Stability of Ballasted Woody Debris Habitat Structures

Abstract

An important component of stream restoration in the Pacific Northwest is the reintroduction of large woody debris (LWD) and construction of LWD habitat structures in degraded systems. A significant problem faced by engineers involved in stream restoration is a lack of physically based guidelines for design and construction of ballasted LWD habitat structures. A simple theoretical approach is developed that forms the basis for determining ballast requirements for three types of LWD structures. Field monitoring and assessment were undertaken to test the approach and to compare predicted and observed stability for approximately 90 ballasted LWD structures. The results indicate that the stability of single-LWD and single-LWD with root wad structures can be successfully predicted by the theory. The stability of the multiple-LWD structures proved to be more complex to predict because numerous design and construction-related factors influence stability. A design approach based on factors of safety is recommended.