Thursday, June 7, 2007 - 8:15 AM
335

Quantification of the structure and function of natural and restored large woody debris jams

Martin Doyle, PhD1, Rebecca Manners, MA2, and Melanie Small, MA1. (1) Department of Geography, University of North Carolina-Chapel Hill, CB #3220, Chapel Hill, NC 27599-3220, (2) Inter-Fluve, 1020 Wasco Street, Hood River, OR 97031

Numerous studies exist on the hydraulics of woody debris jams and the mechanisms driving their geomorphic and ecological effects. While most studies treat jams as single, solid objects, jams are clearly not single cylindrical logs, but rather an accumulation of pieces ranging from leaves and twigs to entire trunks. Here we treat debris jams as porous accumulations of heterogeneous material. We systematically dismantled three natural debris jams, removing 17,783 individual wood pieces to experimentally manipulate jam porosity.  We measured the surrounding velocity, shear stress, and drag force (FD). Results show that assuming non-porosity can result in a 20% over-estimation of FD and a comparable over-estimation of available benthic habitat created by the jam.  Back-calculated values of the combined drag coefficient and frontal-area term in the momentum-extraction equation for each jam at each stage of removal quantified the cumulative hydraulic effects of size and composition of the jam. Only low porosity jams, containing substantial fractions of small woody material, produced the geomorphic and hydraulic characteristics commonly associated with woody debris jams. Common applications of LWD in river restoration projects poorly reproduces natural variability in wood size distribution, and thus will not induce the processes sought by restoration efforts. 


Web Page: www.unc.edu/~mwdoyle/