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Large woody debris fish habitat structure performance and ballasting requirements D’Aoust, Stephane G.

Abstract

Many stream restoration efforts include placement of constructed large woody debris (LWD) habitat structures. These structures are installed in stable channels to rehabilitate summer habitat and critical overwintering refuges in streams, thus attenuating stresses on the aquatic ecosystem until logged riparian areas naturally supply mature windfalls (Slaney & Martin, 1997). This study addresses one of the main problems faced by restoration practitioners: The lack of physically based design guidelines for LWD habitat structures. This study presents the theoretical basis behind design methodologies for three types of LWD structures: (1) Single-LWD, (2) Single-LWD with intact root wad, and (3) Multiple- LWD structures. A field verification program was undertaken to test the applicability the theoretical basis and to refine the design guidelines. Over 80 LWD structures were assessed after construction and again after the fall 1997 to spring 1998 floods. Results indicate that the design approach for single-LWD and single-LWD with root wad structures, based on a factor of safety against sliding failure, successfully predicted the stability of the structures during the past fall to spring floods. The stability of the multiple- LWD structures proved to be more complex to predict since a greater number of design and construction-related factors influence stability. Nonetheless, a design approach based on a safety factor against buoyant failure is recommended. Recommendations with respect to the design and construction of LWD structures are also presented as part of this study.

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