, which creates productive fish habitat, but can also control the morphology and grade of fluvial systems. As opposed to the engineering rigidity of its predecessors such as rock barbs, spur dikes, and bulkheads, large wood structures are unique in that they are engineering structures that replicate the complexity found in the natural system. The structural complexity and hydraulic diversity associated with log jams provide ideal habitat for a variety of life stages and species of fish. Large wood in the channel provides cover for fish, slows water velocities, creates pool habitat, enhances hyporheic flow, provides nutrients for invertebrate populations, and gathers spawning gravels. These project features also provide shading for the river, which decreases summer maximum water temperatures. This technology is an evolving science as lessons learned from ongoing monitoring activities are continually being incorporated into design. Various federal and state agencies are advocating that Reclamation use these types of "soft" engineering approaches to meet endangered species requirements while maintaining water delivery needs.
An understanding of the importance of large wood in river systems has gained significant strength in the research and applied studies of eco-hydraulics within recent history. One major ecological benefit of the use of large wood in rivers is the improvement to floodplain connectivity and creation of habitat and cover for endangered fish species. These improvements to habitat may reduce in-stream flow requirements, thereby increasing the amount of water delivery to downstream water users. Design of large wood structures that can sustain habitat features and meet program objectives for water users are essential components of successful project implementations.
Large wood structures are being incorporated into project designs at a more frequent rate today than ever before. The Trinity River Restoration Program is incorporating 400-500 pieces of wood into the river annually. There are 22 large wood structures, each with several hundred pieces of wood, currently being implemented on the Elwha River. There is a habitat restoration project on the Entiat River that includes the placement of 13 large wood structures on a stretch of river less than a mile in total length. The list goes on. Rivers such as the Columbia tributaries, Trinity, Klamath, Rogue, Yellowstone, Bighorn, and Colorado headwater streams all have a need for a better understanding of the effects of implementing these types of structures, which is what this research intends to provide.
This particular topic of designing with multiple, smaller ELJs including their relative benefit, failure modes, fate/damages, and habitat performance could benefit from a literature review, analysis, and further development of design and implementation techniques. This niche or field within large wood could be relevant to conducting detailed performance assessments of constructed wood projects or as part of developing techniques to model linkages between biologic benefit/use and hydraulics, both identified as high priority research items in the large wood research workshop (Reclamation, 2012). This information can also be incorporated into the Large Wood Design Guidelines, currently funded by the S&T program.
A technical memorandum describing the research efforts and results will be composed. In addition, as appropriate, S&T progress reports and a research bulletin will be prepared and the results will be presented at a professional conference.
This information was last updated on October 24, 2014
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