Development of Design Considerations for Engineered Large Woody Debris (LWD) Structures
Naturally occurring in-stream LWD jams, where trees or logs have entered and collected in channels, are found on many streams. Much of the natural LWD in channels was removed during the 20th century; however, replacement efforts are occurring due to the many varied benefits of LWD. Engineered LWD structures are used for habitat restoration and enhancement, bank protection and stabilization, and to alter flow within a channel. Current design methods are based upon a wide variety of incomplete methods that are not applicable in all conditions. As a result, scientifically supportable methods for hydrologists and engineers are not available to size structure components, determine wood stability, and estimate pool development in order to design stable, cost effective, and sustainable LWD structures. This research proposes to determine the size and configuration of wood required for specific river hydraulic conditions and to develop design methods for LWD structures.
Need and Benefit
The delivery of water and generation of hydroelectric power often requires additional steps to comply with the Endangered Species Act (ESA). Mitigation of past project impacts and improvement of fish habitat is often required. Engineered LWD structures or engineered log jams (ELJs) are used in river restoration to form complex assemblages of hydraulics and substrates, protect streambanks, improve fish habitat, and mitigate for past riprap--all of which work in combination to create diverse habitat for use by various species of differing life stages. 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 LWD 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 LWD within rivers is the improvement to flood plain connectivity and creation of habitat and cover for endangered fish species. These improvements to habitat may reduce instream flow requirements, thereby increasing the amount of water delivery to downstream water users. Design of LWD structures that can sustain habitat features and meet program objectives for water users are essential components of successful project implementations. A document that clearly defines existing resources for ELJ design and implementation may lead to more effective and sustainable projects.
Failures of LWD structures are fairly common and result from a lack of specific guidelines, misapplication of existing conceptual guidelines, or poor implementation and design strategies. Reclamation is incorporating ELJs into its project designs at a more frequent rate today than has previously occurred. An essential component of project success is a clear understanding of application limitations for each type of LWD structure and each scenario within which LWD structures are considered for use. The Pacific Northwest (PN) Region is well suited for the addition of engineered LWD in many of its rivers. However, several other regions including the Mid Pacific, Great Plains, and Lower Colorado regions have seen reduced LWD loading and would also benefit from a clear understanding of LWD projects. Rivers such as the Columbia tributaries, Trinity, Klamath, Yellowstone, Bighorn, and Colorado River headwater streams have need for soundly engineered LWD structures.
A literature review (Project #3987, Literature Review of Large Woody Debris (LWD) Guidelines and Implementation Strategies, 2008) has been performed to identify the current guidelines and application limitations. Based on the review, the existing information does not provide complete design methods for engineering and hydraulic performance criteria. Information does exist providing guidelines for certain aspects of LWD design. Several data gaps, such as defined sizing and configurations, placement locations, and habitat creation and value of the LWD, exist. Further research is necessary to:
* Perform field reconnaissance of existing engineered structures to measure the size, vertical, and horizontal location of the LWD structural components and to draw conclusions on the most effective practices and system interactions
* Use the field investigations and a stability force balance analysis to determine stable LWD configurations and sizing
* Use the field investigations to determine the habitat value of engineered LWD structures
* Perform physical modeling to determine the most effective practices in sizing, configuration, vertical extent, and placement of LWD structures to create scour for habitat enhancement
* Develop comprehensive methods that can be used for LWD design and implementation.
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