Watts Savery, Anne. 2000. A comparison of the hydraulic effect of large woody debris and an engineered alternative. M.S.
A study was conducted comparing the relative hydraulic effects of an engineered wood alternative to large woody debris in stream restoration. The wood alternative (ELWdTM) is available in kits, with a maximum length of 7.3 m and diameter of 60 cm. ELWdTM structures consist of several small diameter logs, which combine to form a single structure. The engineered structures were developed as an alternative to comparatively cumbersome large woody debris for habitat restoration projects. Engineered structures have low mass, high roughness and no rootwad. Engineered structures were paired with large woody debris of similar size in two study sites in western Washington, the Mashell River and Griffin Creek. Griffin Creek engineered wood was given engineered 'roots' in order to be comparable to the large woody debris (LWD) that had rootwads. Logs were placed with the tip of the log in the stream at a 30 - 42° angle from the bank and the butt end on the bank. The logs were tethered with 1.5 cm steel wire rope to anchor trees with 9.1 m slack. A 0.5 x 0.5-m survey was performed on the channel bottom at each log before installation and one year afterward in order to determine influence of logs on the stream. The ability of the logs and the engineered structures to maintain position over the course of the study was compared in two ways, change in angle and movement downstream. The differences in the mean depth scoured by the two log types in Griffin Creek were not statistically significant. The lack of a rootwad comparable to that of native wood and low mass affected the stability of the engineered large woody debris. Engineered structures were pushed downstream with a greater frequency than LWD. Differences in the means of the final angles of LWD and ELWdTM structures in Griffin Creek were not statistically significant. The ability of the engineered structures to maintain channel position was affected by the low mass and lack of a significant root wad. The mobility of large woody debris was limited mainly to change in angle to the stream bank.
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