American Society of Civil Engineers


Flume Analysis of Engineered Large Wood Structures for Scour Development and Habitat


by C. D. Svoboda, (Bureau of Reclamation, Technical Service Center, Hydraulic Investigations and Laboratory Services Group 86-68460, P.O. Box 25007, Denver, CO 80225. E-mail: csvoboda@usbr.gov) and K. Russell, (Bureau of Reclamation, Technical Service Center, Sedimentation and River Hydraulics Group 86-68240, P.O. Box 25007, Denver, CO 80225. E-mail: krussell@usbr.gov)
Section: Hydraulics and Waterways, pp. 2572-2581, (doi:  http://dx.doi.org/10.1061/41173(414)267)

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Document type: Conference Proceeding Paper
Part of: World Environmental and Water Resources Congress 2011: Bearing Knowledge for Sustainability
Abstract: Engineered large woody debris (LWD) structures are being introduced into rivers due to the many realized benefits of woody debris, particularly with respect to salmonid habitat restoration. In this study, a 1:11 Froude scale physical hydraulic model of a gravel bed river channel was constructed to observe the scour patterns and extent of erosion and deposition produced by six LWD configurations. The model represented a "typical" river in the Pacific Northwest as determined by hydraulic geometry relationships. The material size was scaled such that the median grain size (d50 = 8 mm) was near incipient motion for bankfull flow in order to identify bed movement occurring only as a result of interaction with the LWD structure. The scaled logs were simplified to include the trunk and rootwad; smaller branches were excluded. The LWD configurations contained between two and seven logs that were placed in a variety of orientations such as X-shape, trapezoidal shape, or free-form shape. All other parameters (slope, bed material, log size) were kept constant. Each configuration was tested at two discharges: near bankful (90%) and mid depth (50%) flow conditions. Changes in bed topography were recorded using photogrammetry techniques. Aggradation/degradation maps were produced to compare physical changes from the different configurations. Rootwads facing upstream deflected flow around the structures producing less scour within the structure. Downstream oriented rootwads produced localized scour zones. Slower velocities downstream of the structures allowed for deposition of the scoured material. Results provide information to designers of LWD structures to assist in selecting the most effective structure(s) to meet their project objectives.


ASCE Subject Headings:
Flumes
Wood structures
Scour
Debris
Floods